The Geology of the Region about Preservation and Chalky Inlets, South-west Fiordland, N.Z.
[Read before the Otago Institute, June, 1933; received by the Editor, July 7, 1933; issued separately, October, 1935.]
A. Ordovician Sediments and their Metamorphism.
1. The Least Altered Rocks.
Quartzites; greywackes; graptolitic slates; grey argillites; spotted argillites.
2. Regional Metamorphism.
Spotted phyllites; chloritic sericite-schists; biotite-hornfelses and schists; porphyroblastic biotite-schists and paragneisses; quartz-sericite-amphibole-schists; hornblende-biotite-schists or amphibolites.
3. Contact Metamorphism.
(a) With little dynamic influence.
Hornfelsic greywackes; biotite-argillites; quartz-biotite-hornfels; cordierite-biotite-hornfelses; inclusion of slate in granite.
(b) With strong dynamic influence (“injection-schists”).
Quartzose schists; quartz-felspar-biotite-(chlorite)-schists; quartz-biotite-tourmaline-schists; coarse-grained quartz-biotite-sericite-schists; coarse-grained quartz-biotite-sericite-epidote-schists; andalusite-cordierite-mica-schist; pinite(?)- or sericitised andalusite-schist; sillimanite-mica-schist; calc-silicate-hornfels-schist.
(c) The Long Sound Series.
4. Summary: Progressive and retrogressive metamorphism.
B. Plutonic Rocks.
Granites; quartz-biotite-syenite; granodiorites (or soda-tonalites); quartz-mica-diorite (or tonalite); diorites; gneissic microdiorites; hornblende-norite
C. Dyke Rocks.
Granite-pegmatite; granodiorite-pegmatite; diorite-pegmatite; greisen; granite-aplite; syenite-aplite; granodiorite-aplite; uralitised dolerite; lamprophyre.
D. Summary of the Igneous Rocks.
List of References.
The Ordovician Sediments and their Metamorphism.
1. Least Altered Rocks.
The least altered Ordovician sediments occur in the Cape Providence promontory and the northern part of Gulches Head, and form both the northern and the southern shores of Preservation Inlet. The eastern shores of this inlet, however, show the effects of thermal metamorphism adjacent to the granite, and throughout the whole of the area herein described (with the exception, so far as is yet known, of Gulches Head Peninsula) effects of metamorphism are displayed at least in the spotting of the most responsive pelitic beds, and the very general distribution of small crystals of authigenetic
tourmaline. North-westwards from Cavern Head a slight rise in metamorphic grade may be recognised, which may be traced with increasing intensity northwards along the shore of Chalky Inlet to Cunaris Sound. The following are the characteristics of the least altered types of rocks.
The quartzites of the Lancefieldian Series range in average grain-size between 0.7 mm. and 1.5 mm. The quartz grains are sub-angular, sometimes associated with small fragments of chert (1596, 1599),* and set in a subordinate matrix of minutely granular, partially reerystallized quartz and sericite. Where this matrix is more abundant it may be darkened by carbonaceous particles (1700). Grains of glauconite together with a little albite are occasionally present (1604); small flakes of chlorite (1700, 1701) and accessory zircon, apatite, sphene, rutile, ilmenite or magnetite may be seen in various rocks. The first traces of mechanical change are seen in the incipient marginal granulation of highly strained quartz grains (1701). Chemical change commences with the appearance of minute prisms of tourmaline, and the development of finely-flaky biotite, especially in the more argillaceous layers. This is well displayed in the quartzite at the western end of Kisbee Beach (1604), the laminated siltstone of Cavern Head (1593), and the quartzite in Berg's Claim, Te Oneroa (1702), the last a dark-coloured rock of glassy appearance containing about 3% to 4% of biotite with abundant prisms of tourmaline up to 0.12 mm. in length.
Increase in the amount of feldspar in the siliceous rocks leads by transitions into the greywackes. Samples of these from Southport and the islands in Preservation Inlet (1580, 1641, 1657, 1661) have a more or less fine grain-size, 0.1–0.3 mm. in the finer-grained, 0.3–0.6 mm. in the more coarsely granular. The felspar (Ab94 An6) is in twinned cleavage-fragments, and there is a minutely granular quartzo-feldspathic matrix with tiny flakes of sericite, and rarely larger flakes (0.2 mm.) wrapping about the quartz grains. A little partly chloritised biotite is present, some finely divided carbonaceous material, and a scattering of granular siderite or calcite, with accessory apatite, sphene, and zircon.
The general characters of these rocks are very uniform throughout the Lancefieldian (1684), Bendigonian (1656), and Castlemainian (1660) fossiliferous beds. They contain an abundance of finely divided carbonaceous matter, and very often scattered crystals of pyrites. Minutely granular (0.01–0.03 mm.) quartz and sericite are the chief colourless minerals, and small prisms of tourmaline (0.02 mm.) are occasionally present (1660). A little sodic plagioclase and thin films of chlorite are also seen in the last rock, which comes from the northern shores of Coal Island.
[Footnote] * The numbers refer to slides and specimens in the museum of the Department of Geology, University of Otago.
These rocks differ from the last chiefly in their smaller amount of carbonaceous matter, whilst abundance of sericite gives them a phyllitic appearance, though the fissility is rarely excellent. These rocks are common along the southern and north-western shores of Preservation Inlet and in the adjacent islands. Strain-slip cleavage is often well displayed. Their quartz grains are very small (0.005–0.02 mm. in diameter), and the flakes of sericite are rather larger than in the last-described rocks and more or less in parallel position. There are a little apatite, occasional accessory sphene, needles of rutile, a little pyrites, and rare zircons (1600, 1653, 1672). The proximity of the granite is reflected in the occurrence of scraps of biotite and sparse prisms of tourmaline in the rocks (1672) forming the eastern side of Long Island, in which also are numerous faintly-coloured flakes of chlorite up to 0.1 mm. in length.
Throughout the whole of the least-altered Ordovician sediments, the argillites most responsive to metamorphic influences have become spotted. The earliest stages in the change are displayed in the beds lying west of the fossiliferous Castlemainian rocks in Coal Island. The pale grey rock (1666) has a silky sheen, and is composed of a very fine quartz mosaic (0.015 mm.) with an abundance of flakes of sericite, a very little biotite, and a moderate amount of chlorite. There are scattered rhombs and grains of siderite. The spots (oval nodules less than 2 mm. long) are as finely granular as the matrix, from which they differ in the relative scarcity of sericite within the spot, though it is aggregated into a mantling layer about the spot. Other samples show spots produced by the concentration of carbonaceous material in the sericitic base. As Tilley (1924) has commented, the spots consist essentially of the same minerals as characterise the rest of the rock, but in somewhat different proportions. Their production has been compared with the formation of concretions in calcareous sediments (Flett, 1907, p. 23).
A rather more advanced stage is seen in a rock (1685) interstratified with the quartzites at Te Oneroa, in which there is a greater abundance of sericite flakes (0.02–0.04 mm.) and some chlorite in the matrix; the oval spots (1.0 × 0.7 mm.) consist of a few rather large quartz grains and are wrapped about by matted flakes of biotite. East of this the spotted character is soon lost in the narrow zone of hornfels adjacent to the granite, but the gradual disappearance of this character as the argillites suffer dynamic metamorphism may be traced from Cavern Head to Seek Cove and northwards from Southport into Cunaris Sound.
2. Regional Metamorphism.
In this section is traced a series of changes which are not obviously connected with the proximity of the outcropping masses of granite. In order to link the rocks concerned with the sediments already considered, the most characteristic type, the spotted phyllites, will be considered first.
As noted above, these rocks have been found about Preservation Inlet and extend at intervals along the coast from Cavern Head past Southport into Cunaris Sound, following the southern shore of this Sound to the mouth of Cliff Cove, and occurring also on its northern shore near the entrance to Islet Cove, but further north are represented only by traces of spotting detected in rocks in Edwardson Sound, and have not been seen further east in Long Sound, where the rocks are of higher metamorphic grade. On the shore of Chalky Inlet a mile north of Stripe Head and at Surf Point the rocks of this facies (1544, 1608) have a very fine quartz mosaic with grain-size about 0.02 mm., and a greater amount of sericite in tiny parallel shreds associated with a little biotite. Sandy layers are shown by bands of strained quartz grains up to 0.5 mm. in diameter and a few plagioclase grains. The spots are 2 to 3 mm. long and composed of a quartz mosaic (grain-size 0.1 mm.) and a few flakes of biotite. About the spots is a matted covering of biotite flakes with a little almost isotropic chlorite. 1608 is remarkable in that small flakes of chlorite play a greater part than quartz in the composition of the spots, and are therein associated with a minor amount of biotite. There is no indication that this chlorite is derived from the biotite, and perfectly fresh biotite greatly predominates in the matted husk about each spot. Finely-divided epidote, dark granular zircon, scattered iron ore, tiny grains of leucoxene (?), a few prisms of tourmaline, and a string of pyrites occur in one or other of these rocks, which differ but little from those in Preservation Inlet. These spotted phyllites suffer modifications which cause them to pass into closely allied rock-types exemplified especially by 1582 and 1547.
In Cunaris Sound, a mile to the south-east of the Cunaris Islands, and again three miles further east (1549, 1582, 1609), further changes are apparent. The sizes of the spots, and of their constituent grains, of the mica flakes that envelop them, and of the quartz mosaic and mica flakes in the matrix are all approximately double those in the rocks described above. The biotite is red-brown and is in considerable abundance (about 8% in some cases). Occasionally in the less micaceous but well laminated types, in which the spots are as abundant as in others (1582), the mica is aggregated into indefinitely-bounded layers, and grouped therein into lensoid patches, the layers consisting alternately of quartz and sericite. In general there are present small prisms of authigenetic tourmaline, scattered magnetite, and a little pyrites. Sometimes (1549) there occur thick plates of positive pennine a millimetre in length, fraying out into long thin lamellae which are intergrown with biotite. It resembles chloritoid in many respects, but is not so strongly refractive. It is extremely like the pennine figured and described by Dunn (1929) as possibly pseudomorphous after ottrelite.
The penultimate stage in the disappearance of the spotted texture is seen in 1547, occurring on the eastern side of Edwardson Sound, three miles north of Divide Head. The grain-size of the quartz in the mosaic of the base is here rather irregular, varying from 0.05 mm. to 0.10 mm. in alternating laminae respectively richer
and poorer in sericite, but in certain roughly ovoid patches is 0.2 mm. or more, whilst the biotite matting about them has become coarse and rather irregularly redistributed into the ground mass, is partly chloritised and associated with more or less leucoxenised ilmenite. Like the sericite, the biotite is distributed unevenly in layers respectively containing much or little of this mineral; together these two micas constitute about 20% of the rock. There are a few small (0.3 mm.) euhedral or spongy crystals of garnet, whilst zircon is an abundant accessory in prisms up to 0.15 mm. in length with a little apatite and brownish-green tourmaline. This rock is interbedded with typical finely schistose biotite-hornfels and with porphyroblastic hornfels.
The spotted schists described above are interbedded with the metamorphosed equivalents of the other Ordovician sediments, of which two main facies may be distinguished—the chloritic sericiteschists and the more or less schistose biotite-hornfels—which result from different grades and conditions of metamorphism, the chloritic rock being the product of the retrogressive alteration of biotite-sericite-hornfels. Traced north-west from Cavern Head the finer-grained rocks take on a silky sheen and greenish tinge (the semi-schistose rocks of McKay (1896), and near Southport are developed as chloritic sericite-schists. About halfway between the entrance to Southport and Stripe Head (1523, 1579) they consist of a fine rather uneven-grained matrix of quartz and very subordinate albite (about Ab96 An4) and rarely a little orthoclase in grains 0.02–0.10 mm. in diameter, with larger grains of quartz and feldspar 0.4–0.8 mm. in length, the longer axes of which tend to lie in the direction of schistosity. This latter is marked by abundant minute flakes of sericite and sparse optically negative but almost isotropic pale-green chlorite, which has been developed from biotite. A little irregular granular leucoxene is associated with this last mineral, often distributed along its cleavage. Apatite and zircon occur in small grains, and there are a few surviving flakes of biotite up to 0.3 mm. across. The rocks have evidently been derived from sediments intermediate in character between the argillites and the fine-grained greywackes described above.
Rocks of this type outcrop also on the west side of Southport and in the Garden Islets. Specimens from the most southerly and most northerly of these islets (1520, 1521) have grain-size (0.08 mm.) rather more even than in others of this group, with more untwinned feldspar and zircon (causing dark spots in the chlorite), and show also the presence of many thin bands of crushing almost parallel to the plane of schistosity in which the chlorite flakes tend to accumulate, and grains of leucoxene have been formed. A minor result of this granulation is seen in the transverse fracture and drawing-out of a tourmaline prism.
Biotite-hornfelses and Schists.
The rocks last described are interstratified with bands of more or less siliceous lustrous pale brown material, differing from them in no other essential feature than that they possess abundant
biotite. There are the same ramifying minute shearing zones, along which the biotite has been changed to chlorite, and pyrites and leucoxene have been formed (1519, 1522, 1525). Sometimes veinlets of quartz occur along these zones, or the quartz grains are grouped into lenticular patches. Occasionally, in place of shearing, there is a minute corrugation of the schistosity producing helicitic texture on a small scale. The three rocks mentioned illustrate the very fine and moderately fine-grained argillites and a more arenaceous type in which, however, the sutured margins of the quartz grains give evidence of their recrystallization. Sphene, in compact grains, or leucoxene, zircon, apatite, tourmaline, and pyrites make up the accessories. In the northernmost of the Garden Islets these rocks are interstratified with an amphibolite described below. Those sectioned were collected on the eastern half of this islet, but its western portion consists of a great mass of sheared mylonitised breccia which marks the Southport Fault-zone described in Part I of this paper. The minor brecciation observable in microscope-sections of the adjacent rocks shows that the effect of this great dislocation extended into the schists at least fifty yards to the east. On the west side of the fault-zone, however, are almost unaltered greywackes and graptolitic slates.
North-eastwards from the Garden Islets, the biotite-schists and hornfelses are interbedded with spotted schists and are associated with plentiful transition types. Thus 1513 from Stripe Head shows spotting in a schistose derivative of a fine-grained greywacke. The quartz grains are very irregular in size, and the larger lensoid grains 1 mm. long are oriented in the direction of schistosity and show indented margins. There is a minor amount of feldspar and of finely-flaky sericite. Small biotite flakes are scattered very unevenly through the groundmass and are especially aggregated into irregular patches or spots. Zircon is plentiful in grains up to 0.2 mm. in length, and so also is leucoxenised ilmenite. 1532 from a mile south-west of Surf Point is almost a quartz-schist. The rock has been recrystallized with a decrease in the unevenness of the original grain-size, and lensoid quartz grains with indented margins are elongated in the plane of schistosity and are associated with a little albite. Finely-shredded sericite and biotite occur in small amount, the latter being chloritised along shearing planes.
More characteristic fine-grained schistose biotite-hornfelses occur in the regions of greater metamorphism and are represented by 1547 and 1675, collected three miles east and three miles north-east respectively of Divide Head, by 1624 from Divide Head itself, 1623 from Surf Head, 1622 south-east of Cunaris Islands, 1577 and 1582 one mile and three miles east respectively of Cunaris Islands, 1503 and 1511 on the south side of Last Cove, and 1645 from the injection-complex on the west shore of Edwardson Sound. The last three represent the extreme conditions of metamorphism. Traces of spotted structure still remain in 1582 and also in 1547, which outcrops as a fine-grained band among more coarsely-granular schists and has already been described on an earlier page (p. 111).
In 1675, while the sericite-quartz mosaic has a grain-size of about 0.1 mm., the biotite flakes are larger (0.5 mm.) and tend to porphyroblastic growth and sieve-structure, so that this rock links the fine-grained biotite-schists with the porphyroblastic group. 1577 has a matrix of sutured quartz grains varying from 0.05 to 0.2 mm. in diameter, with a little feldspar and about 8% of biotite in flakes up to 0.2 mm. in length, together with a lesser amount of sericite, rare tiny flakes of chlorite, and the usual accessories zircon, apatite, and tourmaline. In 1623 and 1624 there are elongated porphyroblasts of quartz and rarely of plagioclase up to 1 mm. in length, generally oblique to the schistosity, set in a finely-granular base of quartz, minor feldspar, about 15% of sericite in tiny parallel flakes, and chlorite which appears in some layers as fairly plentiful minute shreds. The feldspar includes andesine (about Ab65 An35) and a little orthoclase which may be perthitic. There is about 7% of biotite in ragged, larger, poorly poikiloblastic flakes which is accompanied by rare crystals of white mica up to 0.7 mm. in length. A little finely-granular epidote occurs in addition to tourmaline, rutile, and the usual other accessories.
The most highly metamorphosed rocks of this class occur around Last Cove in Long Sound, and show transitions into two other groups of schists. 1503 and 1511, adjacent to the granite on the southern side of Last Cove, contain about 10% of deep-redbrown biotite aggregated in layers, the individual flakes being 0.5–1.0 mm. in length. With these is about 5% of poikiloblastic white mica intermediate in optical character between muscovite and sericite (2V = 40°), together with a little finely-shredded sericite replacing acid plagioclase. Quartz makes nearly 80% of the rock in elongated grains 0.2–0.5 mm. long. Zircon, apatite, and occasional relatively large prisms of tourmaline (0.2 × 0.9 mm.) are present. By decrease in the amount of mica these rocks pass into quartz-hornfelses, a transitional type being seen (1502) from the southern shore of Long Sound immediately opposite Last Cove.
A larger amount of mica on the other hand is present in 1524 from the north shore of Long Sound, two miles east of Last Cove. Its quartz mosaic (0.1 mm.) contains a little oligoclase, and deep-red-brown mica makes up 30% of the rock. There is a general dusting of finely-divided, partly leucoxenised ilmenite, and there is no sericite. This type is interbedded with much more coarsely-granular schists, including those containing sericite, and affords a transition into the coarsely crystalline mica-schists. 1514, occurring between the outcrop of this last rock and Last Cove, and 1574, on the opposite shore of Long Sound, are sericite-bearing rocks intermediate in character between 1524 and 1511 and 1503. Transitional types similar to 1524 are well developed on the western side of Edwardson Sound among the rocks of the injection-complex there, and are well exemplified by 1638, which differs from 1524 only in the absence of ilmenite and presence of sericite and much apatite, and by 1645 from the same complex. Both are interstratified with more coarsely-granular types in which the grain-size has grown to 0.4 mm., and are invaded by thread-like veins of pegmatite. There is a considerable resemblance between several of these biotite-quartz-schists and the matrix
of rocks (1339, 1340) obtained by Turner (1933, pp. 216, 217) as boulders in Laschelles Creek, Cascade Valley, one hundred and sixty miles north-east of Chalky Inlet. One of these (1339), however, contains porphyroblasts of muscovite which have not yet been observed in analogous rocks in the southern region.
A peculiar rock (1556) from the southern end of the Edwardson Sound complex occurs as a long thin inclusion in the granite. About 60% of biotite is present in rather puckered layers enclosing between them lensoid masses of ferriferous calcite, whilst there is a matrix of quartz, plagioclase (Ab70 An30) and a little orthoclase with grain-size about 0.1 mm. Plentiful sphene, a little zircon and apatite, and occasional needles of rutile are also present.
The poorly-schistose biotite-hornfelses are especially interesting in that they preserve on their weathered surfaces traces of original current-bedding. As an example of them, 1644, from the entrance to Cliff Cove, may be mentioned. It is characterized by very uneven grain-size of the quartz (0.05–0.4 mm.) inherited from the original greywacke sediment, by irregularly-bunched sericite replacing grains of plagioclase and as yet incompletely redistributed, and by irregularly-scattered often sieve-like biotite porphyroblasts 0.3–0.5 mm. in diameter. Like 1675, described above, it affords a transition into the porphyroblastic schists.
Porphyroblastic Biotite-schists and Paragneisses.
The occurrence of a layer marked by unusually large biotite flakes in 1578 above introduces an important group of rocks wherein these are the distinguishing feature. Two series may be instituted according to the degree to which the flakes are developed—the moderately porphyroblastic biotite-schists and paragneisses in which the larger biotite flakes are still very irregular in outline and not of great size, and the markedly porphyroblastic biotite-hornfelses in which the biotite forms fairly large and thick, more or less idiomorphic crystals, usually with definite sieve-structure. These rocks have a massive or somewhat schistose structure, and have a superficial resemblance to fine or medium-grained gneisses, under which designation they were classes in previous reports on this region.
(a) The moderately porphyroblastic rocks have been obtained from the head of Edwardson Sound (1535, 1585), from the east coast of the same, three and two miles north of Divide Head (1567, 1583), and from a mile east of Last Cove in Long Sound (1514). In the first group the grain-size of the quartz mosaic is 0.05–0.1 mm., and in the rest about 0.2 mm. The biotite forms about 15% of the rocks and is in scattered flakes with a tendency to be aggregated and to be sieve-like. In the first group the flakes are light-reddish-brown and about 0.4 mm. in diameter, and a trace of earlier “spotting” remains. There is none of this structure in the rocks of the second group, where the biotite flakes are larger and a little more sieve-like and aggregated. These differences are still more strongly expressed in 1514, the extreme member of this series, in which the biotite is deep-red-brown and as much as 1 mm. in diameter, with a tendency to extension along the plane of schistosity (Pl. 23, Fig. 1). In all there is a little feldspar and sericite in the matrix, with the
usual accessories of apatite, zircon, and a little tourmaline. In the rocks from Edwardson Sound a very little epidote seems to occur, and in that from Long Sound there is about 2% of platy ilmenite in or about the biotite. The features of this group of rocks are reflected precisely in specimens obtained by Turner (1933, pp. 198, 199), in the Cascade Valley, notably 1208 and 1349, banded paragneisses occurring in situ in the gorge of a small north-eastern tributary of Martyr Creek, one and a-half miles above its confluence with Cascade River. These rocks occur there in association with others exemplified by 1345, which are typical of the markedly porphyroblastic facies.
(b) The markedly porphyroblastic biotite-schists or paragneisses vary one from another chiefly in the size of their constituent minerals. They appear first at the southern entrance to Cunaris Sound as a rock (1581) which resembles a fine-grained gneiss in hand-specimen save for its sericite base, which proves in thin section to be made up of a quartz mosaic (grain-size 0.03 mm.) with an almost equal amount of sericite (0.05 mm.) together with a little chlorite. It is faintly dusted by very finely divided carbonaceous particles and contains scattered sparsely through it small more or less leucoxenized grains of ilmenite (0.1 mm.), a little pyrites, and rarely tourmaline. The characteristic porphyroblasts of biotite have a moderately deep-brown colour at darkest, tend to be oval in basal section with a maximum diameter of 0.7 mm., and have strongly embayed margins fraying out laterally into thin lamellae. They show marked sieve-structure. In 1537 from Divide Head, the porphyroblasts are twice as large as those in 1581 and generally are sharply bounded by the basal plane. In both rocks the frequent obliquity of the lines of inclusions to the schistosity of the matrix, the obliquity also of the lamellar extensions of the porphyroblasts to the cleavage-planes of these latter, and the presence of coarsely recrystallized groundmass adjacent to them show that the porphyroblasts have been rotated to some extent. Thin layers of chlorite have sometimes formed on the edges of the biotites, but there are also in 1537 independent twinned sieve-plates of optically positive pennine up to 0.5 mm. in length, resembling chloritoid, but with refractive index little different from that of biotite, with which they are sometimes closely associated. They appear to be independent crystals and not pseudomorphs after biotite. In 1542, from three miles east of Divide Head, a trace of a biotite-fringed “spot” still remains, and the porphyroblasts are rarely above 0.5 mm. in diameter and are generally elongated on the schistosity plane, whilst the matrix is slightly coarser than in the last two rocks. 1625 from the shore of Cunaris Sound south of Cunaris Islands closely resembles 1542, though its matrix is finer and sprinkled with carbonaceous dust and its porphyroblasts of biotite rounded in outline and with very minute sieve-structure. Increase in the coarseness of the quartz mosaic is marked in 1531, 1533, and 1552 from the east coast of Edwardson Sound, two to three miles north-east of Divide Head, but it is obvious in them that grain-size is a function of original composition as well as of metamorphic grade. Thus 1533 contains two layers, in one of which the matrix has a grain-size of
0.08 mm. and contains but little sericite, while the porphyroblasts are 1 mm. in diameter and have well-marked basal planes and few inclusions; in the other the matrix contains abundant sericite, apatite, and zircon, with rare feldspar and tourmaline, and has an uneven grain-size, usually 0.2 mm., but with a number of quartz grains 0.5 mm. in diameter, while the biotite porphyroblasts are exceedingly irregular in outline and full of inclusions. In 1531 the evidence of rotation of porphyroblasts is clear, and lines of sericite flakes run sinuously between them (Pl. 23, Fig. 3). 1576 from the entrance to Cliff Cove is very similar to the last, and on weathered surfaces still shows traces of the current-bedding of the parent sediment.
The extreme development of the porphyroblastic schists is seen in a rock (1506) outcropping half a mile east of Last Cove interstratified with a fine-grained mica-hornfels. In it the quartz grains of the coarse mosaic (0.2–0.3 mm.) have been elongated in the direction of schistosity and are associated with a minor amount of sericite in thin parallel flakes up to 1 mm. in length, while the porphyroblasts, which tend to have similar orientation, are about 1 mm. broad and 0.5 mm. thick and have a reddish-brown colour.
So very characteristic a rock type as these porphyroblastic hornfelses might well serve, if widely distributed, as a means of correlating schists in different regions, and it is therefore of importance to note that its wide distribution appears probable. Thus a rock (D.5) obtained by Professor Speight (1910) from Pickersgill Harbour, Dusky Sound, seven miles from the head of Edwardson Sound, proves to be truly intermediate in character between the fine-grained argillites of the region now described and such fine-grained porphyroblastic schists as 1581 from the entrance to Cunaris Sound, from which it differs only in the smaller grain-size both of the porphyroblasts (0.3 mm.) and the quartz-sericite mosaic (0.2 mm.). On the other hand, the most coarsely granular of the porphyroblastic schists are matched in nearly every detail by a small group of rocks (1213a, 1249, 1345) termed hornfelsic paragneisses by Turner (loc. cit.) and obtained in situ in the valley of Martyr Creek and as boulders in Laschelles and Colin Creeks, tributaries of Cascade River, nearly a hundred and sixty miles north-east of Chalky Inlet. Turner (1933, p. 200) comments especially on the occurrence of alternating bands of coarsely- and finely-granular rocks, such lamination being apparently a more common feature in the Cascade River area than it is in the southern region.
In three localities the biotite-hornfelses include narrow bands of rock which in the field resemble dykes of feldspathic diorite, namely, near the southern entrance to Cunaris Sound (1578), near the mouth of Cliff Cove (1554), and at the western side of the head of Edwardson Sound (1555). As in each case microscopical features are clearly not of igneous origin, the rocks may be held to represent original thin bands of dolomitic marl. They consist chiefly of quartz in small interlocking grains with a rather subordinate amount of feldspar now completely replaced by sericite. Amphibole forms 5% of the rock and is present in rather spongy grains and prisms up
to 1.5 mm. in length, either elongated in the direction of schistosity or in sub-radiating or sheaf-like aggregates. It has a very pale colour and pleochroism, changing from a pale-straw to an apple-green tint. Grains of sphene are abundant, and apatite forms rather large prisms. There are a few grains that are possibly allanite, together with some epidote, magnetite, pyrites, and rare tourmaline. In 1555 there is rather more amphibole than in the others, and a few grains of sieve-like garnet appear. Garnet also occurs in 1554 as a partly idioblastic grain 1.5 mm. across, but the sieve-amphibole has been replaced in this rock by pale chlorite, and there are also a few sheaf-like aggregates of zoisite prisms.
Hornblende-biotite-schists or Amphibolites.
The sericitic quartz-biotite-schists which make up the eastern half of the islet north-west of the entrance to Southport contain interbedded among them a layer about a score of feet thick in which hornblende is the predominant mineral (1526). It forms about 40% of the rock and is segregated into indefinite layers. The mineral occurs in prisms up to 0.4 mm. in length, the central portions of which are often darker than the margins, and may contain many schiller-inclusions of iron-ores. The pleochroism follows the scheme X = pale-yellow-brown, Y = brownish-green, Z = bluish-green, and the angle Z to C is 20°. There is about 8% of biotite in deep-brown flakes, separate or intergrown with the hornblende, and only slightly chloritised. Iron ores form about 3% of the rock, and there are many small prisms of apatite and grains of sphene. The colourless minerals are chiefly labradorite (Ab40 An60), but so much of this is untwinned that it is difficult to estimate the relative proportions between the plagioclase and the quartz. The structure of the rock is more or less granulitic, and the grain-size about 0.1 mm.
In the southern end of the injection-complex on the western side of Edwardson Sound the granite contains a thin, irregularly-bounded strip of a dark hornfelsic rock of which the mineral composition is approximately as follows: quartz 35%, andesine (Ab60 An40) 30%, biotite 20%, hornblende 11%, sphene 2%, clinozoisite 1%, with scattered minute crystals of apatite, pyrites, and magnetite. The largest grains are those of hornblende, which may reach a length of 1.5 mm., but are rarely a quarter of that size. The biotite flakes are fresh and about 0.2 mm. long. The quartz and feldspar mosaic has a grain-size of about 0.1 mm., and the sphene is in tiny oval granules.
The original nature of these rocks is not quite clear. The first gives no indication of being a sill invading the adjacent formations, and is not a flow. It was possibly a tuff. The second may have been similarly derived, and have been caught up as an inclusion in the invading granite, its general position being still parallel to the schistosity of the adjacent schists. The possibility that these rocks may have been derived from marly sediments must not be overlooked. Attention may be drawn in this connection to Vogt's (1927, pp. 215, 486) account of hornblende-clinozoisite-oligoclase-bearing schists containing a noteworthy amount of quartz and biotite which are derived from lime-rich sediments. In our rocks the greater basicity of the feldspar and the smaller amount of clinozoisite may
indicate a different mode of expression of the lime-content of the original rock. Vogt's rocks occurred within the oligoclase zone of the sequence of regional metamorphism recognised by him, a much higher grade of regional metamorphism than that attained by the biotite-sericite-schists that are associated with the amphibolite of Garden Islet. On the other hand, Sugi (1931) points out that two phases of metamorphism may be effective in the production of amphibolite, actinolitic green-schists with sodic plagioclase and epidote passing into amphibolites with calcic plagioclase as a result of the contact effect of quartz-diorite intrusions. That some contact effect has been exercised in the region of the Garden Islets is shown by the presence of tourmaline in the biotite-sericite-schists, but it may be doubted whether it was sufficient to account for the presence of labradorite in the amphibolite. There is no doubt that such effects may have been significant in connection with 1613, which is actually an inclusion in granitic rocks.
3. Contact Metamorphism.
(a) With Little Dynamic Influence.
The rocks to be described in this section fall into two principal groups—those that owe their metamorphism principally to the heat of an invading rock, viz., the contact hornfelses sensu stricto, and those that express the extreme terms in the series of dynamothermal changes in the zones of intrusion-brecciation and lit-par-lit injection about the batholiths in the regions of crystalline schists. The former are very limited in their distribution, occurring so far as has been observed only along the western flanks of the Treble Mountain-Bald Hills batholith, for they were collected only from the northeastern and eastern shores of Preservation Inlet. Half a mile northeast of Cuttle Cove, a dyke of diorite invades greywacke which has become converted at the contact into a massive hornfels (1592) consisting of quartz in grains 0.3 mm. in diameter or in poikilitic masses of twice that dimension including flakes of biotite, while there are also pseudopoikilitic grains and smaller prisms of plagioclase (basic andesine or acid labradorite) largely altered to sericite. The flakes of biotite are very abundant and of deep-red-brown colour, though in irregular patches they are completely chloritised with separation of rutile.
A more extensive and varied series of contact-altered sediments was obtained along the shore north of Kisbee Bay. Though the introduction of tourmaline into many of the sediments about Preservation Inlet shows the effect of granitic emanations, the change in the rocks of this contact-series is more localised and profound.* The actual distance of each specimen noted below from the nearest point on the margin of the invading granite is uncertain, but as an approximation it may be taken as rather less than half that from
[Footnote] * While Goldschmidt and Peters (1932) have shown that the tourmaline in slates or other rocks derived from marine sediments may often result from “the mobilising, redeposition and local enrichment of the boron in the original sediment” (processes which may well have occurred in the formation of our slates), there is evidence in this region, e.g., in the presence of quartz-biotite-tourmaline-schists near the margin of the granite masses, that boric compounds were given off from the granite magmas.
the nearest outcrop of granite on the shore. Other examples of contact-altered rocks were obtained in the Cording Islets and also at Brokenshore Bay, where, however, most of the rocks are altered greywacke so uniform in general appearance that few were collected.
These are represented by 1683, 1664, 1673, and 1670, respectively 1800, 1100, 800, and 750 yards from the granite contact north of Kisbee Bay, and by 1665 from the north-easternmost point of the Cording Islets. They are compact and hornfelsic though not wholly recrystallized, so that the grain-size is still very irregular. Quartz grains up to 1 mm. in diameter predominate over those of plagioclase (usually Ab95 An5), which may be nearly as large, and with these are fragments derived from graphic intergrowth of quartz and feldspar, flakes of muscovite, and smaller grains of apatite, rutile, and zircon set in a medium- or fine-grained quartzo-feldspathic matrix in which authigenetic biotite flakes, though generally very small, have often grown to a diameter of 0.3 mm., and tourmaline occurs as rare small prisms and still rarer stumpy, spongy crystals as much as 0.3 mm. in length. In some of the rocks (e.g., 1673) there are a few minute flakes of sericite in the base, and the feldspar includes andesine (Ab65 An35) and perthite in addition to the more usual Ab95 An5, whilst the biotite sometimes shows sieve-structure.
The lowest grade of metamorphism in the argillites north of Kisbee Bay is represented by a dark-grey biotite-argillite (1677) occurring 1000 yards south of the granite margin. It consists of exceedingly finely granular quartz so clouded by carbonaceous dust as to be rarely resolvable, but scattered all through it is an abundance of tawny-brown biotite flakes up to 0.05 mm. in diameter.
1669, occurring 100 yards further north than the rock last described, is free from carbonaceous material and consists of an even mosaic of quartz grains 0.02–0.04 mm. in diameter with subordinate white mica, many small tourmaline prisms and bands of red-brown flakes of biotite 0.03–0.1 mm. in diameter.
These are represented by 1630, 1668, 1629, 1654, 1662, 1650, 1655, and 1667, occurring respectively 100, 200, 350, 400, 450, 550, 750, 800, and 850 yards south of the contact between the granite and the sediments displayed on the shoreline north of Kisbee Bay. With the exception of 1630, which is typically hornfelsic, the rocks are lustrous, semiphyllitic, and mottled with dark spots. The amount of cordierite varies; usually it is confined to isolated prismatic crystals, rounded or more or less rectangular, or to irregular spots about 2–3 mm. in diameter. These are set in a fine-grained hornfelsic matrix composed of quartz, biotite, and sericite in descending order of abundance. Lamination of the matrix is visible to a varying degree. The grains or crystals of cordierite are crowded with minute inclusions of quartz and to a less extent of biotite. Where
the host forms entirely irregular grains the inclusions are without trace of orientation, but in the oval or prismatic crystals there is a tendency for arrangement of the inclusions parallel to the longer axis of the host. There is evidence (e.g., in 1655) that the prismatic grains were in some cases rotated in the matrix after they had grown almost to their present size, and that as they moved they left at times a wake filled with coarsely crystalline quartz. Their inclusions are elongated athwart the schistosity of the rock, and the laminae in the matrix tend to be bent around the prisms (Pl. 23, Fig. 6). Lamellar twinning of the cordierite may be seen occasionally, and the mineral appears to be of the rare optically positive type (cf. Krishnan, 1924). Sometimes the cordierite is replaced by sericitic “pinite.” It is colourless or a very pale yellow, often passing on the margins into a more yellowish almost or entirely isotropic substance leaving a very irregular birefringent core (cf. Tilley, 1924, p. 33).
In 1630 the cordierite occurs in irregularly-bounded grains making up the greater part of the rock. Near fracture-lines into which a little pyrites has been introduced, the cordierite has been pinitised or changed to the isotropic substance, while the mica has been bleached or chloritised, with the separation of dusty rutile. Such retrogressive metamorphism is also displayed by 1650, a grey-white knotted rock, in which the cordierite is entirely pinitised, and chlorite not only replaces all of the original biotite, but has been introduced into the veinlets which traverse the rock.
The authors are indebted to Dr Marshall for the opportunity of seeing slides of an erratic of cordierite-hornfels found by him in 1930 on Te Oneroa Beach, Otago's Retreat. The slides differ in no essential feature from 1630, which was found by the present expedition in situ adjacent to the granite over a mile north of Kisbee Bay, and exhibit both the fresh and the pinitised forms of the cordierite.
On the northern shores of Preservation Inlet, about 800 yards west from the granite contact in Brokenshore Bay, there is a spotted rock (1627) which apparently indicates an early stage in the contact alteration of argillite. The fine-grained matrix is clouded with carbonaceous dust and rich in wispy aggregates or minute flakes of shimmering sericite, but the spots are sharply prismatic or rhomboid in outline and consist of a fine-grained quartz mosaic either with scattered or centrally-aggregated biotite flakes. It is not clear, however, whether or not these spots are pseudomorphs as their form suggests. On either side of this rock the sediments are greywackes and in hand-specimen give little sign of alteration until the contact with the igneous rocks is reached.
Inclusion of Slate in “Granite.”
Reference may here be made to the discovery by Hutton (1875, p. 40) of a number of masses of slate included in the “granite” on the eastern side of Isthmus Sound. “The junction here is quite abrupt, and jagged and angular fragments of slate are seen embedded in the granite. The slate near the contact has been considerably altered and converted into a finely crystalline rock of a dark grey colour. The minerals can be separated and mica and quartz recognised with a lens. This alteration does not, however, penetrate very
far, and the great mass of the slates in the neighbourhood are quite unaltered.” Two analyses of the greenish-grey slate adjacent to the contact were made by Liversidge (1877; 1884) and are here cited, though it may be surmised that the high figures for manganese reflect the common over-estimation of that element by the older analytical methods. The analysis of the “granite” (granodiorite-aplite) adjacent to this slate is cited on a later page.
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(b) With Strong Dynamic Influence (“Injection-schists”).
The foregoing descriptions are of contact rocks which have not suffered much pressure, their metamorphism being the result chiefly of thermal activities. There are now to be described other groups of schists occurring adjacent to the plutonic rocks, but in regions that have been subjected to very considerable compression, and in which the rocks have a markedly schistose character. They form masses of schists permeated by igneous intrusions in the form either of intrusion-breccias or lit-par-lit injections, and many of the rocks considered occur actually as inclusions at the margins of the intrusive masses. The following types have been recognised: quartzose schists; quartz-feldspar-biotite-(chlorite-) schists; quartz-feldspar-biotite-tourmaline-schists; coarse biotite-sericite-schists; mica-epidote-schists; andalusite-cordierite-mica-schist; pinite(?)- or sericitised andalusite-schist; sillimanite-mica-schists; calc-silicate-hornfels-schists.
Rocks of this lithological facies representing former siliceous greywackes reach their coarsest crystallization on Long Sound near the igneous intrusions at Last Cove (1502, 1505) and in large blocks included in the granodiorite of the mid-southern shore of that Sound (1570). In the two former the quartz is in strained sutured grains 0.6 mm. across, and in the last in somewhat drawn-out grains up to 2 mm. in length. In 1505 quartz makes 95% of the rock and is associated with biotite, large sericite flakes (0.2 mm.) and tourmaline. In 1502 there is about 4% of feldspar (oligoclase) and about 6% of biotite and sericite in irregular poikiloblastic flakes. In 1570 the feldspar (6%) is apparently albite, and there is about 3% of more or less chloritised biotite. Zircon, apatite or sphene, and rarely tourmaline, occur as minor accessories in these schists.
[Footnote] † Al2O3, includes TiO2. Specific Gravities: A, 2.713; B, 2.72.
A small group of schists occurs in which there is much more feldspar than in those of the preceding division. They are represented by the various facies of a single thick band of apple-green schist of rather coarsely granular texture which forms part of a complex with pegmatite and aplite at Blind Point, Northport. Lamination is made apparent in hand-specimen by differences both in grain-size and in proportions of mineral components. Of the three slices prepared from different specimens 1539 shows quartz and feldspar nearly equal in amount and in equant grains about 0.2 mm. across, whilst in 1636, 1682 the grains are larger (0.5–0.7 mm.) and the quartz is relatively rather more abundant, somewhat elongated, and may contain numerous needles of rutile. In the first there is no strain effect; in the other two strain effect is marked. The plagioclase is sodic (Ab85–90 An15–10) and is rather turbid. Oriented flakes of optically negative pennine 0.3 mm. long replace biotite, making up 10% of the rock, and are associated with a smaller amount of nearly uniaxial sericite either as intergrowths or as separate flakes; rarely a little biotite remains unaltered. In 1636 there are shearing planes along which much matted sericite has formed. Apatite occurs sparingly in large grains (0.1 mm.), whilst there is a little pyrites and here and there a veinlet of fine-grained quartz. The distinguishing feature of these rocks is the abundance of their feldspar, and as there is no evidence that this is derived from the invading igneous rocks, the parent sediment must have been a highly feldspathic greywacke. The presence of chlorite and of sericite along shearing planes indicates that some retrogressive metamorphism has taken place. It is noteworthy that rocks (1200, 1207) similar to these in all respects were obtained by Turner (1930, pp. 175–6; 1933, pp. 191–197) included among the pegmatites of the Martyr Creek Gorge in the Cascade Valley. They form small irregular patches in rocks of similar nature which retain their biotite unchloritised.
Though tourmaline is a very widespread accessory mineral present in minute grains in nearly all the schistose rocks of the region, it occurs but rarely as one of the major constituents, and only two instances of such occurrence have been noted. 1626 is a brown biotitic schist of medium grain-size found near the margin of the granite a mile north of the small anchorage-cove on the mid-western coast of Edwardson Sound. It is traversed by many lines of shearing and granulation, and consists chiefly of highly-strained quartz with patches of matted sericite, possibly replacing feldspar, and a little oligoclase. There are also a few larger flakes of sericite with small ones of biotite (0.2–0.8 mm.), the two micas together making up less than 10% of the rock. It is, however, noteworthy for the occurrence of scattered magnetite with accessory zircon. Schistosity is rather poorly indicated in thin section. The characteristic feature of the rock is the occurrence of tourmaline in irregular prismoids or poikilitic masses extending in optical continuity over a distance of about 3 mm. The mineral is a strongly pleochroic schorlite with
X = pale yellow and Z = deep brown; it is irregularly distributed, and makes up less than 5% of the rock. 1686, on the south side of Long Sound, nearly a mile east-north-east of Only Island, strongly resembles in hand-specimen some of the schists of the Long Sound Series, and is again adjacent to granite. It shows long parallel lensoid grains of quartz and is mineralogically almost identical with 1626, save that the feldspar is entirely replaced by pseudomorphous sericite. The abundance of magnetite between the quartz grains is noteworthy, and the tourmaline occurs in comparatively rare patches, though in one instance it is present as a group of grains apparently separate in the slide but nevertheless in optical continuity over a distance of 6 mm.
In addition to being markedly analogous with the tourmaline-schist of Edwardson Sound (1626), this rock shows a noteworthy resemblance in abundance of magnetite and of sericite pseudomorphs after feldspar to 1516, a quartz-mica-schist occurring about 2000 yards further to the north-east.
These are represented by 1538, 1551, 1553, 1571, and 1671 from the great injection-complex on the west side of Edwardson Sound, by 1529 from a mile north of Northport, and by 1536 adjacent to the granite intrusion south of Last Cove, Long Sound. The rocks contain from 40 to 60% of quartz and usually feldspar, the former in highly-strained grains about 0.2–0.3 mm. across, the latter in grains about as large, but in much smaller amount. The feldspar is commonly untwinned, and in part is perhaps orthoclase, though oligoclase is certainly more abundant. Biotite occurs in flakes up to 0.5 mm. in length, usually marking a rather puckered schistosity, and is of a medium-brown colour. Sericite is almost as abundant, forming more or less bent poikiloblastic irregularly-bounded flakes up to 2 mm. in diameter and sometimes 1 mm. thick. In the absence of the characteristic finely flaky habit, this mica might be termed muscovite, and in similar rocks has apparently been often so termed. The optic axial angle is not, however, that distinctive of muscovite, for it is generally small, a number of closely accordant measurements by Becke's method giving 2V = 28°. Accessory zircon is very common, and may range up to 0.06 mm. in diameter. A little apatite is present, and occasionally plates of ilmenite and grains of pyrites, but no tourmaline has been noted in these slides. 1671 shows that adjacent to invading pegmatite the grain-size may be much coarser (1–1.5 mm.); in this rock the mica shows 2V = 36. A type transitional to the schistose biotite-hornfelses is afforded by 1645 from the same complex in which the grain-size is only half the usual dimensions. Strongly schistose types are represented by 1530 from the same region, in which the quartz grains had been drawn out into ribbons prior apparently to the crystallization of the sericite.
An interesting group of rocks 1512, 1575, 1628, and 1640 occurs at the contact between the granodiorite (1507) and mica-schists nearly a mile south of Only Island in Long Sound, and at the most westerly similar contact in the same sound (1574). There has been introduced into the granodiorite a certain amount of sericite which occurs not only in thin plates in the cleavage of the feldspar,
but also in matted aggregates between the grains of quartz and feldspar, in irregularly distributed flakes or in larger poikilitic plates difficult to distinguish from muscovite, but possessing a very low optic axial angle. Vermicular intergrowth of quartz with the colourless mica is frequently visible (Text-Fig. 1). The biotite appears to have been changed in a few rare instances (1507) to a golden-yellow chlorite, but more commonly to very pale almost colourless tangled sericitic aggregates sometimes quite irregular in shape, sometimes with pseudomorphous boundaries. The further change to the normal pale-green chlorite is also visible. The contrast between these igneous rocks and the schists themselves is shown sharply in 1575. In the schists, quartz (25%) is much in excess of feldspar (5%), whilst the remainder of the rock is chiefly mica. Biotite, and to a less extent muscovite, are present in flakes about 0.3–0.4 mm. long, but in the main the mica occurs as tangled aggregates of partially chloritised, very pale-brown sericite or phlogo-pitic mica intergrown with, or derived from, normal biotite. The iron set free during the chemical change, probably reinforced by introduced material, has become pyrrhotite and to a less extent pyrites. A little leucoxenised ilmenite and zircon appear in both the schist and the igneous rock.
Text-fig. 1.—Vermicular (myrmekitic) intergrowth of quartz in muscovite in mica-schist (1512) at contact with granite, southern coast of mid-portion of Long Sound.
The development of vermicular quartz in the coarsely platy sericite (or “muscovite”) is displayed still more elaborately in a contact-schist (1338) obtained by Turner (1933, pp. 207, 208) at the south-western end of the Martyr Bridge in the Cascade Valley, and has been noted also by Tattam (1929) among the injection-schists of north-eastern Victoria. According to Sederholm (1916, p. 131) and Vogt (1927, pp. 252, 415, 524), this peculiar structure results from the action on potash-felspar of the water expelled from the invading magma. The sericitisation of biotite (possibly occurring under the same conditions) also has been noted by Turner in the particular rock cited above, though it is scarcely so clearly displayed as in the specimens from Long Sound.
These rocks form a very distinct group present in the injection-complex on the western side of Edwardson Sound and in the valley between Lake Caesar and Northport, and are illustrated by specimens
1504, 1534, 1540, 1550, 1589, and 1694. Though usually of moderate to coarse grain-size, they are interlaminated with fine-grained types. Thus 1550 shows as part of the one microscope-slide a band of schistose quartz-biotite-hornfels with an average grain-size of 0.2 mm., of which the biotite makes up about 15%. It differs from the normal quartz-biotite-hornfels only in the presence of sparse grains and small prisms of pale-yellow epidote and of accessory apatite, zircon, a little tourmaline, and a string of pyrrhotite grains. The remainder of the rock is typical of the coarse-grained types in which the sieve-porphyroblasts of biotite reach a diameter of 1.5 mm. and make up 25–30% of the rock. Sericite occurs in thin laminae 1 mm. long, sometimes arranged in sinuous bands or else aggregated in narrow shearing zones. It amounts approximately to 10–20%, and its optic axial angle as usual has a low value, for 2V = 26°–30° according to several determinations. The quartz mosaic is rather coarse (0.2–0.3 mm.), with still coarser local vein-like streaks and finely granulated shearing zones. The feldspar constitutes only about 1% of the rock; it is largely fresh, untwinned, and with refractive index equal to that of Canada balsam, so that it is an acid oligoclase. The epidote forms irregular or poikiloblastic grains and euhedral crystals up to 1 mm. in length amounting to about 4% of the total constituents. It is intermediate in character between epidote and clinozoisite and markedly pleochroic, with the rather abnormal absorption scheme Z = canary yellow, X and Y = pale yellow to colourless. Its optic axial angle is large, and its optical character apparently negative, though near to the transition point. The birefringence, however, has the low value of 0.010–0.011 characteristic of clinozoisite and is sometimes slightly anomalous. The mineral is twinned apparently on the 100 plane, giving lamellar and geniculate twins. The accessory minerals are haloed zircon in the biotite, apatite, rare tourmaline, a little platy ilmenite, and occasional sphene and pyrrhotite.
Immediately adjacent to invading pegmatite the grain-size is increased, yielding mica plates over 2 mm. in diameter, and the shearing bands of sericite are very marked, as in 1504 (Plate 23, Fig. 5). In this rock also the largest and best-developed epidotes appear, and jets of pegmatitic quartz and oligoclase project into the schist, whilst grains of epidote in turn may become isolated and included in the pegmatite. 1607 from the Edwardson igneous complex illustrates the effects of shearing and retrogressive metamorphism on rocks of this group, for it is traversed by many shearing planes oblique to the original schistosity and marked by bands of shimmering sericite which cut through epidote porphyroblasts. The grains of quartz are reduced in places to lenticles of highly strained and crushed granules, and the biotite has been changed to an almost isotropic pennine.
It is interesting to note that 1611 from the same locality as the last rock also probably yields an example of retrogressive metamorphism whereby, in addition to other changes, what probably originally was andalusite has been converted to pseudomorphous sericite, as will be described more fully on a later page. There is some suggestion that the retrogressive change has been effected by solutions bringing silica and perhaps alkaline sulphides.
This type is represented by 1680, obtained adjacent to the granite by the Last Cove Fault on the south side of Long Sound and by 1632 on the northern shore of Last Cove near its entrance. 1680 is a richly micaceous, glossy-brown, knotted rock with marked schistosity, the larger knots consisting of almost idiomorphic prisms of andalusite up to 7 mm. in length, which are exceedingly crowded by inclusions of biotite and quartz and sometimes show the faint pleochroism (pink to colourless) distinguishing some varieties of andalusite. There are also much less definitely bounded roundish spots 2–4 mm. in diameter composed of shimmer aggregates of white mica or of a very pale brown or colourless almost isotropic substance with a refractive index much less than that of quartz. These patches probably are pinitised cordierite and, besides being dusted with carbonaceous particles, contain abundant plates of red-brown biotite and of muscovite possessing an optic axial angle (2V = 36°–42°) measurably greater than that of sericite. About them there is a little accumulation of biotite forming an enwrapping zone, whilst beyond this there extend long out-drawn streaks of quartz grains about 0.2 mm. in length with wisps of sillimanite sometimes strung out in association with flakes of biotite into irregular ropes. Magnetite, rare rutile, and a little tourmaline occur with them in minute prisms. The matrix of the rock is very schistose and consists of somewhat elongated quartz grains (0.1 mm.) with an abundance of small flakes of biotite and sericite with accessory zircon, tourmaline, apatite, magnetite, and carbonaceous dust. This rock has certain analogies with a specimen of banded paragneiss (1212) obtained by Turner (1930) among the boulders of Colin Creek in the Cascade Valley.
1632, though similar in many respects to 1680, is strikingly different in possessing about 15–20% of red-brown biotite in thick stumpy poikiloblastic porphyroblasts from 0.5 to 0.7 mm. in diameter, which have frequent haloes around tiny grains of zircon. These are enclosed, along with large irregular knots of andalusite and cordierite, in a schistose base thickly dotted with granules of carbon and composed of quartz in elongated small lenticles from 0.04 to 0.15 mm. in length, with about 15 to 25% of sericite in parallel flakes 0.06 to 0.12 mm. in length. There are relatively numerous tiny prisms of tourmaline with other accessories such as zircon and rutile in small amount, but with fairly frequent coarse grains of pyrite.
The irregular knots of andalusite are as much as 5 mm. across and lie obliquely across the schistosity. They generally show striking sieve-structure, being densely crowded by specks of carbon and grains of quartz, whilst they may enwrap and enclose porphyroblasts of biotite. A curious feature of the mineral, however, is its tendency to pass abruptly from parts showing exaggerated sieve-structure to other large ones free from this and from all inclusions, thus yielding a strong resemblance to relict structure. In association with it there are irregular strings of coarse granules of quartz. The cordierite present not only forms rare irregular knots over 4 mm. across, but occurs also bordering these as small irregular crystals. It is unaltered
and is crowded with the usual inclusions of grains of quartz and specks of carbon, added to which there are sometimes flakes of sericite and rarely of biotite.
Pinite(?)- or Sericitised Andalusite-schist.
As suggested on an earlier page, 1611, a grey micaceous schist occurring adjacent to 1680 described in the last section, may be a product of retrogressive metamorphism of a rock similar to this last. On the other hand, it seems not unlikely that it is derived from a cordierite-schist. Its distinguishing feature is the presence of large prismatic pseudomorphs as much as 10 mm. long and 3–4 mm. broad with rounded terminations. These consist chiefly of aggregates of small sericite flakes arranged parallel to the length of the prism and dusted with finely-divided magnetite. In one pseudomorph, strips of a pleochroic yellow chloritic mineral [diabantite (?)] occur lying in the one general direction, and differ from clinochlore in having optically positive elongation and yellow tint. Around the pseudomorphs is a bordering zone of felted sericite about 0.3–1.0 mm. thick. It is difficult to decide whether these pseudomorphs replace andalusite or cordierite: their size and association suggest the former, but they may also be considered as composed of gigantolite surrounded by pinite.
The matrix of the rock contains a number of large, irregular, and highly-strained grains of quartz embedded in a tangled mass of sericite, with larger plates of muscovite and chloritic pseudomorphs after biotite and with a little secondary sphene. Throughout is an abundance of finely granular magnetite generally aggregated into streaks.
These rocks have been observed in Long Sound, three miles east of Last Cove (1545, 1546), in the creek north of Northport (1646) and at the western entrance thereto (1541, 1557). The first two (1545, 1546) (Pl. 23, Fig. 4) represent closely interlaminated medium- and coarse-grained types, in both of which the schistosity is strongly marked by parallel mica plates. In the former or medium-grained type, deep-red biotite makes up about 20% of the rock and, with a smaller amount of sericite, rests in a matrix of elongated highly-strained quartz grains and a little acid plagioclase with also a little tourmaline, apatite, zircon, and strings of pyrrhotite. The sillimanite occurs in lenticular patches 2–3 mm. long, in which the fibres of the mineral are densely felted, though fraying out at either end along the plane of schistosity, and bent by shearing movements, a little of the biotite also being involved in the movement. The more coarsely crystallized type approaches an “augen-schist” on account of the development of eye-like porphyroblasts of perthitic orthoclase up to 5 mm. in diameter, which contain abundant inclusions of rounded quartz and biotite, fibres of sillimanite, and grains of zircon, pyrrhotite, and tourmaline which, by maintaining the general direction of schistosity, indicate the late period of growth of the orthoclase. The surrounding quartz mosaic has a grain-size of 0.3 mm. and contains much acid oligoclase in addition to quartz, whilst it enwraps large plates of biotite and sericite up to 3 mm. in diameter.
The rocks (1541, 1557) from the south-western end of North-port differ from these last in their scarcity of sericite. The quartz grains of the matrix average 0.3 mm. in diameter, with larger grains grouped in layers; with them there is small amount of fresh oligoclase in grains which may contain small rounded inclusions of quartz and are of the same size as those of this latter mineral. Biotite forms rather dark, slightly-reddish-brown flakes about 1 mm. long with abundant deeply-haloed zircons, and makes up from 25% to 35% of the rock. The sericite flakes range up to 0.5 mm. in length and scarcely make up 3% of the rock, whilst there are long streaks of sillimanite bunched here and there into tangled masses. Apatite occurs in fair amount, and there are rare needles of rutile in the biotite, but no tourmaline or iron ores. The rock from the creek north of Northport (1546) differs from these last chiefly in its smaller grain-size and in the presence of large porphyroblastic plates of sericite with which the sillimanite is associated in sheaf-like or radiating aggregates. It contains a little pyrites.
The members of this series occur only among the highly schistose rocks of Long Sound in the vicinity of plutonic igneous rocks, and are therefore placed in this division rather than among the normal contact rocks. They show a lamination, however, which results from variation in composition of the marly sediment from which they were derived, and their schistosity is parallel to the bedding planes. They are tough, dark-grey rocks yielding fluted surfaces on weathering, and have been recognised at four points, viz., immediately adjacent to the granite south of Last Cove (1509), at two points on the north coast of Long Sound a mile further east (1663, 1634), and on the islet between Only Island and the mainland (1508). In all four occurrences they form narrow bands only a few feet thick in the normal schists.
1509 is a banded rock in which there are relatively subordinate pale-coloured layers consisting of microcline and quartz, together with small granules of diopside and a minor amount of wollastonite (?), calcite, and sphene. The bulk of the rock, however, consists of diopside in small granules and more particularly in large irregular poikiloblasts about 2 mm. in diameter with a rather small optic axial angle (2V = 56°). Subordinate highly irregular sieve-masses of vesuvianite have been formed by retrogressive metamorphism (c.f. Osborne, 1932, pp. 221–2).* With them are associated twinned prisms of wollastonite (?) which generally mark the schistosity and are accompanied by grains of quartz and microcline sometimes strung out along the plane of schistosity.
1663 is very fine-grained (0.03 mm.), and more or less intermediate in composition between the previous rock and the normal argillites. It consists of quartz with some orthoclase, diopside, biotite (more or less replaced by pennine), and a little muscovite. The quartz and orthoclase tend to be aggregated into long streaks,
[Footnote] * We are indebted to Dr. Osborne for confirming this observation.
in the centres of which are strings of coarsely granular pyrites, while finely-divided iron ores and a few small grains of red-brown sphene are dotted about.
1634 is markedly laminated, with considerable variation in the mineral composition of the several layers. Some consist essentially of a very fine mosaic of quartz with a little acid labradorite, the quartz being sometimes aggregated into lensoid patches. Scattered ragged grains of diopside, a little clinozoisite and vesuvianite, both possibly diaphthoretic, some more or less chloritised biotite, and many tiny grains of iron ore (magnetite or ilmenite), and pyrites are also present. Other layers differ from these in the scarcity of the iron ores and the greater abundance of the ferromagnesian silicates, chiefly diopside, which forms irregular masses, over a millimetre across, lying athwart the lamination of the rock. A little sphene may occur in such bands. Other layers in which the ferromagnesian minerals predominate contain both diopside and pale green amphibole, the latter forming large irregular sieve-crystals occasionally exceeding 4–5 mm. in diameter. In these layers are thin streaks rich in chloritised biotite and clinozoisite almost to the exclusion of the microcline, perthitic orthoclase, and occasional plagioclase grains which form the bulk of the colourless minerals in the predominantly ferromagnesian layers. In these layers sphene and pyrites may be abundant. The whole character of the rock is suggestive of derivation from a laminated dolomitic marl.
1508 has a base of sutured quartz grains 0.1–0.2 mm. in diameter, with centrally-placed specks of iron ore and a large amount of basic plagioclase, usually untwinned, which forms rounded grains, also often with centrally-accumulated specks of iron-ore. Pale-yellow phlogopite is scantily distributed in small flakes, but diopside with marked sieve-structure is abundant in stumpy prisms up to 0.8 mm. in length and in irregular grains. Sphene in small rounded red-brown grains is a common accessory together with apatite, a little ilmenite, and much pyrites in fairly evenly-scattered minute crystals. In hand-specimen this rock resembles dark-green slate and forms only a thin layer.
That these rocks resemble in a number of features those of the Long Sound Series will appear from the subjoined description of the latter.
(c) The Long Sound Series.
The coarsely granular rocks of the Long Sound Series are represented by 1696, 1697 from the crumpled beds on the north-west side of the Sound a mile from its head, 1518 from a mile further to the south-west, and 1561 two miles further to the south-west in the region of lit-par-lit injection in the cove opposite to Only Island. The rocks of medium grain-size are represented by 1510, 1516, 1631, and 1690 from among the crumpled beds before mentioned, and are associated with a strongly micaceous type 1517.
1518 and 1561 are practically quartz-schist, consisting of strained, rather elongated sutured grains of quartz about 1.0 mm. long, with about 5% of plagioclase (Ab70 An30), a little orthoclase in irregular grains (0.2 mm.), and a like amount of chloritised biotite, with
secondary sphene, a few plates of muscovite, and grains of apatite, zircon, and pyrites. 1696, 1697 have a more distinctive composition, the nature of which varies in the different layers of the rocks. The basis of the rocks is a mosaic of quartz with subordinate microcline and has a grain-size normally of 0.2 mm., rising to 1.0 mm. in the quartzose streaks. Long lenticles of granular dusty oligoclase or isolated more or less sericitised grains of the same mineral are also present. Scattered through this base are grains of diopside (0.1–0.3 mm.) or prisms of this mineral (0.5 mm.) elongated in the bedding plane and tending to be aggregated in the lenticles of plagıocıase. Irregular, very imperfectly oriented sieve-prismoids of pale-green amphibole may in some cases (1697) predominate over the diopside. Streaks rich in zoisite in grains or prisms up to 1.0 mm. across occur, and, especially in association with these, there is a noteworthy amount of ilmenite forming irregular plates 0.1 mm. long oriented along the general structure plane and associated with a little pyrites. Small red-brown grains of sphene are scattered through the rock, apatite is common, and zircon fairly abundant. Averaging the two slides, the general proportions between the minerals are: quartz 53%; microcline 15%; oligoclase 8%; diopside 8%; hornblende 8%; zoisite 4%; ilmenite 3%; apatite, sphene, pyrites, and zircon together 1%.
1510 and 1690, which come from medium-grained layers between coarse bands such as are represented by the above, have the structure of pyroxene-granulite. The average grain-size of the quartz-feldspar mosaic is a little less (1690) or more (1510) than 0.10 mm. This mosaic makes up about 70% of 1690, which is uniform throughout, but there is some banding and development of layers enriched in colourless minerals in 1510. The feldspar in the first rock is untwinned, but its basicity is shown by its refractive index; in the second, extinction-angles prove it to bytownite. Diopside (about 25%) occurs in grains of the same size as those of the matrix, and often shows lamellar twinning in 1510, where it also forms streaked aggregates of larger grains (0.6 mm.). Besides these minerals there occur abundant small plates of ilmenite, strings and grains of pyrites, and many rounded grains of red-brown sphene. In 1510 there is a layer almost free from such iron-ores and poor in diopside, but containing poikiloblastic vesuvianite and a noteworthy amount of calcite.
The lamination of 1631 is determined chiefly by the occurrence of thin bands of grain-size (0.3–0.5 mm.) larger than that of the rest of the rock, and by the parallel orientation of the irregular platy grains of ilmenite, which, with the granular pyrites, tend to be specially abundant in particular laminae. Strongly-sutured strained quartz grains 0.1–0.3 mm. in diameter form the main constituent of the rock in company with about 5% of basic plagioclase. Some bands have a notable amount of phlogopite in oriented flakes 0.25 mm. long, together with numerous grains of red-brown sphene and flakes of sericite. In place of diopside there is a very pale amphibole in grains about 0.2 mm. in diameter. Rarely tourmaline accompanies the accessory minerals zircon, apatite, and finely-granular clinozoisite.
1517, which also outcrops among the crumpled rocks near the head of Long Sound, is more micaceous than the above-described rocks. Its mosaic of slightly-elongated quartz grains (0.1–0.2 mm.) is associated with a minor amount of acid plagioclase and about 20% of deep reddish-brown biotite (0.2 mm.) in regularly-oriented flakes more abundant in certain layers than in others and associated with irregular aggregates of finely-flaky sericite. Here and there are minute prisms of epidote (?), rare small prisms of tourmaline, needles or short prisms of rutile, zircon, and apatite. In addition, the richly micaceous layers contain aggregates of irregular but elongated grains of pyrites. 1516 is richer in sericite, which is generally in matted cloudy aggregates forming 10% of the rock, and sometimes appears to replace feldspar. It also occurs in strings or lenticular masses with platy ilmenite, a little pyrites, and haematite. Pale phlogopite occurs in small amount and contains a little rutile, whilst zircon, sphene, and apatite are also present. Quartz forming elongated (0.2–0.8 mm.) sutured grains is the chief constituent of the rock, and there is 10–15% of oligoclase.
The petrological features of the rocks of the Long Sound Series are important because of their resemblance in detail to the calc-silicate-hornfels-schists—a resemblance which macroscopic inspection could scarcely have substantiated. The remarkable association of microcline with calc-silicates both in 1696 and 1697 of the Long Sound Series and 1509 at Last Cove is very noteworthy, and so is the association of diopside, basic plagioclase, abundant fine ilmenite, and red-brown rounded grains of sphene both in 1510 and 1690 from the Long Sound Series and 1508 from Only Island. Such marked similarities based on two very diverse mineral associations are very suggestive of genetic relations between the various rocks concerned.
There is, however, evidence supporting on petrological grounds a correlation between the rocks of the Long Sound Series and more distant formations. As noted in Part I, the mass of calc-silicate-hornfels which was discovered by Dr. G. J. Williams included in granite at Blaikie's Hill in the south of Stewart Island (Williams, 1934) shows the closest resemblance to the rocks of the Long Sound Series. Through Dr. Williams' courtesy we are permitted to include here the description of the specimen from Blaikie's Hill placed by him in the Otago University collections. It is banded, with fine hornfelsic layers (1703) alternating with sheared coarsely-granular layers (1703a). The former have the texture typical of slightly schistose granulites, with a general parallelism of the longer axes of the grains (average size 0.2 mm.). Quartz is the dominant mineral and is associated with orthoclase, bytownite (often intergrown with quartz), diopside, elliptical grains of sphene and scattered minute grains of apatite and larger crystals of pyrites. The coarsely granular rock shows long ribboned layers of quartz, between which are sheared aggregates of diopside, dusty orthoclase, bytownite (or anorthite), zoisite, and sphene.
4. Summary: Progressive and Retrogressive Metamorphism.
It is now possible to generalise concerning the distribution throughout the region of the more or less metamorphic rocks. It is evident that the whole area lay within the influence of subjacent batholiths, since spotting of the responsive pelites and introduced tourmaline are seen even in the outlying parts of Cape Providence and Coal Island. That such spotting occurred before the effects of regional metamorphism were produced is shown by its exaggeration beyond the boundary of the area exhibiting these latter effects, and its gradual obliteration in the regions of more intense alteration as the rocks take on a schistose or a poikiloblastic texture. The data are not available to permit charting of lines of isogradic metamorphism, but apparently the south-western limits of the areas of regional metamorphism run from near Surf Point south-eastwards to Brokenshore Bay, and consequently obliquely to the strike of the folded Ordovician rocks. Traced in a direction perpendicular to this hypothetical isograd, or north-east, the intensity of metamorphism increases. The quartzites become more coarsely crystalline and vitreous in appearance, and the original argillites either more markedly schistose and more coarsely flaky, or, with the development of pophyroblasts, more gneissic in appearance, reaching the maximum grade of metamorphism in the middle part of Long Sound. There does not, however, appear to be any regular increase in the lime-content of the plagioclase in the schists during this progressive metamorphism such as Turner (1933) has discovered in the Haast and Cascade Valleys. But, whereas in this last region the original argillites were remarkably rich in epidote, which yielded its content of lime to the feldspar during the progress of the metamorphism, no such source of lime was constantly present in the argillites of Preservation and Chalky Inlets.
There is no evidence of the development to any appreciable extent of a chloritic grade of phyllites and semi-schists, in which biotite may subsequently form by the union of chlorite and sericite, as has been shown to be the case in many regions, notably in the Scottish Highlands (Tilley, 1925) and in Norway (Goldschmidt, 1920; Vogt, 1927). Such chloritic rocks have indeed been shown to be the dominant metamorphic formations in South Westland and north-western Otago a hundred and fifty to two hundred miles northeast of this area (Turner, 1930, 1933). Here, however, as in the Singhbhum District of India (Dunn, 1929, p. 38), the biotitesericite phyllites have developed directly from argillites containing little or no recognisable chlorite, and the chlorite-sericite schists of the Garden Islets, Southport, have resulted from a regressive metamorphism of the biotite-sericite phyllites occurring in a region of marked shearing stress adjacent to the Southport Fault (cf. Knopf, 1931).
The presence of garnet in rocks near the head of Edwardson and Cunaris Sounds and at one intermediate locality is a hint of the position that would be occupied by the garnet isograd were the composition of the sediments sufficiently often suitable for its expression, and it is interesting to note that this line runs in a
N.W.–S.E. direction and is, therefore, parallel to that noted as forming the south-west margin of the area affected by regional metamorphism.
Further, it would appear that, though the limit of regional metamorphism conforms broadly with that of the area occupied by the major batholiths, it is not parallel to the boundaries of the individual intrusions, which have risen in some places through the depth-zone of schists into the overlying slightly metamorphosed rocks. The distinction between the contact-altered sediments south-west of the granites and the contact-schists and injection-schists among or to the north-east of them is thus one of situation and not of period of metamorphism. Hutton (1875), McKay (1896), and more recently Park (1921) had assumed this distinction to be significant of age, and believed the schists to be pre-Cambrian. Turner's recent studies (1933) in the Haast and Cascade Valleys lead to the view that the metamorphism therein was produced during a single period of plutonic activity, with some retrogressive changes during later earth-movements. The observations made in the Preservation Inlet area accord with this conclusion.
Profound metamorphic activity at the deep-seated contacts led to the formation of the very coarse biotite-sericite (“muscovite”)-schists, in which, instead of finely-flaky sericite, large plates of white mica were developed, differing from muscovite only in the possession of a small optic axial angle, or led, by the introduction of foreign material, to the formation of augen-masses of feldspar or of epidote (clinozoisite). There has sometimes been the formation of sillimanite (probably as a result of the breaking-down of micas) or the occasional conversion of biotite into sericite and iron ores or the development of vermicular quartz in coarsely-flaky sericite (“muscovite”). All these features are analogous with those found by Turner (1933) one hundred and sixty miles to the north-east, by W. R. Browne (1914) in south-eastern New South Wales, and by Tattam (1929) in north-eastern Victoria.
Retrogressive metamorphism expresses itself in two ways—the production of pinite and gigantolite pseudomorphs after cordierite and the development of some chlorite from biotite. These changes may well have occurred during the steady approach of the rocks towards the earth's surface during prolonged elevation and erosion, but much more commonly the development of the chlorite from biotite is associated with evidences of shattering or shearing and of the percolation of solutions through the channels so formed, with the deposition of more or less pyrites. This chlorite is very characteristically pale in colour, and is associated with finely-granular or sagenitic rutile or granular sphene. Change of ilmenite to leucoxene and of feldspar to sericite may accompany the development of chlorite. The region where this retrograde metamorphism is most strongly marked is that adjacent to the Southport Fault, where the strongly chloritic character of the sericitic schists of the Garden Islets is its result and is accompanied throughout by evidence of shearing stress. To a less extent the effects of shearing can be seen in certain rocks from the western coast of Edwardson Sound.
The occurrence of calc-silicate-hornfels-schist analogous to rocks of the Long Sound Series among the normal regionally metamorphosed schists and paragneisses near Last Cove and Only Island, Long Sound, is one of the most interesting features discovered during the petrological study of the sediments; its stratigraphical significance is discussed elsewhere in this paper. Here it may be noted only that the calc-silicate rocks show the effect both of dynamic and of thermal metamorphism, and have developed vesuvianite and possibly clinozoisite during retrogressive changes.
B. Plutonic Rocks.
The most potassic and siliceous granites in the region described form the latest intrusions in the Northport region and are represented by 1688 from the north coast of Long Sound near Only Island, 1652 from Northport, and 1601 from a point about a quarter of a mile north-east of the beach at Landing Bay near Cape Providence. Probably similar rocks are represented by much of the pinkish granite in the cliffs facing Edwardson Sound and stretching northeast from Northport, which Hector (1864) noted as likely to yield an excellent building stone, but insufficient collecting was done to confirm this.
The Long Sound rock (1688) is fairly uniform in grain-size (1.0 mm.) save for the presence of a few larger grains (3.0 mm.) of perthitic orthoclase. Quartz is the most abundant mineral, but there is almost as much orthoclase, the smaller grains of which in particular have been to a considerable extent changed into sericite. Microcline, generally in fresh equant grains, is rather less abundant, and there is also a small amount of albite. Biotite occurs in small scanty largely chloritised flakes. There is a general approach towards gneissic structure. The rock from Northport differs from this in the character of the plagioclase. Quartz is again the most abundant mineral (40%) and forms grains up to 5 mm. in length. Microcline is almost as abundant in grains up to 2 mm. across, which occasionally contain a little myrmekite or irregular intergrowths of plagioclase. The individual grains of plagioclase are up to 1.0 mm. in length and are of basic oligoclase. There is a little flaky biotite, still less muscovite, a few large grains of apatite (0.3 mm.), and much zircon. The last rock (1501) forms the bed of the stream that enters Landing Bay and in hand-specimen shows very clearly the difference between white plagioclase and flesh-pink perthitic orthoclase, which both form grains 4–5 mm. in diameter, orthoclase being in excess and showing both Carlsbad and Manebach twinning. The plagioclase (Ab65 An35 to Ab75 An25) is slightly zoned and often shows myrmekitic marginal zones where in contact with orthoclase. The quartz is greatly crushed and includes numerous needles of rutile; it is in part drawn out into zones of shattering parallel to which are flakes of biotite changed for the most part to chlorite and carbonates. Microcline occurs only in a few small grains. Magnetite, apatite, and zircon are present as accessories.
The portion of Treble Mountain massif that extends from Isthmus Sound towards Bald Hills is made up of a very coarse-grained granite with pink orthoclase crystals up to 2 cm. in length, white plagioclase, clear quartz, and flakes of biotite. It would make a most handsome building stone, and could readily be quarried alongside sheltered bays, as Marshall (1929) has pointed out. This rock was described by Hutton (1899) as a syenite, but corrected accounts have been given by Marshall (1907) and Speight (1910). Slices of a specimen from Isthmus Sound (1658) and of an erratic from Te Oneroa, Otago's Retreat (1603), show that orthoclase is the chief constituent, forming 35–40% of the rock in large irregular perthitic prismoids. Plagioclase is almost as abundant in irregular grains up to 6 mm. across and in rather smaller prismoids, which are markedly zoned and apparently a little more basic than in the rock last described, for they range from Ab55 An45 to Ab65 An35 with a marginal film of Ab78 An22. Quartz makes up nearly 20% of the rock in grains sometimes 6 mm. across and slightly strained. There is a little interstitial microcline, amounting to 5% or less, and biotite is still less abundant, though occasionally forming plates as much as 2 mm. across. There are a few minute prisms of apatite and a little secondary epidote and sericite. According to Hector (1864) the rock of Red Head, north of Gulches Head, is of a similar character, though with a finer base, and so also is the granite from Green Islets described by Professor Park (1926), the analysis of which is quoted below.
A pebble (1558) from the east side of Landing Bay, near Cape Providence, appears as a modification of the last type of granite, for quartz occurs in relatively small amount and usually in shatter-zones. This syenite invades a richly hornblendic rock and consists chiefly of microperthitic orthoclase in crystals sometimes as much as 5 mm. in diameter, with irregular grains of quartz and oligoclase and rarely hornblende and biotite. The rock has been intensely sheared, yielding shatter-zones composed of crushed feldspar and quartz in sutured, ribboned, intensely strained grains, and myrmekite has developed in the feldspars adjacent thereto, or the orthoclase is replaced by microcline (Pl. 23, Fig. 2). Plates of biotite are drawn out into ropes in these shearing zones and are there associated with apatite, magnetite, sphene, and a little hornblende, while thin films of epidote occur rarely in the cleavages of the feldspar. Both as regards mineral composition and granulated structure, this rock is very similar indeed to a gneissic rock (D.6) obtained by Professor Speight (1910) at Pickersgill Harbour near the southern entrance to Dusky Sound.
Granodiorites (or Soda-tonalites).
The bulk of the plutonic rocks studied belong to this group, which is represented by 1689, a rather gneissic rock on the margin of the Kakapo massif, two miles south-west of the head of Edwardson Sound, by specimens from the vicinity of Northport (1528, 1573, 1587, 1591, 1619, 1643, 1651), by a very narrow selvedge between the greywacke and the invading granite in Brokenshore Bay (1679),
and by the rocks from the middle portion of Long Sound, namely 1564, 1566 from its northern shore and 1507, 1562, 1565, 1575, and 1647 from its southern.
The characteristic minerals of these granodiorites are oligoclase, quartz, and biotite, with rarely subordinate amounts of potashfeldspar, muscovite or hornblende and accessory apatite, zircon, iron ore, sphene, pyrites, epidote, and rarely allanite. The rocks are either massive or gneissic, sometimes being crushed with granulation or elongation of the quartz crystals in the direction of foliation. The grain-size varies between 1 mm. and 4 mm., and is usually about 2 mm. Plagioclase forms 50% to 75% of the rock and varies in composition from Ab65 An35, or more often Ab70 An30, to Ab80 An20. It occurs in irregular, equant grains or elongated prisms in which zonal structure is but slightly marked, whilst the grains are usually more or less clouded by minute scales of sericite. Quartz forms from 15% to 35% of the rock, averaging about 30%, the lowest figure being reached only in a highly feldspathic rock (1564). It occurs in interstitial grains moulded on feldspar or in aggregates of sutured individuals usually showing marked strain-effect. Potash-feldspar occurs only in small amount. It forms prisms of moirée orthoclase-perthite in a rock on the margin of the intrusion south of Only Island in Long Sound (1507) and appears as microcline in a sheared rock (1647) invading diorite two miles north-east of the last, in 1566 on the north shore of Long Sound immediately opposite to 1647, in 1679 from Brokenshore Bay, in 1643, a small dyke invading the diorite at Northport, and in 1619, a dyke intrusive into the schists near Mosquito Point, Northport. The presence of myrmekite along the margin of the plagioclase in the crushed portions of the rock is exhibited in 1566, 1619, and 1647. Biotite occurs in amounts varying from 5% to 25%, the latter in a dark rock (1643) invading diorite at Northport. The biotite in 1566 and most of the rocks of this type adjacent to Northport, where they are found north and west of Mosquito Point and in Great Island, has as its darkest tint a rather greenish brown; that in the most micaceous of the Northport rocks (1528) and in all those of Long Sound is at darkest a deep-red-brown. Its flakes are rarely as much as 1.5 mm. long and only rarely show sieve-structure (1651). They are more or less parallel in the gneissic types, and in the sheared ones may be bent and streaked out along the zones of shearing. They are slightly chloritised even in the freshest rocks, and in the most altered (1564) are completely replaced by pennine with the separation of rutile in imperfect sagenitic form. In 1564 the chlorite is almost colourless, its former content of iron having been removed save for the little that remains as siderite and limonite in its cleavage planes, while some muscovite occurs among the chlorite. In other rocks, e.g., 1691 from Northport, the pennine is apple-green and strings of sphene have formed in its cleavage cracks. Muscovite occurs in certain of the granodiorites irrespective of the area from which they come, but is absent from others. When present it is usually in amounts less than 5%, though it may make quite large flakes up to 3 mm. in diameter, upon which the biotite flakes are often moulded. In a group of peculiar rocks (1507, 1512, 1565,
1575) from the margin of the granodiorite half a mile south of Only Island, there has been so intimate a lit-par-lit injection of igneous rocks into the quartz-mica schists that it is difficult to make a distinction between the two sets of rocks, and, as a result, there is an abundance of extremely irregular pseudopoikilitic or else confusedly-matted sericite in portions of rock which otherwise have a granodiorite composition. This feature has been discussed at greater length in describing the injection-schists.
Text-fiq. 2.—Crystal of allanite surrounded by epidote (dotted) producing dark halo in enclosing biotite. Granodiorite (1528), Northport.
Hornblende is present in a single gneissic rock (1651) which in invaded by a vein of aplite, and was collected as a pebble in Shallow River at Northport. It forms less than 3% of the rock in grains usually less than 1 mm. long, which rarely show sieve-structure and are moulded on or intergrown with biotite. It is very strongly pleochroic with X = pale-brown, Y = deep-brownish-green, Z = deep-bluish-green. Apatite is a very common accessory mineral in rounded grains or idiomorphic prisms 0.1–0.3 mm. in diameter; in the rocks from the zone of lit-par-lit injection mentioned above, it is commonly zoned, having a grey kernel surrounded by a colourless outer layer. Zircons are fairly plentiful, rare crystals reaching as much as 0.2 mm. in length, and when included in biotite have given rise to very dark haloes. Allanite occurs, but is infrequent. A few idiomorphic almost colourless crystals of this mineral, 0.1–0.2 mm. long, appear in 1528 from Northport surrounded by a thin film of epidote, and where this is thinnest there is a broad and dark halo in the enveloping biotite (Text-fig. 2). The allanite here appears to be primary in origin, and has a birefringence of 0.013. 1566 from Long Sound, however, shows another variety of the mineral corresponding with Larsen's (1921) description of a secondary allanite produced by the alteration of the primary
mineral. It has a slightly higher birefringence, and a strong pleochroism (X = yellow-brown, Z = dark-reddish-brown) the extinction direction X being inclined about 25° to the elongation of the grain. This mineral may also be bordered by epidote, which is considerably more abundant than in other rocks of this group, amounting to nearly 2%, and occurring in more or less poikilitic masses up to 0.5 mm. across. It is in close association with biotite and very probably of primary origin. The same possibly is true of the epidote in 1651, in which allanite is also present.
The iron ore of the granodiorites is commonly ilmenite with a tendency to form elongated or platy grains often surrounded by leucoxene, which, if enclosed in biotite, may form long threads extending along the cleavage lines. In some cases the oxide appears to be magnetite only. Pyrites is present in small grains in only a few of the rocks examined, and the ores as a whole are very sparsely developed.
The classification of the rocks of this group is complicated by lax petrographic usage. Few of them have sufficient potash feldspar to fall within the limits of granodiorite as defined quantitatively by Iddings or Shand, the possible exception being 1507. The predominant plagioclase is more sodic than is normal for quartz-diorites or tonalites and there is usually no hornblende. On the other hand, there is usually more biotite than occurs in the otherwise similar trondhjemites. They are, however, similar to rocks which by an extension of the strict usage have been termed granodiorites by Lindgren, Hatch and Wells, and others, but strictly appear to fall within Shand's definition of soda-tonalite.
Related rocks in neighbouring regions are but little known. There is, however, considerable resemblance between 1528 above and a rock (D.14) obtained further north by Professor Speight (1910), whilst another from Green Islets described by Professor Park (1926) as a diorite has probably some connection with the granodiorites of the present area, but differs from them in the presence of hornblende with potassic feldspar. Seelye's analysis (see Park, 1926) shows that the closest analogues of Professor Park's rock are among the quartz-monzonites.
Quartz-mica-diorite or Tonalite.
In the members of this group, as compared with the granodiorites, quartz is present in smaller amount, there is little or no potash feldspar, the plagioclase tends to be more calcic, and hornblende may occur with or in place of biotite. The rocks occur on the margins of the batholiths or as inclusions in the granite of these latter; commonly they have been invaded by granodiorite or granite. They are represented by 1586 and 1588, which are massive rocks invaded by granite on the northern side of Northport, by 1606 and 1616, pebbles from the same region, by 1614, a dyke on the south side of Cunaris Sound near the entrance to Cliff Cove, by 1695 forming the promontory a mile north of Only Island in Long Sound, by 1633, a dyke occurring about three miles east-north-east of Last Cove in the same Sound, and by inclusions in granodiorite on the southern side of Long Sound (1572) and in granites on the mid-west
shore of Isthmus Sound (1595). There is about 65% of plagioclase, the major portion of which is Ab60 An40 with a narrow outer zone of Ab70 An30 and rarely a thin fringe reaching Ab80 An20. In some of them sericitisation is far advanced. The quartz varies in amount from 5 to 15% and is usually highly strained. In 1588, 1633, and 1695 biotite is the chief or only coloured mineral and makes about 20% of the rock, in plates up to 2 mm. in length which are either fresh or chloritised. In the absence of hornblende a little muscovite may be present, as in 1588. In 1572, 1586, and 1614 the biotite is associated with an equal amount of hornblende, often in very intimate intergrowth with this latter, a particularly good example being afforded by 1572. In 1572 and 1586 the hornblende has the usual pale-yellow-brown to bluish-green pleochroism, and in the latter is occasionally sieve-like. In 1614 brownish tints predominate in the fresher crystals of this mineral, but there is a tendency to change into pale actinolitic pseudomorphs. In 1595 the change to colourless tremolite is complete, and has been accompanied by the separation of magnetite in grains arranged along the cleavages, and by the formation of very finely divided carbonates and of marginal outgrowths of chlorite. This alteration of hornblende is perhaps connected with the propylitisation accompanying the formation of Bradshaw's copper-bearing lode in the immediate vicinity, but it is peculiar that the biotite surrounding the amphibole-grains should have suffered little alteration. In 1606 hornblende is much more abundant than biotite and forms slightly-schillerised brown crystals with greenish margins. Epidote often occurs in small grains, but in 1588 it makes large irregular masses up to 3 mm. in diameter containing a core of deep-brown allanite, a mineral which also appears sparsely in 1572 and 1586. Sphene is an important accessory in the strongly hornblendic rocks, and apatite is very abundant throughout the quartz-diorites in prisms sometimes as much as 0.40 × 0.05 mm. in size. The iron ore is magnetite, and occasionally there is a little pyrites.
Gneissic structures and shearing are more common among the tonalites and diorites than among the acid plutonic rocks. A striking example is afforded by a boulder (1606) from Shallow River, Northport, which macroscopically appears to be a dark fine-grained amphibolite containing lenticular patches of coarse-grained diorite. In the latter the grain-size reaches 3.0 mm. and feldspars dominate over the coloured minerals, namely hornblende and grass-green flakes of pennine which replaces biotite oriented in parallel bands. The finer-grained portions are composed of hornblende and feldspar in equal proportion, oriented and sheared along the direction of schistosity. 1616, a pebble from the same locality, is a relatively fine-grained gneissic quartz-diorite or microdiorite noteworthy for the substantial parallelism of the deep-brown to greenish-brown flakes of biotite 0.5–0.8 mm. in length, which make up about 25% of the rock along with about 5% of grass-green hornblende, generally in small prisms, and for the abundance (15%–20%) of coarse epidote. This last mineral is usually in well-formed crystals as much as 3 mm. long, often with a core of allanite, but may be in less regular masses. It is strikingly poikilitic and appears to be of primary origin.
As an appendix to these quartz-diorites may be cited 1563, an inclusion in the granodiorite near the creek north of Mosquito Point, Northport. It consists of about 70% of sericitised indeterminable feldspar in grains about 0.5 mm. long, with about 20% of pennine replacing more or less oriented biotite, a little sphene having been developed in the change. Quartz grains form nearly 10% of the rock, and others of leucoxenised ilmenite are fairly abundant. Small prisms of apatite appear and grains of pyrites, while films of the last mineral often surround the grains of quartz. Veinlets and scattered masses of calcite and an occasional grain of epidote are also present. Probably this rock is a much-altered fragment of a feldspathic quartz-mica-diorite.
Comparisons with rocks from other parts of Fiordland show that, though there are differences in texture and grain-size, there is similarity in all essentials between D4, a gneissic diorite from Duck Cove described by Professor Speight (1910), and 1586 and, to a lesser extent, 1572 noted above.
This group differs from the quartz-diorites in the smaller amount or absence of quartz and biotite, though the distribution of its members is the same as that of these tonalitic rocks. Diorite occurs as dykes extending out from the batholiths, as small masses at the margins of these latter, or as inclusions in them. It is represented by 1560 and 1515 from the southern shore of Long Sound 3000 yards north-east of Only Island; by 1659, included in the granites of Isthmus Sound; 1592, a vein invading the sediments half a mile north-east of Cuttle Cove; 1610, a narrow dyke a mile east of Cunaris Islands; 1559, on the northern shore of Northport; 1589a, a mile up the creek draining from Lake Caesar; 1615, a pebble collected from the same creek; and 1568 and 1678, from the eastern shores of Landing Bay near Cape Providence.
In 1568 the coloured constituent forms about 10% of the rock and is a pale-green pennine replacing biotite; with it there is leucoxenised ilmenite, much sphene having been developed by the change of both biotite and this latter mineral. The rock has been a good deal crushed, and the original mica has been arranged in and parallel to shearing-zones. The plagioclase is approximately Ab76 An24 and occurs in prismoids up to 4 mm. in length dusted with sericite and containing small inclusions of quartz; its grains have been separated by the shearing, and a small amount of crushed quartz fills the interspaces. The feldspar of the other diorites is usually more basic than in this rock; it is sometimes turbid owing to the presence of zoisite and sericite, and in 1559 has been partly replaced by prehnite along definite fracture lines, the change working sporadically into the feldspar from its margins. In 1515 the grain-size is a little smaller, and the shattering not so marked. The pennine is again in oriented flakes, but is not so abundant as in 1568. On the other hand, there is about 6% of epidote in aggregates of irregular grains of moderate size closely associated with biotite or strung out along fracture lines, and scattered as a cloud of minute grains through the feldspar (Ab65 An35). Quartz is absent. 1560 is still more basic, for zoned plagioclase makes up 75% of the rock
and is Ab50 An50 in the centres of the grains. It further contains nearly 20% of hornblende in prisms nearly 4 mm. long, which show the normal pleochroism, X = brownish-yellow, Y = deep-green, Z = brownish-green with a bluish tinge on the margins. Sometimes this latter mineral is lighter in colour, as in 1559 and 1589a, where it forms 45% of the rock and is associated with 3% of chloritised biotite in small oriented flakes. Its pleochroism in these two rocks is X = very pale yellow-brown, Y = grass-green, Z = weak blue-green. It forms grains about 1 mm. long sometimes moulded upon the feldspar, sometimes idiomorphic against it. In 1678, on the other hand, the hornblende forms prisms 2–4 mm. long, darker and browner than normally, and often schillerized, as also in 1560.
In the diorite (1659) included in the granite at Bradshaw's Mine, Isthmus Sound, propylitisation is more advanced than in the tonalite (1595) which forms a similar inclusion in this granite, and the hornblende has been replaced by finely-divided carbonates and distributed chlorite. In 1610, a dyke of diorite occurring a mile east of Cunaris Islands, the hornblende is present as large poikilitic masses of the pale-green actinolitic type, mingled with a little chlorite and associated with a greater amount of biotite in irregular aggregates of small confusedly-matted plates.
Ilmenite is a frequent accessory, and is often more or less leucoxenised; it is especially fresh and abundant in 1610. Separate grains of sphene are occasionally present, and in rare cases may be quite large (0.8 mm.). Epidote is not infrequent, and is occasionally accompanied by allanite (1615). Apatite is plentiful in nearly all the diorites, and is sometimes in relatively large prisms.
The gneissic microdiorites constitute a small group of rather fine- or medium-grained rocks which are probably sill-like apophyses of the batholiths and which have most of them been subsequently invaded by pegmatite, with consequent reactions leading to the formation of biotite. 1527, which occurs at the head of the bay by Mosquito Point, Northport, where it is invaded by granite or granodiorite, has an almost granulitic texture and even grain-size (0.3–0.5 mm.) for both feldspar and hornblende. Flakes of biotite are less abundant than either of these minerals, but larger (up to 1.5 mm.), and are usually oriented in one plane and along the lines of shearing which traverse the rock are completely chloritised with the development of sphene. The scattered grains of ilmenite have also been changed to sphene along these lines. The plagioclase, averaging Ab45 An55, is slightly zoned, and there is about 5% of interstitial quartz. The hornblende is rather pale in colour and slightly schillerized.
Interesting features are exhibited by a series of boulders obtained from Shallow River, Northport. Of these, 1548 shows a contact between microdiorite and pegmatitic granodiorite. The former is composed of labradorite, slightly schillerized hornblende, and a smaller amount of biotite together with a little ilmenite and apatite, the longer axes of all these minerals being more or less oriented. There is also some interstitial quartz. The invading pegmatitic granodiorite occurs in long lenticles consisting essentially of quartz and
andesine 2–4 mm. across, sieve-plates of biotite (3–6 mm.), large prisms of hornblende and sieve-garnets over 6 mm. across. In addition, it contains ilmenite, a little sphene, tawny yellow epidote, and zircon. In 1692 and 1693, a vein of aplite invading gneissic microdiorite breaks across the foliation of the latter, either penetrating or causing the feldspathisation of the invaded rock for a distance of one or two centimetres from the intrusion. A thin section (1692) taken 10 cm. from the vein shows a normal gneissic microdiorite, consisting of 45% of andesine (Ab60 An40), 28% of hornblende, 24% of biotite, and 3% of quartz in oriented grains about 0.3–0.7 mm. long, with accessory sphene, ilmenite, rare epidote, and a few large feldspar phenocrysts or phenocrystic aggregates. Immediately adjacent to the aplite, however, the hornblende has completely disappeared, and the plates of biotite which continue the foliation fray out among the crystals of quartz and oligoclase (Ab68–72 An32–28) which constitutes the vein of granodiorite in proportions approximately 40% and 60% respectively. This replacement of hornblende by biotite immediately adjacent to an acid intrusion is a well-known phenomenon (cf. Bowen, 1926, p. 198). Indeed, the coarse flakes of biotite in the rocks 1548 and 1527 described above may owe their development in large measure to the influence of the adjacent granodiorite intrusions.
Though these rocks are not precisely similar to any described by Professor Speight (1910) from the adjacent Sounds, there are points of an analogy between 1548 and D.12 from an unrecorded locality in Dusky Sound, for the latter rock (of which no description has yet appeared) has also streaks of large oriented flakes of biotite in a general base of gneissic hornblendic microdiorite.
This type of rock is represented by 1649 and 1687 from the north side of Last Cove, about two and three-quarter miles east of Last Cove. 1649 is a grey rock of medium grain-size, of which slightly sericitised plagioclase (Ab35 An65) in prisms 1–2 mm. long makes up about 50%. Hypersthene forms about 30% of the rock, occurring in rather dusty faintly pleochroic ragged grains about 1 mm. long and usually roughly prismatic, though sometimes moulded on the plagioclase. Though the pleochroism is slight, the negative optical character is distinctive Brown hornblende also occurs in poikilitic plates usually small, but sometimes up to 3 mm. in diameter. It is a little schillerized, and has usually altered marginally into a colourless tremolitic amphibole in optical continuity with it and without observable change in the extinction angle, though with some indication of increased birefringence. Where this alteration has been complete, there are individual masses of the colourless mineral, either in compact grains or tending to fray out into confused aggregates of fibres. Such colourless amphibole may project into the hypersthene obliquely to the cleavage of the latter, or may lie enclosed within its cleavage-planes, or may form poikilitic masses enclosing it. Red-brown biotite may also occur in little flakes in the cleavage planes of the hypersthene, or in larger ones lying within the amphibole, quite unaffected by this change of host, or again may be present in flakes independent alike of hypersthene and amphibole.
There are a sprinkling of pyrites and a few small grains of sphene. In 1687 the biotite, like the hornblende, may occur in extended poikilitic patches enclosing grains of feldspar and pseudomorphs of pyroxene, though the actual amount present is small. The hornblende is partially bleached as before, and the feldspar rather more sericitised, but the hypersthene has been entirely replaced by talc. The occurrence of a large mass of pyrite suggests that hydrothermal solutions may have effected the last change. It is worthy of note that the schiller-inclusions in the hornblende consist of very fine brown translucent fibres in a close rectangular lattice.
The two rocks just described come from an apparently small mass of norite which has invaded the most highly metamorphosed of the schists. They exhibit, however, no sign of strain effects, and there is, therefore, no direct analogy with the development of colourless amphibole in hornblende schists at points of special stress as noted in the Singhbhum district (Dunn, 1929, p. 93–5), though the bleaching of the margins of a hornblende in an epidiorite invaded by granite-gneiss in the same region may be noted (loc. cit., p. 98). Again, a rather similar change of brown hornblende has been noted by Turner (1933) in a diorite-pegmatite (1333) invading hornfels in the Arawata River in South Westland; here again the feldspar is only slightly sericitised, and no pressure-effects occur. On the other hand, in the flaser-gabbros of the Lizard, though the secondary hornblende may be colourless “often the original brown hornblende is not modified, but remains even when the diallage with which it is surrounded is entirely transformed” (Flett, 1912, p. 88). In view of the small amount of information as yet available, further consideration of the relation of these two types of amphibole must be postponed.
It is, however, of interest to note the distribution of possible analogues to this hornblende-norite in adjacent regions. These are briefly a mica-norite from the Darran Mountains (Cleddau River), Milford Sound, containing augite rather than hornblende (Marshall, 1907), a similar but gneissic rock from Bligh Sound (Speight, 1910), with which may be mentioned a hypersthene-amphibolite from the same Sound (Speight, 1910), and a dioritic rock with a little enstatite from Harrison Cove, Milford Sound (Hutton, 1891). Hutton (loc. cit.) noted the presence of a plagioclase-hypersthene-rock (“enstatite-diorite”) in Wet Jacket Arm, Dusky Sound, and Speight has collected but not yet described a quartz-diorite (D.8) containing a minor amount of brightly pleochroic hypersthene with augite and hornblende, occurring at an unrecorded locality in the same Sound. None of these rocks appears to be closely analogous with the Long Sound norite, and the norite of the Bluff Hill (Wild, 1911) is again different in several respects from any of the rocks cited. There is, therefore, no clear indication as yet of the magmatic affinities of the Long Sound norite, and it must still remain an open question as to whether it belongs to the same series of plutonic intrusions as the diorites, granodiorites, and granites, though much less gneissic than the earliest and most basic members of that series, or whether it may be relegated to a series of intrusive masses injected at a much
later date, which according to Turner's hypothesis (1933) may be represented by the ultrabasic and basic plutonic rocks of Cascade Valley and the Red Hills.
C. Dyke Rocks.
An example of this group of rocks is afforded by 1612, which invades schistose aplite at Blind Point, Northport. It is a somewhat sheared type consisting of Carlsbad twins of rather perthitic orthoclase, a subordinate amount of dusty plagioclase (oligoclase-albite?) with myrmekitic selvedges against crush-zones, abundant highly-strained quartz, and muscovite in large plates, with scraps of chloritised biotite. The distinction between this rock and the normal coarse-grained granite of the area is not sharp.
1620, a dyke invading granite at the cove near Mosquito Point, Northport, is closely similar to the last, but contains microcline in addition to abundant largely perthitic orthoclase. The micas are in very small amount. Pink garnets 5 mm. and more in diameter were noted macroscopically in some of the pegmatitic dykes of this district; some of these dykes proved in section, however, to be granodiorite-pegmatite.
This type is commoner than more alkalic pegmatites among the specimens collected. As an example, 1674 from the intrusion-complex on the western side of Edwardson Sound may be noted. It is intimately associated with coarse mica-schists and has caught up fragments of them. In section it is coarse-grained (2–3 mm.) mixture of 45% of highly-strained quartz, 35% of slightly sericitised plagioclase (about Ab78 An22), 10% of orthoclase in very irregular grains, and about 8% of muscovite in bent flakes up to 3 mm. in length with an optic axial angle approximating 2V = 40°. In addition, there is about 2% of biotite in much smaller flakes containing haloed zircons, which has probably been derived from the invaded schists. An intimate injection of pegmatite into coarse epidote-biotite-schist is illustrated by 1504 from the same region. In this rock large plagioclase grains are in irregular intergrowth with strained quartz and also contain many “droplets” of quartz, especially about their boundaries, and sometimes myriads of needles of what appears to be sillimanite. 1571 furnishes a further example of this type of pegmatite from the same injection-complex.
As with the granite-pegmatites, there is difficulty in drawing a sharp distinction between these pegmatitic phases and the less quartzose but also very coarse-grained granodiorite (1573) which invades the schists near Mosquito Point, Northport; indeed, this last rock was termed pegmatite in the field.
Rather different features are shown by 1637, a boulder from Shallow River, Northport, for it is a gneissic and pegmatitic granodiorite with irregularly-streaked aggregates of coarsely-foliated biotite containing rare large crystals of garnet as much as 8 mm. across. It is not clear whether this rock illustrates an extreme case of injection-foliation or, as is more probable, is a very coarse orthogneiss.
Rocks of this type have been observed in several areas, notably in the screes from the bluff at the entrance to Cliff Cove, at the head of Cunaris Sound, among the pebbles on Kisbee Beach, and on the south shore of Long Sound a mile to the north-east of Only Island. None of these rocks has been sliced. That from the first locality is probably a quartz-diorite-pegmatite, whilst the others contain prisms of hornblende 6–8 cm. long in a matrix apparently of much-altered plagioclase.
In the injection-complex of Edwardson Sound are veins, consisting chiefly of quartz and mica, which traverse the granodiorites and coarse mica-schists and may be illustrated by 1569, a specimen from the southern end of the complex. It consists of quartz (55%), muscovite (35%), and feldspar (10%). The quartz occurs in highly strained, sutured grains about 1 mm. across. The mica forms plates 2–3 mm. across and has an optic axial angle (2V = 48°) definitely distinguishing it from the sericitic mica in the invaded schists. The feldspar is often rather turbid; in part it is an unstriped and optically negative variety which is probably orthoclase, and in part striped and optically positive, with a low extinction angle perpendicular to 010, but with higher refractive index than Canada balsam, and thus probably oligoclase, though its exact composition and relative proportions are not determinable. It occurs in irregular and sometimes poikilitic grains occasionally reaching a diameter of 1.5 mm.
This rock is a greisen only in the sense of its mineral composition, for it does not give evidence of formation by hydrothermal alteration of aplite.
Rocks of this type are represented by 1676, a rather coarse-grained vein invading the schists at the southern head of the western entrance to Northport, and 1617 from opposite Mosquito Point, Northport. The first is intermediate in character between pegmatites and aplites and is somewhat gneissic, strained, and a little sheared. Quartz makes 30% of the rock in grains from 0.5 to 1.5 mm. in diameter, and there is a little under 40% of microperthitic orthoclase in irregular or prismoidal grains up to 3 mm. in length which often contain round or graphic inclusions of quartz and plates of muscovite arranged parallel to their cleavage. Microcline (15%) forms rather smaller grains with similar inclusions, and oligoclasealbite (5%) occurs in prismoids 1–2 mm. in diameter, occasionally showing myrmekitic boundaries against the potash-feldspar. All the feldspars are rather turbid, so that the determination of the composition and abundance of the plagioclase is not absolute. About 9% of muscovite (2V = 36°) occurs in plates about 1 mm. long and in sheared aggregates of small flakes, whilst chloritised biotite scarcely exceeds 1%. Pink garnet in irregular grains about 0.6 mm. across forms loose aggregates amounting to about 3% of the rock or less, and there are small prisms of zircon scattered through the colourless constituents.
There is a considerable resemblance in mineralogical composition between this rock and the aplite of Green Islets described by Professor Park (1926), though in the latter the muscovite is less abundant and there is no garnet.
1617 is a type with a considerably greater quantity of chloritised biotite (8–10%) than the last, in flakes generally about 0.4 mm. in length, and with very little muscovite. Its feldspar includes plentiful microcline and orthoclase which is generally perthitic, and a little plagioclase (about Ab65 An35). It is traversed by a narrow (3–5 mm.) veinlet of granite in which there is little mica.
This type is represented by 1621 from the headland about half a mile south-west of the granite-aplite 1676 near the south-western extremity of Great Island. It is a pale-pink rock consisting of irregular grains and prismoids of very variable size (0.1–2.0 mm.) with 40% of very sodic albite (Ab97 An3), orthoclase (40%), microcline (10%), and nearly 10% of calcite in veinlets. There are a few grains of quartz and some small flakes of muscovite, as well as others of chlorite which has been introduced into the veinlets.
An example of this type of rock is furnished by 1519 from a pebble in Shallow River, Northport. As noted on an earlier page, this rock forms a thin vein invading, and at the margins permeating, a gneissic microdiorite. It is pale pink with a grain-size of 0.5–1.5 mm. and consists of about 60% of oligoclase (Ab68 An32–Ab72 An28) in irregular grains and nearly 40% of quartz. There is a minor amount of orthoclase sometimes containing droplets or vermiform inclusions of quartz, which also occur in the oligoclase. A few small flakes of biotite and minute prisms of apatite are also present.
The rock termed by Hutton (1889, p. 123) muscovite-syenite or granite, which invades the slate at the entrance to Isthmus Sound, may be placed in this group. According to the published description, it is a fine-to-coarse-grained white or pink rock consisting of quartz, equal amounts of plagioclase and orthoclase, muscovite, and pink garnet. Liversidge's (1877) analysis given below would correspond approximately to this mineral composition only if the oligoclase were considerably in excess of the orthoclase. There is, however, too much potash in the rock to allow it to be classed as a trondhjemite-aplite (Goldschmidt, 1916, p. 89).
A dyke of dolerite thirty feet wide occurs in the promontory between Te Oneroa and Powell's Beach, Preservation Inlet, and exhibits a fine-grained selvedge against the invaded quartzite. Its central portion is represented by 1501, which was originally a typical ophitic coarse-grained dolerite. The partially sericitised plagioclase, which makes about 55% of the rock, is in almost idiomorphic tabulae, about 1.5 × 1 × 0.4 mm. in dimensions, which are slightly zoned, consisting of Ab50 An50 for the major part, but reaching Ab60 An40 on the margins in some cases. The original augite formed
about 35% of the rock and was in ophitic patches 2–3 mm. in diameter; it is now completely changed to a pale tremolitic uralite, with separation of abundant minute plates of ilmenite and of distributed chlorite which invades the cleavage planes of the feldspars. The remainder of the rock consists principally of ilmenite in grains and plates up to 0.5 mm. in length, with a little biotite in small, rarely chloritised flakes included either in the feldspar or uralite. There are a very few interstitial grains of quartz and many small needles of apatite. Pyrites has formed secondarily, usually in the uralite about the margins of the feldspars. Strain-structures and signs of shearing are absent.
In the middle of the northern coast of Coal Island there is a narrow dyke of pale ochreous rock (1597) which proves to be of lamprophyric character, though very deeply decomposed. Its fine-grained base is pilotaxitic, with the feldspars replaced by quartz, sericite and carbonates, and with small granular or else elongated pseudomorphs, which possibly replace augite and hornblende respectively, composed of dusty carbonates, either massed or as a border around almost colourless pennine and flaky biotite. There is also an abundance of finely granular ilmenite (?). In this base are phenocrystic pseudomorphs of olivine up to 1 mm. in length composed of quartz and carbonates, and others 1–2 mm. long which are believed to replace hornblende on account of the typical hexagonal shape of cross-sections and the profile of the longitudinal section of twinned prisms. There is no trace of cleavages within these prisms, and their constituent minerals are the same as those of the smaller pseudomorphs with the addition of a little rutile. Possibly the rock was originally a type of camptonite. It contains fragments of carbonaceous argillite and shows no sign of strain or shearing.
D. Summary of the Igneous Rocks.
Though the rocks that have been described herein fall into several series, there are many links which bind the granites to the quartz-mica-syenites, on the one hand, and to the granodiorites, quartz-mica-diorites, diorites, and micro diorites on the other. In broad generalisation, the granites when present form the major and central portions of the batholiths, whilst the others are marginal facies only, and the rule that a more acid rock invades a less acid is maintained without known exception in the region studied, though no opportunity was available for the investigation of a possible exception thereto not far beyond this district at Green Islets (Park, 1926). The marginal rocks show most markedly the effects of stress displayed by features that are produced during crystallization, such as banding and orientation of the constituents and by crushing or shearing along the same directions. Sometimes the granite breaks through the less acid selvedge and comes in contact with the surrounding rocks, where it develops its more obviously porphyritic facies, with a finer-grained base contrasting with large orthoclase phenocrysts. Effects of crushing tend to be more marked, also, in such situations, but neither here nor elsewhere is the quartz free from strain-effects,
whilst the same is true of the various hypabyssal rocks belonging to this series, even to the latest members, the granite-pegmatites. Comparisons with rocks obtained from Dusky Sound to the north and Green Islets to the south-east show the continuation and extension of the same comagmatic series.
In regard to the range of chemical characters of this granitic series, there are available only an old analysis from Preservation Inlet (Liversidge, 1877) and three modern ones of the rocks from Green Islets (Park, 1926). The following are the figures:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
A. “Muscovite-syenite or Granite” (Granodiorite-aplite) invading slate, Isthmus Sound, Preservation Inlet. Analyst, A. Liversidge.
B. “Diorite” or Quartz-monzonite, Green Islets. Analyst, F. T. Seelye.
D. Aplite, Green Islets.
C. Granite, Green Islets.
The age of these rocks must still remain uncertain. It has been noted in Part 1 that Hutton (1900) and Park (1921) considered that the diorites, granodiorites, and granites of the Fiordland area were formed at the close of Palaeozoic times. They declared also (though without supporting petrological investigation) that pebbles of these rocks were present in the Triassic conglomerates. Marshall (1904) described pebbles of granites, diorite, porphyries, and porphyrites from the Triassic conglomerates of Nelson, but could draw no comparison between these and the igneous rocks occurring in situ in the Nelson province, as nothing was known at the time of the petrographical features of the latter. Detailed studies of pebbles in the Triassic and Jurassic conglomerates of eastern Southland, carried out in Otago University by J. B. Mackie (1935), show that, though acid volcanic and hypabyssal rocks predominate among
[Footnote] * Also TiO2 not separated from Al2O3.
[Footnote] † Also undetermined constituents.
[Footnote] ‡ Recorded as 100.000 in the original paper.
these, there is a minority of acid plutonic rocks of which a small proportion have some features reminiscent of those displayed by certain of the rocks described herein, a resemblance which is, however, as yet insufficient to justify direct correlation therewith. Hutton and Park, citing different features, held that the plutonic rocks of the Fiordland region were intrusive into “Maitai” sediments of presumably late Palaeozoic or even early Triassic age, and believed that these last were separated by an unconformity from the continuous sequence of Triassic and Jurassic sediments. Other writers have held that there is no unconformity between these Mesozoic sediments and the formations invaded by the plutonic rocks, and have concluded that the intrusions must at the earliest have occurred at the close of the Jurassic period, a view which Speight (1910) and Benson (1921) accepted. The series of ultrabasic and basic intrusions in northern Fiordland and South Westland was considered by them to belong to this same period of igneous activity which accompanied the Hokonui orogeny. On the other hand, Turner (1933), while accepting the epi-Jurassic (early Cretaceous) age of the ultrabasic and associated basic plutonic rocks, is inclined to assign the diorites and granites to an older period, probably that assigned to them by Hutton and Park. The evidence in the Long Sound region is insufficient to permit the recognition there of two distinct eras of plutonic activity.
There is no evidence of the age of the dolerite between Te Oneroa and Powell's Beach. The presence of a diabase-porphyrite near Mount Pinder, Dusky Sound (Park, 1924) may be cited as a possible analogue. The lamprophyre of Coal Island may well be one of the series of dykes scattered here and there through the Southern Alps, notably near the Haast Pass (Turner, 1932), to which an early Tertiary age has been tentatively assigned (Morgan and Bartrum, 1915).
List of References Cited.
[In addition to those noted in Parts I and II.]
Benson, W. N., 1921. Recent Advances in New Zealand Geology, Rept. Aust. Assoc. Adv. Sci., vol. xv, pp. 45–128, esp. 53–4.
Bowen, N. L., 1928. The Evolution of the Igneous Rocks, Princeton University Press
Browne, W. R., 1914. The Geology of the Cooma District, N.S.W., Part 1, Journ. and Proc. Roy. Soc. N.S.W., vol. xiviii, 172–222.
Dunn, J. A., 1929. The Geology of Northern Singhbhum, including part of the Ranchi and Manbhum Districts, Mem. Geol. Sur. India, vol. liv.
Flett, J. S., 1907. The Geology around Lands End, Explanation of Sheets 351, 358, Mem. Geol. Sur. England and Wales.
——, 1912. The Geology of the Lizard and Meneage, Explanation of Sheet 359, Mem. Geol. Sur. England and Wales.
Goldschmidt, V. M., 1916. Geologisch-Petrographische Studien im Hochgebirge des südlichen Norwegens iv. Ubersicht der Eruptivgesteine im Kaledonischen Gebirge zwischen Stavanger und Trondhjem, Videnskapsselskapets Skrifter, 1. Mat. Nat. Kl. No. 2, pp. 1–140, esp. 89–93.
——, 1920. Ibid. V. Die Injectionsmetamorphose in Stavanger Gebiete, loc. cit. No. 10, pp. 1–142, esp. 55–60.
——and Peters, C. I., 1932. Zur Geochemie des Bors II, Nachrichten Gesell, Wiss., Gottingen, Mat. Phys. Klasse, iii, No. 28, iv, No. 31, pp. 528–545, esp. 543.
Hutton, F. W., 1892. On the Foliated Rocks of Otago, Trans. N.Z. Inst., vol. xxiv, pp. 359–365.
——, 1900. The Geological History of New Zealand, Trans. N.Z. Inst., vol. xxxii, pp. 159–183, esp. 164.
Knopf, E. B., 1931. Retrogressive Metamorphism and Phyllonitisation, Am. Journ. Sci., vol. xxi, ser. v, pp. 1–28.
Krishnan, M. S., 1924. Note of a Cordierite Gneiss from Madura District, Madras, India, Min. Mag. vol. xx, pp. 248–251.
Larsen, E. S., 1921. The Microscopic Determination of the Non-opaque Minerals, U.S. Geol. sur. Bull. 679.
Mackie, J. B., 1935. The Geology of the Glenomaru Survey District, New Zealand, Trans. Roy. Soc. N.Z., vol. lxiv, pp. 275–302.
Marshall, P., 1904. Boulders in a Triassic Conglomerate, Nelson, Trans. N.Z. Inst., vol. xxxvi, pp. 467–471.
——, 1929. The Building Stones of New Zealand, N.Z. Dept. Scientific and Industrial Research, Bull. 11.
Osborne, G. D., 1932. The Metamorphosed Limestones and Associated Contaminated Igneous Rocks of the Carlingford District, Co. Louth, Geol. Mag., vol. lxix, pp. 209–233.
Park, J., 1924. The Pre-Cambrian Complex and Pyrrhotite Bands, Dusky Sound, Econ. Geol., vol. xix, pp. 750–755.
——, 1926. Granite Inclusions in a Quartz-biotite Diorite at Green Islets, Southland, Trans. N.Z. Inst., vol. lvi, pp. 384–386.
Sederholm, J. J., 1916. On Synantetic Minerals, Bull. Comm. Geol., Finland, No. 48, p. 131.
Shand, S. J., 1927. Eruptive Rocks, Murby, London.
Sugi, K., 1931. On the Metamorphic Facies of the Misaka Series in the vicinity of Nakagawa, Province Sagami, Japanese Journ. Geol. and Geography, vol. lx, pp. 38–142.
Tilley, C. E., 1924. Contact Metamorphism in the Comrie Area of the Perthshire Highlands, Quart. Journ. Geol. Soc., vol. lxxx, pp. 22–71.
——, 1925. Metamorphic Zones in the Southern Highlands of Scotland, Ibid., vol. lxxxi, pp. 100–112.
Turner, F. J., 1930. The Metamorphic and Ultrabasic Rocks of the Lower Cascade Valley, South Westland, Trans. N.Z. Inst., vol. lxi, pp. 170–201.
——, 1932. Tinguaites and Camptonites from the Vicinity of Haast Pass, Trans. N.Z. Inst., vol. lxii, pp. 215–229.
——, 1933. The Metamorphic and Intrusive Rocks of Southern Westland, Trans. N.Z. Inst., vol. lxiii, pp. 178–284.
Vogt, Th., 1929. Sulitelma-feltets Geologi og Petrographi, Norges Geologiska Undersokelse, No. 121.
Wild, L. J., 1912. The Geology of the Bluff, New Zealand, Trans. N.Z. Inst., vol. xliv, pp. 317–339.
Williams, J. G., 1934. A Granite-schist Contact in Stewart Island, New Zealand, Quart. Journ. Geol. Soc., vol. xc, pp. 322–353.
Description of Plate 23.
Fig. 1.—Moderately porphyroblastic paragneiss (biotite-hornfels) (1514) from one mile east of Last Cove, Long Sound, characterised by a relatively coarse-grained matrix of quartz and very subordinate feldspar with poikiloblasts of biotite (basal sections stippled), sericitic “muscovite,” and occasional ilmenite.
Fig. 2.—Crushed quartz-syenite (1558) from eastern side of Landing Bay, Cape Providence. Orthoclase (in left hand top corner) with biotite and granulated quartz.
Fig. 3.—Markedly porphyroblastic paragneiss (biotite-hornfels) (1531) from near junction of Cunaris and Edwardson Sounds. Biotite with quartz and sericite (lath-like flakes) in base, and occasional ilmenite.
Fig. 4.—Biotite-quartz-muscovite-sillimanite augen-schist (1546) from near granite intrusion three miles E.N.E. of Last Cove, Long Sound. Augen of orthoclase in centre with rounded inclusions of biotite and quartz (not shown). Matrix is quartz with occasional ilmenite.
Fig. 5.—Epidote (clinozoisite)-biotite-“muscovite”-schist (1504) with matrix of quartz and a little feldspar. From injection-complex, mid-west shore of Edwardson Sound. Inset: Idioblast of clinozoisite on the same scale.
Fig. 6.—Cordierite-hornfels (1655) adjacent to granite, a mile north of Kisbee Beach, Preservation Inlet. Oriented inclusions of biotite in the large rounded crystals of cordierite. Quartz, sericite, and biotite in the matrix.