The Pomona Island Granite and Associated Gneisses, etc..
(a) The Invaded Gneisses.
Eastward from the head of South Arm to the western border of the Tertiary covering strata, the ancient gneisses* previously correlated with the Dusky Sound Series are the dominant rocks, though broken at intervals by tongues of Pomona Island granite. Interruption of this sort is least within two miles of the head of the South Arm, and again along the western shore of Hope Arm south of Stockyard Cove.
Two broad lithological divisions may conveniently be distinguished, viz., coarse-grained gneisses and finer dark-coloured amphibolites, though there is considerable variation in each group. Certain features are common to the rocks of both groups, especially the universal association of plagioclase (acid oligoclase to medium andesine) with green hornblende, biotite or both minerals, and the constant abundance of sphene and apatite as accessory constituents. Rough foliation may usually be observed, and is parallel to the bedding when the latter is recognisable.
The most widely developed members are coarse hornblendeplagioclase-biotite-gneisses, often containing between 10% and 20% of quartz and small amounts of colourless highly birefringent epidote. Many of these rocks are closely similar to gneisses of the Western Province and from the area on the north coast of the lake opposite Pomona Island (cf. Part 1, pp. 95, 96; Part 2, pp. 227, 228). Typical examples are described briefly below:—
[Footnote] * These belong to the same group as the gneisses invaded by trondhjemitic granite further west.
No. 4512 (East coast of South Arm, 1 ml. from head; widely distributed). A coarsely gneissie rock with average grain-size of 3 mm. The composition (estimated by inspection) is blue-green hornblende 20%, biotite 20%, oligoclase 50%, quartz 5%, epidote 5%, abundant sphene and apatite. The biotite is partly chloritised, and also shows replacement by aggregates of prehnite.
Nos. 4546 and 4547 (South side of first small bay inside western headland at entrance to Hope Arm) represent light and dark bands of the same hand-specimen. Fine-grained dark material (No. 4547), comparable in composition and structure with the amphibolites, predominates. This consists of hornblende 35%, biotite 15%, medium andesine 40–45%, epidote 5%, sphene 1%, iron-ore 1%, and accessory apatite. The average grain-size is 0.5 mm., but there are occasional large feldspars 4 mm. across; much of the feldspar shows albite twinning, an unusual feature in rocks of this type. While much of the hornblende is the usual blue-green type, there is also a variety having deep greenish-brown for the Z vibration-direction. Light-coloured bands in the same rock (No. 4546) contain less hornblende and only minor biotite. The amphibole is in coarse ragged crystals with marked sieve-structure, the central portions of which may be bleached to pale-green or almost colourless.
Nos. 4550 and 4551 (Western shores of Hope Arm) are plagioclase-hornblende-biotite-gneisses of finer grain, resembling the predominating rocks of Holmwood and adjacent islands (cf. Part 1, pp. 84, 85). Sphene is plentiful and pyrite usually is present, while in No. 4451 there is about 15% of quartz. On the southern side of Stockyard Cove they give place to very coarse gneisses of dioritie aspect (No. 4548) consisting of oligoclase-andesine, coarse biotite, sieved green hornblende and accessory apatite and sphene.
Associated and interbedded with the hornblende-bearing rocks are gneisses containing little or no amphibole, typical examples of which are described below:—
No. 4543 (South shore half-way between South and Hope Arms) is a rather fine-grained rock consisting of untwinned basic oligoclase and biotite in subequal proportions, accompanied by minor colourless epidote, relatively abundant granular sphene and accessory apatite. The biotite is a greenish-brown variety. The rock occurs as a constituent of large masses completely surrounded by Pomona Island granite. Interbedded with it are a hornblende-plagioclase-gneiss with little biotite (No. 4542) and a coarse gneissie rock of most unusual composition consisting almost entirely of biotite and epidote (No. 4541). In the latter rock, the epidote is the usual colourless highly birefringent type and makes up about 60% of the total composition. The biotite is deep greenish brown (Z) to pale yellow (X). Accessories include sphene, apatite, pyrite and occasional aggregates of sericite enclosed in the epidote.
No. 4553 (West coast of Hope Arm, 1¼ mls. from head). An unusually light-coloured phase, interbedded with plagioclase-biotite-gneiss (No. 4552) and plagioclase-quartz-hornblende-biotite-gneiss (No. 4551). The composition is oligoclase 50%–60%, quartz 30%–40%, biotite 5%, coarse granular colourless epidote 5%, accompanied
by sphene, apatite and much pyrite as accessories. The biotite is intensely pleochroic from pale yellow to deep red-brown, in contrast with the greenish-brown tint of the mica in associated rocks. It is partially replaced by colourless chlorite. An unusual feature shared also by the associated plagioclase-biotite-gneiss is the presence of small amounts of interstitial highly irregular orthoclase.
Along the eastern shore of South Arm, from the entrance to a point about 3 mls. from the head, the predominating members of the gneissic series are fine-grained, dark-green amphibolites of almost hornfelsic aspect (Nos. 4513, 4514, 4516, 4520, 4530, 4534). Small masses of contaminated epidiorite are associated with these rocks near the entrance to the Arm, and throughout their whole extent interruption by extensive outerops of granite is frequent. Full petrographic descriptions are unnecessary since there is a close mineralogical resemblance to the amphibolites from Holmwood and other islands as already described (Part 1, p. 85). The main constituents are plagioclase (usually andesine) and deep blue-green hornblende in about equal quantity, accompanied by accessory epidote, sphene, iron-ore and apatite. Reddish-brown biotite may be present in small quantities (e.g. Nos. 4520, 4534), and in one case (No. 4534) the percentage of epidote is as high as 10%. Patches of well crystallised prehnite were noted in a single section (No. 4513). Slender prisms of yellow rutile are very plentiful in No. 4519. Pyrite was observed in most sections. In contrast with the amphibolites described in Part 1, the rocks from the South Arm usually show perfect parallelism of the hornblende crystals, which typically are prismatic and idioblastic. In this respect they may closely resemble certain of the hornblende-schists that occur on Pomona Island as major inclusions surrounded by granite (Part 1, pp. 93, 94).
(b) Basic Intrusive Rocks.
From a point about ½ ml. inside South Arm to the headland at the entrance of Hope Arm, basic intrusives predominate among the rocks invaded by the granites, and are associated with various members of the basement gneiss series. Some are indistinguishable from rocks outeropping on Pomona Island and the northern shore of the lake, and shown in Part 1 (pp. 90–92) to be contaminated phases of the Bechive epidiorite; others, while still retaining traces of igneous structure, have no counterpart among rocks previously described from Manapouri. There are also one or two specimens of doubtful origin, which should perhaps be included with the hornblendic members of the ancient gneissic series (e.g. No. 4539).
Many of the amphibolites already described are probably derivatives of basic lavas or tuffs (cf. Part 1, p. 86), and it is even possible that some of the rocks classed above as amphibolites may in reality be completely reconstituted members of the epidiorite group. The difficulty in making a sharp distinction arises from the fact that regional metamorphism of the ancient lavas and tuffs and partial granitisation of the epidiorites both lead to the development of the mineral assemblage oligoclase-hornblende-biotite-sphene-epidote. In the amphibolites the original structure has been obliterated and a crystalloblastic structure substituted. In the contaminated
intrusives, however, the original structure tends to be preserved in part, and especially the coarse tabular crystals of plagioclase usually retain their habit even though changed in composition. In all except a few doubtful cases therefore, amphibolites and contaminated epidiorites may be distinguished on grounds of structure.
Typical contaminated epidiorites are represented by Nos. 4532 (5 ch. E. of entrance to South Arm), 4535 (25 ch. E. of entrance to South Arm) and 4544 (headland at western side of entrance to Hope Arm). Coarse tabular plagioclase (medium to basic oligoclase) sometimes enclosing clusters of epidote prisms makes up 50% to 60% of the composition. Deep green hornblende is plentiful, but in more altered rocks (e.g. No. 4532) tends to be replaced by aggregates of flaky biotite, while the remaining crystals develop pronounced sieve-structure. The percentage of biotite varies from 10% to 30%. Sphene and apatite are exceedingly plentiful. The former characteristically occurs as granular rims surrounding grains of iron-ore, or else as rounded aggregates with roughly radial structure resulting from complete replacement of that mineral. Epidote is a minor constituent. In No. 4544 nests of granular quartz make up 10% of the composition and there are small amounts of interstitial orthoclase. Locally the structure may be considerably modified by shearing (No. 4533). The above rocks are all invaded or surrounded by the Pomona Island granite or its contaminated derivatives.
A somewhat different type of basic intrusive rock, probably belonging to the epidiorite series, occurs in association with amphibolite and granite at a point about half a mile inside South Arm (Nos. 4528, 4529). No. 4528 is a holocrystalline rock composed principally of long prisms of hornblende (40%) and equidimensional twinned grains of plagioclase (50% to 60%). Minor constituents are pyrite, coarsely crystalline chlorite (optically +, biaxial) and apatite, together making up about 5% of the composition. The plagioclase is a medium andesine and locally is altered to kaolin or sericite. The hornblende occurs in poorly terminated subidiomorphic prisms (1 mm. × 0.5 mm.) with strong pleochroism as follows:—
X = pale yellow
Y = deep yellowish-green
Z = deep blue-green
Z > Y > X.
The extinction angle Z to c = 17°. The central portions of the grains are crowded with schiller inclusions. No. 4529 is obviously related to the rock just described, but is finer in grain and contains about 5% of pale biotite. Local recrystallisation has considerably reduced the grain-size. Both rocks may be classed as diorites. From their field association with an amphibolite (No. 4530) of similar mineralogical composition but with no trace of igneous structure it is possible that all three rocks have a common origin. Alternative grouping with the epidiorites is preferred by the writer however.
(c) Pomona Island Granite.
Extensive outcrops of granite of the Pomona Island type occur at frequent intervals along the eastern side of South Arm from a point about three miles from the head to the entrance, and thence eastward along the southern shore of the lake to Hope Arm. The individual outcrops are often five or ten chains in width, and alternate with equally extensive exposures of gneiss and epidiorite. Toward the head of South Arm the granites are represented only by sparsely distributed pegmatitic and aplitic dykes cutting the gneiss. Granites are well developed in Hope Arm only around Stockyard Cove and again on the eastern shores on a small peninsula half a mile north of the Monument. Also in one or two places on the western shore of Hope Arm massive pegmatitic dykes invade the prevailing gneiss.
In composition representative specimens (Nos. 4523–4525, 4527, 4538, 4540) are similar to the Pomona Island granites described in Part 1 (p. 89): microcline, albite-oligoclase and quartz are about equally plentiful, while the dark constituents together total less than 5% of the composition. In structure there are certain differences however. Typically there is definite foliation due partly to segregation of quartz into narrow lenticles and partly to parallel development of streaky aggregates of dark minerals; this will be discussed more fully in the section dealing with fabric analysis. The quartz is coarse-grained and undulose. The habit of the feldspars varies. In most specimens from South Arm (e.g. Nos. 4523–4525) small grains of microcline and plagioclase (0.2 mm. in diameter) enclose a few larger crystals of both minerals, those of plagioclase being charged with epidote or sericite. In No. 4540 (halfway between South and Hope Arms) coarse composite crystals composed of microcline and oligoclase in subequal proportions are accompanied by a few large grains of oligoclase alone. In other instances (e.g. No. 4538) clear coarse grains of perthitic microcline and cloudy crystals of oligoclase often riddled with small flakes of sericite occur independently. The range of structures suggests that in some cases microcline and plagioclase have crystallised independently while in others microcline has replaced crystals of plagioclase at a late stage.
The dark streaks observed in the hand-specimen in most cases are aggregates of black opaque iron-ore, very strongly coloured chlorite, sphene and epidote. The chlorite is uniaxial, negative and intensely pleochroic; X = yellow, Y = Z = very deep green. Apparently it is a product of hydrothermal alteration of biotite, for residual flakes and laminae of that mineral were noted in association with chlorite in one section (No. 4538). The sphene often mantles the grains of iron-ore but also occurs as independent rounded crystals. Allanite with intense pleochroism from pale yellowish-brown to deep red-brown is often associated with the epidote. In some rocks aggregates of muscovite and chlorite or muscovite, iron-ore and minor chlorite also occur (Nos. 4527, 4540). Comparison with the contaminated granites of Pomona Island (Part 1, p. 99) suggests that these dark composite streaks are largely of xenolithic origin, derived from the disintegration of granitised epidiorite and amphibolite.
The aplites and pegmatites associated with the Pomona Island granite are normal rocks worthy of only brief comment. The former (Nos. 4521, 4522) consist largely of oligoclase and quartz, with small quantities of biotite and accessory chlorite, muscovite, epidote and apatite. The pegmatites are coarse-grained rocks in which perthitic microcline, medium oligoclase and quartz are the main constituents; accessories include biotite, muscovite, apatite and sphene. Crystals of the two varieties of feldspar seem to be quite independent.
(d) Contaminated Derivatives of Granite Magma.
Intrusive rocks believed to be contaminated derivatives of the granite magma were collected from three localities. These occurrences are described separately below so that the evidence bearing on petrogenesis may be recorded fully.
The most southerly mass of Pomona granite exposed in South Arm forms an extensive outcrop about 15 to 20 chains in length flanked on either side by amphibolite, about 3 mls. from the head of the arm. The typical granite collected from the middle of this outcrop (No. 4515) is much poorer in potash-feldspar than the normal Pomona granites. The main constituents are coarse albiteoligoclase (enclosing much epidote and sericite) 60%–70%, orthoclase with local microcline structure 10%; quartz 15%, chloritised biotite 5%, and epidote 5%. Allanite, apatite and pyrite are accessories. An unusual feature is the presence of pinkish garnet in highly irregular granular masses between the boundaries of the large feldspars. Border phases collected at the northern contact are coarse blotched rocks of dioritic appearance (Nos. 4517, 4518). No. 4517 differs from the granite just described in complete absence of potashfeldspar, greater abundance of chloritised biotite (10%) and the presence of plentiful green hornblende (20%). The biotite is almost completely replaced by chlorite and prehnite, and grains of ironore enclosed in the hornblende crystals are rimmed with granular sphene. In composition the rock is a tonalite. No. 4518 appears to be a typical diorite composed essentially of medium andesine (90%) and coarse blue-green hornblende (10%). Accessories include black iron-ore, biotite, epidote and apatite.
About five chains east of the entrance to South Arm a coarsely blotched rock (No. 4531) of rather similar composition occurs as fairly extensive masses invading contaminated epidiorite and enclosing major inclusions of amphibolite. Plagioclase approximating to oligoclase-andesine makes up 75% of the rock. The remainder consists of aggregates of coarse blue-green hornblende in all stages of replacement by intensely pleochroic yellowish-brown biotite. These aggregates enclose coarse prisms of apatite and large grains of iron-ore rimmed with granular sphene. In the duplicate section from the same hand-specimen (No. 4531a) replacement of hornblende by biotite is complete and the biotite itself has locally been altered to chlorite.
Rather similar rocks occur again halfway between the entrances to Hope and South Arms, as masses 50 yds. in width alternating with contaminated epidiorite. Of these No. 4537 resembles No. 4531a in consisting almost entirely of medium oligoclase (75%) and deep
yellowish-brown biotite (25%). The latter occurs in aggregated masses enclosing granular sphene, iron-ore and apatite. Small amounts of quartz occur in nests between the crystals of feldspar. No. 4536 differs from the rock just described in the presence of residual hornblende enclosed in the biotite aggregates, and in the development of microcline and orthoclase interstitially and as patches replacing the large grains of plagioclase. The latter enclose a good deal of epidote and sericite. Quartz is absent.
In considering the genesis of the rocks just described certain additional facts must be borne in mind: (a) in all cases the rocks in question definitely bear an intrusive relation to the amphibolites and epidiorites; (b) exposures of typical Pomona Island granite occur at intervals between the three localities enumerated above; (c) the coarse hornblende of the “dioritic” rocks seems in all cases to be a primary product of magmatic crystallisation and could not have been derived from the adjacent amphibolites and epidiorites by marginal disintegration of these rocks; (d) the proportion of dark constituents to plagioclase is much too low to allow the possibility of derivation by contamination of rocks of the epidiorite group. It is therefore suggested that the “dioritic” rocks such as Nos. 4518 and 4531 are products of crystallisation of a locally contaminated magma derived from the parent Pomona granite magma as a result of reaction between the latter and the invaded rocks (cf. Part 1, p. 99). Replacement of hornblende by biotite, introduction of quartz, and in one case late crystallisation of microcline at the expense of earlier-formed plagioclase are attributed to the action of residual potassic silica-rich liquids emanating from the adjacent relatively uncontaminated granitic magma. The microcline-poor granite (No. 4515) associated with dioritic hybrids in South Arm is the product of crystallisation of a granite magma in the earlier stages of contamination when potash was being removed progressively to allow conversion of hornblende to biotite in adjacent rocks (cf. Part 1, p. 99).
Elsewhere in the contact zone of the granite intrusion marginal reaction has affected the bulk composition of the magma to a less extent, and microcline-rich granites containing plentiful altered xenolithic clots of biotite, sphene and iron-ore make sharp contacts with the invaded rocks. An intermediate condition has already been described on the southern end of Pomona Island, where normal granite and altered epidiorite are separated by a zone of granite poorer in microcline and richer in xenolithic biotite than is usually the case (Part 1, p. 94).