Metamorphic Rocks and Albite-Rich Igneous Rocks from Jurassic Conglomerates at Kawhia.
[Read before Auckland Institute, June 20, 1934; received by the Editor, July 8, 1934; issued separately, October, 1935.]
During a short visit to Kawhia a few years ago the writer noticed pebbles of mica-schist in the Jurassic conglomerates at Ururoa Point near Te Maika, and collected chips from them and associated igneous rocks which have proved upon microscopic examination to be of considerable interest and of no little importance in helping to establish the pre-Triassic date of the metamorphism by which the rocks have been affected, for it must be recollected that these Jurassic conglomerates are members of a conformable Triassic-Jurassic sequence.
The similarity of the metamorphic rocks, and indeed also of the igneous, to those of Western Otago and Southland is noteworthy, and helps to substantiate the probability that a metamorphic terrain of early Palaeozoic age, comparable with that in the South Island, underlies the North Island (see Bartrum, 1921).
An interesting feature of the igneous rocks represented is that they add a good albite-rich series to that already described by Professor Speight (1923) from Banks Peninsula. Other New Zealand records of such albite-bearing rocks are rare, though not unknown (see, for example, Ongley and Macpherson, 1923, p. 24); rocks comparable to those now described are, however, now being brought to light in Otago and Southland, as later pages will show.
The writer has had no opportunity of making a systematic collection from the Kawhia conglomerates, and there is no doubt that, if this were done, many new types would be disclosed. The delay in publishing the information now set forth is, in fact, due largely to the writer's desire, so far unfulfilled, to improve his collection.
Grateful acknowledgment must be made to the welcome and necessary assistance given by Dr F. J. Turner, of Otago University, in determination of the feldspars, which has proved very difficult on account of their considerable alteration or decomposition. The determinations have been checked wherever possible by reference to the various convergent-light figures, but, in spite of this, in some cases considerable uncertainty as to the proportions of various varieties must remain.
Dr Turner has very kindly reported upon such comparisons as are possible between the various metamorphic rocks described in this paper and those of the Otago and South Westland areas with which he is so familiar, his conclusions being published later on in this paper. Here again his assistance has been invaluable.
List of Types of Rocks Represented.
A. Plutonic Rocks:
1. Hornblende-granite (U.1).*
2. Soda-granites (U.12, 15, 21,?26).
3. Quartz-diorite and Tonalite (U.5, 20).
4. Diorites (U.14,?19).
B. Hypabyssal Rocks:
5. Aplite (U.3, 28).
C. Volcanic Rocks:
6. Soda-trachytes (Ceratophyres) (U.6, 11, 17, 23).
7. Albite-quartz-andesite (or? Dacite) (U.22).
8. Hornblende-augite-andesite (U.25, 27).
D. Metamorphic Rocks:
9. Granitic Orthogneiss (U.7).
10. Quartz-sericite-chlorite-epidote-clinozoisite-schist (U.4).
11. Quartz-albite-mica-epidote-zoisite-garnet-schist (U.8).
12. Granoblastic Quartz-albite-sericite Hornfelsic Schist (U.16).
13. Quartz-oligoclase-biotite-muscovite-?cordierite-schist (U.13, 18, 24).
14. Blastoporphyritic Andesine-hornblende-quartz-biotite-schist (U.2).
15. Contact-altered Basic Intermediate Igneous Rock (U.10).
E. Greywacke: (U.9).
It must be noted that certain rocks not listed amongst the metamorphic types have nevertheless been affected by metamorphism, notably U.3 and 19.
Description of Rock-types.
A. Plutonic Rocks:
1. Hornblende-granite (U.1).
This is a moderately fine-grained granite with 25–30%† of quartz, about 2–3% of bluish-green to grass-green hornblende in small often idiomorphic crystals, and the balance mainly perthite with a minor amount of acid lime-soda-feldspar.
2. Soda-granites (U.12, 15, 21,?26).
These are fairly coarse-grained rocks with albite as the dominant feldspar. U.12 has about 7% of slightly chloritised deep-bluish-green to pale-greenish-brown hornblende, which is inclined to be grouped in clusters of crystals averaging about 0.8 mm. in length and to be enclosed in the feldspar. In addition, there is a little over 30% of strained somewhat shattered quartz, whilst the balance is feldspar, but for a little accessory magnetite and apatite, secondary calcite and chlorite, and one large crystal, originally 2 mm. in diameter,
[Footnote] * This is the number of the corresponding thin section in the Auckland University College collection.
[Footnote] † The percentages given are estimated, and not determined by micrometric analysis.
of pinkish garnet. The feldspar is dominantly nearly pure albite showing considerable sericitisation, with occasional acid lime-bearing plagioclase and a very little unweathered clear orthoclase. U.15 is another acidic type with about 40% of quartz. Advanced weathering makes it impossible to determine how much of the feldspar, if any, is, a lime-soda variety. Albite, however, is certainly the only important feldspar. Muscovite occurs in scattered long flakes; a little chlorite and clinozoisitic epidote are also present. U.21 and 26, in contrast with the last, must lie very near the lower border of the acid class, for quartz is not in excess of 10%. U.21 has a few small (0·5 mm.) crystals of chloritised brownish-green hornblende with a moderate number of chlorite-and-sphene replacements of biotite in addition to several crystals up to 0·8 mm. in length of almost colourless augite, which also is sometimes partially converted to chlorite. The feldspar is albite, and has its larger crystals sometimes bordered by fringes of micropegmatite. There are the usual accessory minerals, apatite, zircon, sphene and iron-ore in small quantity, and little pyrite, whilst secondary epidote and clinozoisite are commonly associated with the feldspar.
U.26 has a little over 3% of somewhat chloritised greenish hornblende and shows an approach to porphyritic structure, the “phenocrysts” sometimes as much as 5·5 mm. in diameter and being in the main albite, though refractive index tests suggest that a little acid lime-soda feldspar is also present. They are usually surrounded by feldspars showing stout rectangular shapes in section and from 0·5 mm. to 0·8 mm. in length. These are albite and perthitic orthoclase, the former slightly in excess of the latter, and are in turn enwrapped by a zone of fairly abundant micropegmatite (see Pl. 21, Fig. 1). In common with the “phenocrysts” they show advanced kaolinisation. In the granitic portions of the section around the “phenocrysts,” both albite and perthitic orthoclase are prominent along with the usual accessories, amongst which the iron-ore is somewhat coarse and ragged and sometimes fringed by small grains of sphene.
3. Quartz-diorite (U.20) and Tonalite (U.5).
U.5 is a medium-grained dioritic rock with about 5% of quartz and 20% approximately of coloured minerals. Of these latter rich-brown biotite (slightly chloritised) is the most abundant, but there are also plentiful pale-green augite and greenish or greenish-brown hornblende in large part or wholly derived from the augite. So far as the weathered state of the feldspar permits of its determination, it appears to be albite-oligoclase. There are also present numerous small prisms of apatite, a little sphene, and about 2% of iron-ore (? ilmenite).
U.20 has about 10% of quartz as small interlocked crystals probably developed by recrystallization, along with shreds of chlorite (probably after biotite), around the margins of the large (1·5 mm.) crystals of weathered feldspar (approximately Ab63 An37). There are also a few crystals of apatite and zircon and secondary chlorite and calcite.
4. Diorites (U.14,?19).
U.14 contains about 20% of green hornblende locally converted to chlorite and generally in small widespread crystals, about 3% of ilmenite in irregular crystals and frequent irregular strings of coarse sphene, the balance being feldspar (andesine-labradorite—Ab50 An50) in crystals which in places have been shattered and in others bent by pressure.
U.19 is so greatly altered that its original nature is very doubtful. Its plagioclase crystals are now largely invaded by small granules of very pale green pyroxene, whilst there is also about 20% of brownish-green hornblende in ragged shreds with some attempt at parallel orientation. Occasional larger crystals of this mineral, some as much as 2·5 mm. in length, may poecilitically enclose laths of feldspar; at times it has faint schiller structure. Granular pyroxene and iron-ore tend to be concentrated in more or less indefinite wide bands with irregular strings of pyroxene transverse to them, the larger crystals of the fairly abundant iron-ore sometimes having a fringe of leucoxene. Pyrite is not infrequent, and there are also a little apatite and sphene. The feldspar is considerably kaolinised; it is nearly all labradorite of medium character (about Ab40 An60).
The peculiarities of this rock suggest strongly that it has suffered contact metamorphism.
B. Hypabyssal Rocks:
5. Aplites (U.3, 28).
The two sections examined probably represent the same rock, which is a fine-grained aplite with a moderate quantity of quartz and microperthitic orthoclase micrographically intergrown. Quartz is slightly less in amount than the other main constituent, microperthitic orthoclase; there is also minor albite.
In U.3 there is a little chloritised biotite. A very little iron-ore and sphene occur as accessories, and in U.28 a narrow string of secondary epidote and a few tiny flakes of chlorite are present.
C. Volcanic Rocks:
6. Soda-trachytes (Ceratophyres) (U.6, 11, 17, 23).
Macroscopically U.6 shows fine-scale fluxional banding co-ordinated in thin section with well-shown flow-structure in the micro-crystalline groundmass. It is a porphyritic type with somewhat sparse idiomorphic phenocrysts of feldspar which generally are not over 0·5 mm. in length, but occasionally reach nearly 2 mm. There are rare small crystals of colourless monoclinic pyroxene represented also by occasional pseudomorphs of chloritic or serpentinous nature. The accessory minerals are abundant apatite, with iron-ore fringed by leucoxenic sphene.
The phenocrystic feldspar is practically all fairly pure albite, though possibly with some acidic lime-soda variety also present, for occasional crystals appear to be optically negative. The groundmass is feldspar with a few scattered tiny grains of what appears to be pyroxene. Its variety could not be determined satisfactorily, for its
crystals are very minute, but orthoclase is possibly important, for some of the microlites have their mean refractive index lower than that of adjacent albite.
U.11 is a considerably chloritised and weathered porphyritic type with sub-idiomorphic phenocrysts consisting of important albite, some cryptoperthite, a little andesine (Ab63 An37) and scattered chloritic pseudomorphs after augite which are associated with tiny strings and grains of sphene. A few small remnants of the augite still survive. There are a few prisms of apatite and a moderate quantity of fairly coarse magnetite.
In the groundmass albite is present as tiny fluxionally-arranged microlites which are very commonly enclosed micropoecilitically in patches of alkali feldspar (? orthoclase) with much lower mean refractive index than the albite.
U.17 again is porphyritic, with large idiomorphic phenocrysts of albite in which at times there is separation of orthoclase to yield antiperthite. A few small phenocrysts of augite and rare orthoclase are also present, and a little accessory magnetite and sphene. Secondary minerals include a few shreds of chlorite and abundant calcite in wide strings and irregular patches. The finely trachytic groundmass is much weathered, and it proved impracticable to decide whether its feldspar is albite or orthoclase.
The inclusion of U.23 with the trachytes is doubtful, because advanced weathering and chloritization has made determination of the proportions of the different feldspars impracticable. Its abundant large idiomorphic phenocrysts of feldspar include both albite and a soda-lime plagioclase near acid andesine, the former apparently dominant. Coarse secondary epidote is abundant in them as well as in other parts of the rock. There are occasional small (about 0·4 mm.) phenocrysts of pale-green augite in various stages of conversion to chlorite, with larger chloritic pseudomorphs after this same pyroxene. The subtrachytic groundmass consists of laths of lamellar-twinned feldspar determinable as albite in rare instances, scattered tiny squares of magnetite, and chlorite both disseminated and as pseudomorphs after probable pyroxene.
It is not unlikely that this facies is really transitional to the albite-bearing andesite exemplified by U.22, and is, therefore, in effect a soda-trachyandesite; this is the more likely since the advanced epidotisation evidenced indicates that the lime molecules of the feldspars in part at least have been abstracted.
7. Albite-quartz-andesite (or? Dacite) (U.22).
Large phenocrysts of abundant idiomorphic feldspar, some as much as 4 mm. in length, are set fairly closely along with occasional chlorite replacements of earlier ferromagnesian mineral and a few crystals of quartz in a finely crystalline groundmass predominantly of feldspar laths, but with some recognizable quartz. The rock is considerably chloritised and epidotised, the epidote generally in coarse irregular crystals enclosed in the feldspar, though also frequent throughout the fabric of the rock, sometimes in coarse strings. It is often closely associated with the chlorite. The form and cross-fracture of the chlorite pseudomorphs at times suggest original hypersthene, but more often euhedral outlines are lacking. The
accessory minerals include fairly plentiful apatite, partially leucoxenised ilmenite in large skeleton crystals, and a little pyrite. There are also one or two small (0·8 mm.) xenoliths of a trachytic rock.
The phenocrystic feldspar includes both albite and an acid plagioclase (? oligoclase), the latter apparently, but doubtfully, the more abundant. Some rare orthoclase also is suggested by the data obtained. No satisfactory tests could be obtained for the feldspar of the groundmass, but it appears not to be alkali-feldspar.
8. Hornblende-augite-andesites (U.25, 27).
Both sections examined illustrate typical porphyritic andesites with abundant idiomorphic phenocrysts of plagioclase which are generally about 0·2 mm. in length, but on occasions are as much as 2 mm. They are labradorite (Ab45 An55) with strong zonal structure and well-shown albite and occasional Carlsbad twins. The fairly finely crystalline groundmass (about 50% of the rock) is mainly of feldspar in stumpy irregular crystals along with a little magnetite and, in U.27, pale-green augite in fairly frequent small prisms.
Sharply idiomorphic hornblende is abundant (15 to 20%) in both rocks in relatively large crystals reaching as much as 2 mm. in length; pale-green augite is present, but in much smaller, less numerous crystals. In U.25 it is faintly pleochroic in pale-green to pale-brown tints. The hornblende of U.25, is a deep-brown variety, often with zonally arranged bands of included dust-like iron-ore; at times this latter is indiscriminately scattered in its host, and at others has schiller arrangement (see Pl. 21, Fig. 2). The hornblende of U.27 is deep-brownish-green in colour and is not characterised by separation of iron-ore particles.
D. Metamorphic Rocks:
No attempt has been made to classify the various rocks on the basis of their mode of metamorphism. It will be seen that some are definitely contact types, others exhibit various grades of regional metamorphism, whilst some demonstrate the fact that they have suffered retrogressive alteration in response to shearing stress subsequent to an earlier higher-grade metamorphism in manner similar to that demonstrated by Turner (1933) for certain rocks of South Westland.
9. Granitic Orthogneiss (U.7).
This type represents an earlier granitic rock which has been considerably sheared by pressure, with the production of curved irregular zones of granulation along which there has been developed by recrystallization a fine-grained mosaic of quartz and feldspar with fairly numerous small flakes of biotite in part converted to chlorite. Associated also with this mosaic are small myrmekitic inter-growths of quartz and feldspar. The other portions of the thin section are comprised by large uncrushed remnants of feldspar and about 30% of quartz in irregular large grains. A few crystals of greenish hornblende are present and a relatively large amount of sphene in particularly large grains. Apatite, zircon, and iron-ore are the other accessories. In certain areas secondary calcite in fair quantity occupies the crevices between the fragments of shattered
quartz. The large feldspars are considerably clouded by weathering, with the production also of much sericite, so that the determination of the proportions of albite and oligoclase, both of which are present, is impossible, though convergent-light tests suggest the dominance of oligoclase. Occasional small crystals of orthoclase, clear in contrast with the cloudy plagioclase, are recognizable.
10. Quartz-sericite-chlorite-epidote-schist (U.4).
This is a remarkably well banded rock, the various bands characterised, as expectable, by richness in one or more special minerals.
Iron-ore, generally in small stumpy rod-like forms, is prominent in most layers and at times is aggregated to form nearly 25% of certain lensoid bands, though locally eliminated in others. Similarly in the leucocratic layers sutured grains of quartz, larger than in other bands and averaging about 0·12 mm. in diameter with occasional increase to 0·6 mm., are the dominant constituent, though permeated by narrow strings of epidote, a little clinozoisite, iron-ore, chlorite, and calcite (seePl. 21, Fig. 3). Epidote often is very plentiful, and layers rich in this mineral and often also chlorite, with a little clinozoisite, alternate with the quartz-rich bands. Its small prismoids at times crowd together almost to the exclusion of other minerals, though in other layers it is much less abundant. Rare prisms of bluish tourmaline occur here and there, elongated parallel to the schistosity. Feldspar is represented merely by a small number of kaolinised grains of indeterminate variety, and occasional apatite also is to be found.
The chlorite is almost ubiquitous, though only in important quantity where aggregated into bands with epidote as its common associate. It is present as two varieties: a common greenish pennine and a rarer brownish variety which appears to be in process of replacement by the green one. Its smaller flakes generally are sub-parallel to the schistosity, but the larger ones of the aggregations tend very commonly to lie at considerable angles with this latter (see Pl. 21, Fig. 3).
Dr. Turner* regards this rock as a member of the chlorite zone, produced by contact rather than by purely dynamic metamorphism.
11. Quartz-albite-mica-epidote-zoisite-garnet-schist (U.8).
This rock is strongly schistose with leucocratic layers alternating with others less so but enriched in biotite. The general mosaic-like mesostasis consists of quartz and alkali feldspar in sub-equal amount, the former in grains averaging about 0·1 mm. in diameter and the latter in somewhat smaller grains much clouded by weathering products. About 8% of biotite occurs in small rather poorly oriented flakes at times slightly chloritised, whilst small flakes of muscovite are in less quantity though numerous. Epidote and zoisite are both in small grains, the latter abundant only in certain bands. The garnet is evenly distributed as small not infrequent subidiomorphic crystals colourless in section.
[Footnote] * Personal communication.
Calcite is plentiful in some bands, whilst there is also a little myrmekitically intergrown quartz and feldspar and the unusual occurrence is presented of the poikiloblastic enclosure of small grains of quartz in an alkali feldspar of cryptoperthitic character (see Pl. 21, Fig. 4). Sphene occurs rarely.
The feldspar is an alkali variety somewhat rarely showing lamellar twinning. Determinations of its optical character suggest that it is positive, so that it appears to be albite. This probability is made the more secure by the fact, as Dr Turner notes in a letter to the writer, that orthoclase would come in only at high grades of metamorphism, whereas this rock is a medium-grade type, probably a dynamothermally metamorphosed quartzo-feldspathic schist of the biotite or almandine zone, as defined for pelites, for the associations of albite + epidote and calcite + quartz are improbable in contact metamorphism of medium grade.
12. Granoblastic Quartz-albite-chlorite-muscovite Hornfelsic Schist (U.16).
Macroscopically, this rock is noticeably schistose, though this fact is not discernible in the thin section, which shows essentially the granoblastic structure of a coarse hornfels with grain-size averaging about 0·20 mm.
About 8% of ragged chlorite, enclosing tiny grains of sphene and occasional minute zircons, evidently represents original biotite, of which only a few minute flakes now remain. Muscovite (2 E = approximately 55°) is in widely scattered large flakes, up to 2 mm. in length, which are responsible for the macroscopic appearance of schistosity, though much less abundant than the chlorite. Occasionally they show poor poikiloblastic structure (seePl. 22, Fig. 5). An [ unclear: ] idioblast of brown tourmaline 0·4 mm. in length, rare calcite, apatite, and very rare granules of epidote were also noted. Otherwise the rock consists of unsutured grains of quartz and feldspar, the latter deeply clouded by weathering products and in slightly greater quantity than the quartz. In all but a few of the grains tested, the mean refractive index is less than that of Canada balsam, whilst the maximum symmetrical extinction angle given with the somewhat infrequent 010 twin is 17°. It thus must be fairly pure albite. Oligoclase, however, is also present, and, indeed, Dr. Turner, who has examined the section, is inclined to think it is much more important than the present writer suspects. Clear crystals of orthoclase show up occasionally, with refractive index much less than that of adjacent cloudy feldspar, and negative optical character.
The retrogressive metamorphism indicated by this rock is paralleled by that of several of the types described from South Westland by Dr Turner (1933), who compares it in particular with the chloritised hornfelses he describes from the Arawata and Martyr Rivers.
13. Quartz-oligoclase-mica-?cordierite-schist (U.13, 18, 24).
U.13 and 18 contain a little garnet, almost colourless in thin section, which is absent from U.24. All have abundant biotite and muscovite with a little chlorite derived by retrogressive metamorphism from the biotite, for all stages of chloritisation of this latter mineral are evidenced and, further, the rare haloes present in the biotite
reappear in the chlorite. The usual granular sphene and rutile appear only in the chlorite of U.24, though discarded iron-ore is common. In all three sections there are patches, which may exceed 2 mm. in length and commonly have a stout rectangular shape, which are a felt of interwoven tiny flakes of a pale-greyish-green muscovite poikiloblastically enclosing grains of quartz and flakes of biotite or normal white mica. These patches exactly resemble the mica described in Victorian rocks by Tattam (1929) and undoubtedly represent pinitic replacements effected by retrogressive metamorphism of earlier cordierite or andalusite, probably the former. Comparable replacement of cordierite is shown by certain of the contact rocks of the Preservation Inlet area (Benson and Bartrum, 1935).
The feldspar is in important amount, though slightly subsidiary to quartz; it is fairly acid oligoclase with lamellar twinning freely shown.
U.13 exhibits great irregularity in the distribution of its constituents. It is relatively coarse in grain (grain-size generally between 0.2 and 0.4 mm.) and has large leucocratic belts fringed or invaded irregularly by sinuous bands or tongues enriched in deep-brown biotite sometimes to the extent of nearly 40%, with the grains of quartz and feldspar much smaller than elsewhere in the section and predominantly of the former. Sheaf-like aggregates of mica weave around eye-like lenses of coarsely crystallized quartz and feldspar or around the pinite pseudomorphs (see Pl. 22, Fig. 6). In such biotite-rich tongues or sheafs there is little muscovite, though the relations between the micas are reversed in the leucocratic portions and muscovite may locally reach 30%, accompanied then by more chlorite than is usual elsewhere.
Garnet is not plentiful; there is one crystal 0.5 mm. across and several smaller ones. A little pyrite and iron ore are also present.
U.18 is very similar to U.13, but resembles a hornfels in its granoblastic structure and lack of definite orientation of its micas. As in the previous rock, coarse areas (grain-size about 0.6 mm.) rich in quartz, feldspar and muscovite interweave irregularly with others of finer grain (grain-size about 0.10 mm.) with biotite locally reaching as much as 25% which are relatively depleted in feldspar. A very little pyrite, a few small garnets, and granular iron-ore also occur, the latter generally closely associated with the biotite or with chlorite developed from this latter.
U.24 is a particularly coarse-grained granoblastic type with grains of quartz as much as 0.7 mm. or more in diameter quite numerous and enwrapped by a much finer-grained matrix. Biotite does not exceed from 5 to 10% in most portions of the thin section, though it is particularly abundant in a central triangular area. Apart from its occurrence in the pinitic replacements, muscovite constitutes only a very small percentage of the rock, although occasional sieved crystals as much as 1.x8 in length are present. In addition to the general oligoclase, a little orthoclase apparently is represented. No garnet appears, but there are small amounts of epidote, apatite, pyrite, and disseminated iron-ore, as well as iron-ore associated with a relatively noteworthy amount of what appears to be rutile in the chlorite replacements of biotite.
14. Blastoporphyritic Andesine - hornblende - quartz - biotite - schist (U.2).
The schistosity is here well evidenced by the sub-parallel arrangement of the andesine porphyroblasts and of the prisms of hornblende (see Pl. 22, Fig. 7). The latter is a green variety with pleochroism X = pale-golden-yellow, Y = deep-green, Z = deep-bluish-green, and constitutes about 25% of the rock, usually in small ragged prisms not exceeding 0·1 mm. in length, though much larger similar crystals are scattered sparsely throughout the thin section; with it there is about 4% of brown biotite in tiny flakes. The remainder of the rock consists mainly of andesine (Ab65 An35) with quartz in what appears to be small amount, though ready separation of this latter mineral from the andesine is difficult on account of the similarity of the refractive indices of the two minerals and the unweathered condition of the feldspar in the mesostasis. That of the porphyroblasts, however, has been partially converted to kaolin, sericite, and a little calcite; at times it encloses prisms of hornblende.
A little clinozoisitic epidote, small patches of a pale-green chlorite or serpentine, occasional apatite, pyrite, and well-shaped small crystals of magnetite may be identified here and there, whilst there is a noteworthy quantity of sphene in numerous small grains.
The constitution of this schist shows that it represents a medium-grade contact-altered igneous type. Dr. Turner compares it with certain contact-altered lavas from Bluff.
15. Contact-altered Basic Intermediate Igneous Rock (U.10). (Pl. 22, Fig. 8).
But for the nature of its mesostasis, the minutely poikiloblastic character of occasional crystals of its hornblende, and the striking manner in which small narrow prisms of this latter mineral often marginally penetrate its feldspar porphyroblasts, or are included in them, this rock shows close resemblance to certain igneous types. The writer was uncertain as to whether or not its structure was in large part a heritage of its igneous ancestry, and therefore sent the thin section of it to Dr. Turner, who reports that he considers it a basic or basic intermediate igneous rock partially altered by contact metamorphism.
It consists essentially of somewhat lath-shaped poorly-defined porphyroblasts of albite which are clouded deeply by decomposition products and are generally not over 0·7 mm. in length, but at times very much larger, and then bent by pressure. Along with smaller ragged porphyroblasts of green hornblende, these are enclosed in a finely granular matrix consisting of feldspar, a little quartz, fairly plentiful prisms of hornblende, and about 3% of brownish biotite. Groups of coarse irregular grains of iron-ore are fairly prominent, and small long needles of apatite are numerous, whilst sphene is rare and secondary minerals include a little clinozoisitic epidote and plentiful calcite. The hornblende is of the same variety as in the hornblende-schist described above and forms about 15% of the rock. Its occasional poikiloblastic nature and its relations to the feldspar porphyroblasts already have been noted.
Albite is the only plagioclase definitely determinable, but there are a few clear small crystals of orthoclase. In addition, a colourless, perfectly cleaved mineral of particularly low refractive index occurs locally in very small amount as tiny plates enwrapping other minerals of the mesostasis. It is either monoclinic or triclinic and has very low birefringence, sections showing the cleavage remaining dark under crossed nicols. Such sections appear to give discordant results with convergent-light tests, for they are approximately normal to a bisectrix and in two instances gave an angle of 12° between the cleavage and either the optic normal or the optic axial plane; in another instance this angle was 54°. The interference figures, however, were poorly shown.
The only suggestion that the writer can proffer is that this mineral is a zeolite, and, if the optical tests have any value, it appears to come nearest to scolecite.
E. Greywacke (U.9).
Most of the rock is comprised by grains (0.04 mm.) of quartz and a feldspar which appears to be orthoclase, though plagioclase also is present. Scattered flakes of chlorite after biotite occur in small number with tiny grains of epidote, whilst remnants of the biotite locally survive. The granular material enwraps a few fragments of a finely microcrystalline feldspathic rock and includes also some grains of sphene, rare apatite, a little magnetite, and some veinlets of calcite. There is also a vein or band, 0.7 mm. wide, of somewhat granular quartz and cloudy orthoclase (grain-size 0.16 mm.) with rare striped plagioclase and coarse sphene. It is not improbable that this represents a tiny veinlet of pegmatite.
Summary of Comparisons.
Dr. F. J. Turner, Otago University, very kindly has supplied the following statement:—
The sedimentary schists (U.4, 8, 13, 16, 18, 24) appear to be essentially contact-metamorphosed rocks, though the dynamic factor has obviously entered to a minor extent into their reconstitution in most instances. Only one specimen (U.16) is a true hornfels. Though there is no resemblance whatever to the schists of Central and Western Otago, there is some similarity to the more strongly metamorphosed schists, gneisses, and hornfelses of South Westland. Thus, in metamorphic grade and mineral composition the quartz-biotite-muscovite-oligoclase-schists U.13, 18, 24 are reminiscent of the gneisses of the oligoclase zone in the lower part of the Haast Valley, though garnet is absent in these latter rocks. Again, the hornfels (U.16) from Ururoa Point exactly matches many of the retrogressively metamorphosed hornfelses which are so extensively developed in the Arawata and Cascade Valleys of South Westland.
Two specimens of what seem to be partially contact-metamorphosed basic or semi-basic igneous rocks (U.2, 10) are represented in the material from Ururoa Point. While unlike anything I have seen in Otago or Westland, these rocks
bear a strong resemblance to the metamorphosed lavas of the Bluff district, which are at present being investigated by Mr H. Service. The grade of metamorphism attained by these two specimens is distinctly lower than that of the majority of the sedimentary schists.
The highly sodic composition of many of the igneous rocks in the Ururoa Point conglomerates is perhaps significant in view of the recent discovery by Mr J. B. Mackie* of trondhjemites and ceratophyres in the Mesozoic conglomerates of the Nugget Point district. Ceratophyres have also been found by Mr H. Service* to occur in situ in association with metamorphosed basic lavas in the vicinity of Bluff, Southland.
Date of Metamorphism.
In his paper on the metamorphic rocks of South Westland, Turner (1933) carefully summarises such evidence as is available to indicate the date of progressive dynamothermometamorphism which affected as a combined unit the rocks of that area and of Central Otago, and was later followed by movements which led to retrogressive changes in certain of these rocks. He clearly states two alternative hypotheses that are possible, namely:
(a) That there was an early period of intense metamorphism affecting rocks now represented in Palaeozoic and Mesozoic conglomerates followed by a later, Lower Cretaceous, period yielding the schists of Otago, Westland, and Nelson.
(b) That there was a single Palaeozoic period of intense metamorphism.
In his paper Dr. Turner favours the second hypothesis, and points out that the subsequent retrogressive effects evidenced by many schists could well have been introduced by the Early Cretaceous (post-Hokonuian) orogeny.
It cannot be claimed that the Ururoa Point conglomerates yield evidence at all conclusive in its bearing on this question, though it does focus attention on one aspect of the problem, namely, that, if the first hypothesis be assumed correct, erosion of exposed Palaeozoic metamorphic rocks resulting from the earlier period of pressure and batholithic invasion was proceeding in Jurassic times. It becomes desirable, therefore, to determine if possible the probable date of this “early” metamorphism, and, in this connection, Dr. Turner's (1933) demonstration of the gradual metamorphic transition from the schists of Central Otago to those of South Westland assumes considerable importance, for, under the theory discussed, it involves the necessity of referring the South Westland schists, in conjunction with those of Central Otago, to the later period of metamorphism. The unconformity separating the schists of the two periods would thus have to be below the South Westland schists.
[Footnote] * J. B. Mackie, The Geology of the Glenomaru Survey District, Trans. Roy. Soc. N.Z., vol. 64, pp. 296, 297, 1935.
Fig. 1.—Soda-granite (U 26). Essentially large crystals of albite enwrapped by micropegmatite. Nicols crossed. × 32 diams.
Fig. 2.—Hornblende - augite - andesite (U.25). Large semi-opique phenocrysts of hornblende crowded by dust-like magnetite occur with smaller clear ones of shattered feldspar in a microcrystalline groundmass × 32 diams.
Fig. 3—Quartz-sericite-chlorite-epidote-clinozoisite-schist (U.4) Black opaque portions are enrichments of magnetite and are associated with abundant epidote To the right of central quartz-rich band there is chlorite arranged transverse to schistosity. Nicols crossed. × 32 diams.
Fig. 5—Granoblastic quartz - albite - chlorite-muscovite hornfelsic schist (U 16) A large poikıloblastic crystal of muscovite occupies much of upper half Ragged chlorite (grey) with associated iron-ore, quartz (clear) and weathered albite (grey) also are visible. × 32 diams
Fig. 6.—Quartz - oligoolase - mica -? cordierite-schist (U.13) A large poikıloblastic pinite replacement of ? cordierite is enwrapped by blotite (dark) with grams of quartz and a little feldspar × 29 diams
Fig. 7.—Blastoporphyritic andesine-hornblende-quartz-biotite-schist (U.2). Porphyroblasts of cloudy andesins are enclosed in a mesostasis in which sub-parallel prisms of hornblende are conspicuous × 32 diams
In Preservation Inlet area, the Ordovician sediments show metamorphism comparable with that of the South Westland rocks further north, and, if, as appears reasonable, the metamorphism of the two sets of rocks was one and the same event, the date of the assumed earlier period of pressure and igneous invasion must have been pre-Ordovician.
This provisional conclusion admittedly rests on evidence that is not conclusive, but its implications, nevertheless, are worth consideration: they are that, in the area supply débris to the Ururoa Point conglomerates, either the sediments representing the whole of Ordovician and subsequent pre-Triassic time had been removed by the Jurassic, or else the assumed Lower Cretaceous metamorphism so prominently evidenced in these rocks in the South Island did not affect them in the north, for metamorphic rocks comparable with those of the South Island are not there known, whether in situ or in Upper Cretaceous or Tertiary conglomerates.
Neither alternative appeals to the writer, and he therefore prefers to regard the schists of the Ururoa Point pebbles as a product of an epoch of metamorphism certainly prior to the Upper Triassic. but not pre-Ordovician. The date of this epoch must be placed in advance of that of the Ururoa Point beds because the Kawhia section indicates substantial conformity in the Upper Triassic and Jurassic sequence. Certain similarities drawn attention to above by Dr. Turner between the schists described in this paper and those of South Westland make it appear, therefore, that the major progressive metamorphism of the Otago and South Westland region must similarly be pre-Triassic, a view which conflicts with the belief of Ongley (1933) that it took place at the end of the Jurassic. Evidence from the Preservation Inlet area noted above indicates that this metamorphism must be later than the Ordovician.
In conclusion, it is perhaps desirable to point out that obviously the retrogressive metamorphic effects exhibited by some of the rocks of the Ururoa Point pebbles cannot be due to the early Cretaceous orogeny by which Turner (1933) provisionally explains similar changes in his South Westland schists.
Bartrum, J. A., 1921. A Conglomerate at Onerahi, near Whangarei, Auckland, Trans. N.Z. Inst., vol. 53, pp. 128–30.
Benson, W. N., and Bartrum, J. A., 1935. The Geology of the Region Around Preservation and Chalky Inlets; Part III, Petrology, Trans. Royal. Soc. N.Z., vol. 65 (in the press).
Ongley, M., 1933. Kaitangata-Green Island Subdivision, N.Z. Geol. Surv. 27th An. Rep., pp. 12–18.
Ongley, M., and Macpherson, E. O., 1923. The Geology and Mineral Resources of the Collingwood Subdivision, N.Z. Geol. Surv. Bull. No. 25 (n.s.).
Speight, R., 1923. The Intrusive Rocks of Banks Peninsula, Rec. Cant. Museum, vol. ii, pt. 3, pp. 121–150.
Tattam, C. M., 1929. The Metamorphic Rocks of North-east Victoria, Geol. Surv. of Vic. Bull., No. 52.
Turner, F. J., 1933. The Metamorphic and Intrusive Rocks of Southern Westland, Trans. N.Z. Inst., vol. 63, pp. 178–284.