The Petrology of the Arahura and Pounamu Series in the
Kokatahi River, North Westland
This papers presents the results of the potrological study of the Arahura and Pounamu schists.
The names “Arahura series” and “Pounamu series” were introduced by Bell and Fraser (1906) for rocks within the Hokitika Sheet of the North Westland Quadrangle of the New Zealand Geological Survey. The Arahura series comprised the mica schists which cover a wide area on this sheet, and the Pounamu series designated the comparatively narrow bands of green schists locally inter-bedded with the Arahura series. These names have been adopted by later workers in adjacent areas, and are applied to rocks of these types throughout North Westland and South-west Nelson. These rocks are thus widely distributed, and are well exposed at many places, particularly along some of the deep gorses by which the major rivers cross the schist belt Unfortunately, such localities are also rather inaccessible, and the rocks of these series have been little studied since the original work of Bell and Fraser, and of Morgan (1908).
In view of the wide distribution of the Arahura and Pounamu series, and of the interesting problems concerning the origin and evolution of the rock types, we decided to make a petrological and geochemical study of a typical sequence. The section at the bottom of the lower gorge of the Kokatahi River was selected for study. This section is well exposed in a short distance, and is readily accessible, being an easy walk from the end of the road from Hokitika. Several bands of green schists of the Pounamu series are inter-layered with typical schists of the Arahura series. This paper presents the results of the petrological study of these rocks. The results of the geochemical study will be reported in a separate paper.
The lower gorge of the Kokatahi River is included in the S 58 sheet of the National Grid. This sheet has not been published, but the area is covered by a geological map of the Toaroha Survey District (Morgan, 1908). A good road leads from Hokitika to within three miles of the gorge. After fording the Styx River, a track on the north bank of the Kokatahi River leads to the section.
The rocks form part of the Alpine schist belt, which here averages about ten miles in width, striking north-east parallel to the Alpine Fault. The grade of metamorphism decreases away from the fault and Morgan (1908) placed the boundary between the “mica schists” and the “less, metamorphosed part'” of
[Footnote] 1 The American Museum of Natural History, New York. 24.
[Footnote] 2 Geology Department, Oxford University, England.
the Arahura series four miles south-east of the section, and showed that two and a-half miles north-west a small area of “gneissic and dark schists” occurs hard against the fault.
The nature of the sequence and the location of the samples is shown in Fig. 1. The section extends 700 feet along the Kokatahi River, which here flows northwest across the strike of the beds, which is 50° with some local variation. The dip is fairly uniformly 80° S.E. throughout the section, but it was not possible to determine whether the beds were overturned. The rocks are best exposed toward the downstream end of the section, particularly where the river bed widens, although it one place a small stream entering from the north-east has spread coarse gravel over part of a broad outcrop of the green schists. The beds further upstream are quartz-albite-biotite schists (“mica schists” of the Arahura series). While an excellent section is exposed in the walls of the gorge, this was inaccessible and collecting was limited to the thickly forested banks Near the footbridge, about 200 feet lower ( [ unclear: ] ) in the sequence, thin bands of green schist of the Pounamu series appear, and become more numerous until in the most accessible part of the river bed they form a 150-foot bed. Gradational contacts between the green schists and the Arahura rocks, and the presence of small stringers of green schist in the quartz-albite-biotite schists indicate that the rocks comprising the Pounamu series were not the result of later intrusion, but were formed along with the sediments now represented by the Arahura rocks. The field relations suggest that the sequence was in place prior to the metamorphism. H. W. Wellman (personal communication) states that green schists elsewhere in Westland show similar field relationships.
Six feet below the upper margin of the main green schist, zone occurs a seven foot thick, apparently conglomeratic bed. In outcrop it has what may best be described as a “pebbly” appearance with “pebbles” of 2 to 4 inches diameter. Since there is no difference in composition between the pebbles and the enclosing green schist the structure is probably derived from an agglomerate or a pillow lava.
Below the main green schist belt, at the downstream entrance to the gorge, appear fine-grained calcite schists which in places are practically pure marble.
Alluvium masks further outcrop downstream except for an exposure of quartz-albite-biotite schist in the bed of a small stream on the track a quarter of a mile downstream from the entrance to the gorge.
Schists of The Arahura Series
In hand specimen the schists of the Arahura series all show marked foliation except T10, which has a confused structure suggestive of a crush breccia. Different samples lave a considerable diversity in texture, ranging from coarse-grained types such as T1 and T2, in which the individual minerals are isolated in separate layers and lenses, to very fine-grained types like T17, in which the schistosity is indicated by layering and easy cleavability in one plane rather than by segregation of the individual minerals. The coarse-grained types have a brown (biotite) and white (quartz and feldspar) speckled appearance; the finer-grained types are grey in colour and the only recognizable mineral is brown biotite in porphyro-blasts up to 1 mm. across.
The mineralogy of the schists of the Arahura series shows considerable diversity, although the essential minerals are usually quartz, albite, and biotite
In T17 and T22, which are rich in calcite, biotite is reduced in amount to accessory proportions. Muscovite is present in most, samples, but does not amount to more than 15%, and is usually considerably less. Clinozoisite is ubiquitous (except in T17) as small grains generally included in the feldspar, its amount is [ unclear: ] generally about 5%. Accessory minerals, which may or may not be present and seldom reach more than 5%. are sphene. chlorite, garnet, tourmaline, and opaque material (probably magnetite and hematite).
The albite is untwinned and cannot be readily distinguished from quartz in thin section. However, lenses rich in albite generally contain a considerable amount of muscovite and clinozoisite, whereas quartz lenses are generally rather free from foreign minerals. For a number of samples the percentage of albite was checked by a chemical determination of Na2O. The quartz is also markedly coarser-grained than the albite; the average grain size of the quartz is about 0.1 mm., of the albite about 0.02 mm. The albite contains about 5% of anorthite. as indicated by its refractive indices. The biotite has refractive index γ between 1.63 and 1.64, except for a grepn biotite from T17, for which γ = 1.596.
The following notes apply to the individual samples.
T1: Quartz 16%. albite 25%, biotite 45%, clinozoisite 8%, muscovite 3%. sphene 2%, chlorite 1%. This specimen is noteworthy for the abundance of biotite, which occurs as porphyroblastic flakes 0 5–1 mm. across, refractive index β = γ = 1.630, pleochroism not as strong as in most biotite in these rocks, × = very pale yellow, Y = Z= medium brown. The muscovite occurs interleaved with the biotite. The clinozoisite (or zoisite) appears as small, colourless crystals in the feldspathic lenses, with high relief and low birefringence, often showing anomalous blue-grey interference colours. The small amount of chlorite fringes crystals of biotite, and probably is an alteration product of this mineral. The
sphene occurs in fine-grained mossy aggregates, often surrounding black opaque grains which are probably ilmenite.
T2: Quartz 15%, albite 25%, biotite 40%, muscovite 9%, clinozoisite 7%, sphene 2%, garnet 2%. This specimen resembles T1 in all respects, except for a greater abundance of muscovite and a lesser abundance of biotite, and the presence of a small amount of garnet. The muscovite occurs as small crystals scattered throughout the rock, but is especially abundant in the feldspathic lenses; the biotite is concentrated in layers. The garnet is colourless and euhedral, and is concentrated in narrow bands parallel with the schistosity; its refractive index is 1.793. Spectrographic analysis shows that the major cations present in this garnet are iron, aluminium and calcium, and that manganese is quite low, about 1%; it is thus not a spessartite, as are many low-grade garnets, but is intermediate between almandite and grossularite, the refractive index indicating about 70% of the almandite component.
T3: Quartz 25%, albite 25%. biotite 30%, muscovite 13%, clinozoisite 5%. sphene 2%, small amount of opaque material. T3 is closely similar to T2; it has occasional layers crowded with black opaque dust.
T5: Quartz 30%, albite 30%, biotite 15%, muscovite 10%, clinozoisite 5%. sphene 2%, garnet 1%, magnetite 2%, tourmaline 5%. This specimen is like T3; however, the amount of biotite is lower, and the presence of a small amount of tourmaline, pleochroism E = light brown, O = dark brown, is noteworthy.
T8: Quartz 20%, albite 45%. biotite 10%, muscovite 1%, clinozoisite 10%, chlorite 3%, garnet 6%, tourmaline 3%, hematite 2% This schist is noticeably finer-grained in hand specimen than the preceding samples. The amount of biotite is low, and muscovite is rare. Albite is much more abundant than in any other sample. The tourmaline has different pleochroism from that of T5, E = very pale yellow, O = dark blue-grey. The refractive indices of the tourmaline are ω = 1.626, ω = 1 650, which from Winchell's diagram (1951) indicates a composition approximately midway between a magnesium and an iron tourmaline. The garnet is colourless, euhedral, and shows minute birefringent inclusions; the refractive index is 1.792, and the composition is probably similar to that of the garnet in T2.
T10: Quartz 30%, albite 25%, biotite 10%, muscovite 10%, clinozoisite 7%, chlorite 30%. calcite 15%. This sample does not show the characteristic schistosity of the other specimens, but has a crushed and brecciated structure. The thin section shows numerous small micro-faults. The structure suggests faulting and crushing after metamorphism, and the specimen may be from the crush-zone of a post-metamorphism fault. The mineralogy is similar to that of the schists described above except for the presence of about 15% of calcite, sometime as small individual crystals (about 0.1 mm. across) and sometimes as wispy, highly pressed material between grains of quartz and feldspar. The chlorite is mostly an alteration product from biotite, and may indicate retrogressive meta morphism accompanying the crushing.
T17: Calcite 85%, quartz < 1%, albite 6%, biotite 5%, chlorite 1%, opaque 2%, epidote < 1% This rock is highly calcareous and the silicate minerals little more than accessories. In hand specimen it is finely laminated, but the thin section shows an equigranular texture, with albite and a little quartz occurring as rounded grains uniformly disposed throughout the groundmass of calcite.
The biotite differs in pleochroism from other schists in this group: × = colourless, Y = Z = olive green; its refractive indices are also lower: β = γ — 1.596. The chlorite has γ = 1 603.
T18, T19, T20. Quartz 30%, albite 27%, biotite 15%, muscovite 15%, clinozoisite 5%, chlorite 4%, garnet 2%, opaque 2%, tourmaline < 1%. These are typical schists of the Arahura series. The biotite is not in oriented lenses, but as porphyroblastic crystals randomly scattered throughout the rock. The muscovite occurs as tiny (0.03 mm.) crystals oriented parallel with the schistosity in the feldspathic layers. The rocks contain about 5% of chlorite as wispy layers in the groundmass, these layers being oblique to the main direction of schistosity. The tourmaline occurs as tiny crystals, pleochroism similar to that in T8: E = pale grey, 0 = dark blue-grey.
T21 : Quartz 30%, albite 20%, muscovite 30%, chlorite 8%, clinozoisite 8%, opaque 4%. In hand specimen this rock is a pale grey-green schist showing numerous small quartz-feldspar lenses when fractured at right angles to the schistosity planes In thin section this rock is distinctive for the absence of biotite and the presence of an appreciable amount of chlorite (γ = 1 610). In the field this rock was classified as a Pounamu schist, but its mineralogy is more akin to that of the Arahura rocks; if the Pounamu rocks were originally basic igneous rocks, then T21 may have been a tuffaceous sediment.
T22. Calcite 40%. quartz 20%, albite 20%, epidote 10%, chlorite 5%. opaque 5%, biotite < 1%. This sample is noteworthy for the high calcite content, and the practical absence of biotite. The rock as a whole is highly schistose, with separate layers of calcite, feldspar, and quartz. Epidote rather than clinozoisiter is present in this rock; it is distinctly yellow and has α — 1.730, γ = 1.762: it occasionally forms almost pure lenses in the rock.
Schists of the Pounamu Series
Of the rock samples studied T6. 7. 9, 11, 12, 13. 14, and 15 belong to the Pounamu series. The typical rock type is a pale grey-green schist, often with white lenses and bands of quartz and feldspar Calcite is often present in these lenses and bands, and can readily be detected by testing with dilute HCl. Biotite is generally prominent as lustrous black flakes averaging about 1 mm. across. The presence of epidote is indicated by occasional yellow-green streaks rich in this mineral. Hornblende is present in most of these rocks, but is not prominent macrosrcopically, except in T6 and T9, in which it is present as coarse porphyro-blasts up to 10 mm long T11 is noteworthy for the presence of a considerable amount (about 15%) of magnetite, occurring as octahedrons up to 3 mm across Accessory rutile can be seen in T6 as small red-brown flecks in the amphibole-rich bands.
An average composition of these rocks is not easily determined, because of the segregation of quartz-feldspar lenses from the more continuous bands of ferromagnesian minerals. An overall estimate would be 15% quartz. 20% albite. 15% biotite, 20% hornblende. 15% epidote, 10% chlorite, 5% calcite. The only other minerals noted were rutile in T6 and T7, and magnetite in T9 and T11.
The feldspar in these rocks is albite It is untwinned in very small grains ( [ unclear: ] ea 0.02 mm. diameter), and is not easily distinguished from quartz in thin sections. Its refractive index shows that it contains about 5% of the anorthite
component. The biotite is similar in all these rocks (except T9); its pleochroism is different from that in the Arahura rocks: × = very pale yellow, Y = Z olive green and the γ index of refraction is uniformly between 1.620 and 1.625. The biotite in T9 is exceptional in having pleochroism × = colourless, Y = Z = orange-brown, but its γ index of refraction is also between 1.620 and 1.625. The hornblende varies somewhat from one specimen to another; in most specimens it occurs as a felted mass of small acicular crystals, pleochroic scheme × = colourless to pale yellow-green, Z = blue-green. Epidote is prominent in most thin sections as yellow grains up to 0.2 mm. in diameter, strongly pleochroic from pale yellow to bright yellow and with a high birefringence. The chlorite is pale green, weakly pleochroic, γ = 1 60, birefringence about 0 005, optically positive, with 2V very close to 0°.
The following notes apply to the individual samples:
T6: Quartz 20%, albite 20%, hornblende 30%, chlorite 10%., epidote 8%, biotite 5%, calcite 5%, rutile 2% In hand specimen this has an almost gneissose structure, showing narrow (ca. 5 mm.) white quartz-feldspar bands alternating with broader (ca. 10 mm.) green bands consisting largely of relatively coarsely crystal [ unclear: ] ine amphibole (crystals up to 5 mm. long) The amphibole bands show occasional reddish-brown flecks, which were found to consist of rutile. The quartz-feldspar bands effervesce freely in dilute HCl, as a result of the presence of some calcite. Some amphibole layers are also rich in chlorite, and some have considerable amounts of a brown-grey biotite.
In thin section the foliation is well-marked, although in the amphibole-rich bands the crystals of hornblende are randomly oriented within the bands. The hornblende shows strong pleochroism; × = pale yellow, Y = grass green. Z = deep blue-green, α = 1.649, β = 1.662, γ = 1.669, Z∧c = 16°, (-), 2V = 70°. The optical properties of this hornblende are very similar to an actinolite from the Otago schists analysed and described by Hutton (1940), but spectro-graphic analysis gives an A12O3 content of 14.4%, showing that it is a true hornblende. The chlorite is weakly pleochroic in pale green tints, with low birefringence (0.006). optically positive, α = 1.594 and γ = 1.600. The biotite is pleochroic in tints of green. β = γ = 1.622 The albite is very fine-grained, untwined, and its refractive indices indicate a content of about 5% An Scattered throughout the groundmass (except in the quartz-rich lenses) is a considerable amount of pale yellow epidote in tiny granules (α = 1.725, β = 1.743, γ — 1.760, 2V = 90° ±). The rutile is prominent, occurring in isolated patches, dark yellow in colour, with very high relief.
T7: Chlorite 30%, quartz 25%, albite 22%. clinozoisite 15%, biotite 2%, calcite 5%, rutile 1%. In hand specimen this rock is a fine-grained greenish-grey schist with a silvery sheen. Occasionally black biotite flakes, about 1 mm. in diameter, occur in widely separated layers throughout the rock Broken surfaces show small white lenses of calcite. up to several millimetres in length.
In thin section the rock appears as a fine-grained aggregate of chlorite, quartz, and albite. The chlorite is weakly pleochroic (colourless-pale green) The clino-zoisite mainly occurs throughout the groundmass as tiny crystals with anomalous blue-grey interference colours, but occasionally larger crystals with first-order red interference colours are seen (α = 1.710, γ = 1.720). The biotite is moderately pleochroic (X = yellow, Y = Z = green-brown) and appears to be later than the schistosity.
T9: Hornblende 20%, biotite 10%, quartz 25%, albite 30%, clinozoisite 10%, opaque 5%, chlorite < 1%. In hand specimen this is a striking rock. Its structure is almost gneissose and large black hornblende prisms, generally lying in schistosity planes, but often oblique to them, are prominent. The groundmass of the rock is a fine-grained white or greenish-grey aggregate of quartz and feldspar, with occasional lenses and bands rich in brown biotite. The weathered surface of this rock is very rusty.
The hornblende is not very pleochroic: × = pale yellow, Y — pale grey-green, Z = blue-green, refractive indices α = 1.651, β = 1.665, γ = 1.677, (-), 2V = 86°, Z ∧ c = 21°. The biotite has rather different pleochroism from other biotites in these rocks × = colourless, Y = Z = orange-brown. The opaque material occurs as dust-like material throughout the groundmass, and as larger grams, generally associated with the hornblende. The biotite flakes are strongly bent, twisted and rolled out in the direction of the schistosity. The clinozoisite is non-pleochroic, has mean refractive index approximately 1.717, and double refraction 0.011.
T11: Biotite 40% epidote 25%, hornblende 13%, quartz. 2%. albite 2%, magnetite 18% (some hematite or illiterate included here). In hand specimen this is a fine-grained greyish-green schist. Some lavers arc yellowish-green through a concentration of epidote. Black lustrous flakes of biotite coat the schistosity surfaces, and a characteristic feature of this sample, not present in other specimens from this locality, is the presence of abundant octahedrons of magnetite up to 3 mm. across. The rock also contains occasional narrow (ca. 2 mm.) veins of calcite parallel to the schistosity.
In thin section the rock appears as alternate layers of epidote and of biotite plus hornblende, with large porphyroblasts of magnetite scattered throughout. The pleochroism of the biotite is × = very pale yellow, Y = Z = olive green; β = γ = 1.622. The refractive indices of the hornblende are α — 1.643, β = 1.654, γ = 1.664; pleochroism × = pale yellow. Y = pale green, Z = blue-green. The epidote is pale yellow with refractive indices α = 1.731, β = 1.753, γ = 1.766. There is a very small amount (1%) of quartz and albite in the groundmass, and rare grains of calcite.
T12: Hornblende 60%, biotite 20%, epidote 10%, chlorite 10%. In hand specimen this rock is a very fusile pale grey-green schist with a silvery sheen on the schistosity surfaces. It contains an abundance of lustrous black flakes of biotite, averaging about 1 mm. across. The biotite occurs throughout. The rock, but is concentrated in lenses and layers parallel to the schistosity. A littie calcite is present, as small lenses scattered through the rock.
In thin section the rock appears as a felted mass of small acicular crystals of hornblende, pleochroic × = pale yellow, Y = pale blue-green. Z = blue-green; refractive indices α = 1.627, β = 1.638. γ = 1.647, Z ∧ c = 18°. The crystals of biotite are strongly elongated in the direction of the schistosity, pleochroism × = very pale yellow, V = Z = olive green, refractive indices β = γ = 1.622 The epidote is distinctly yellow, and occurs as strings of tiny crystals. A little quartz and albite, and some chlorite (γ = 1.600) occur in limited areas of the section.
T13: Quartz 10%. albite 10%. hornblende 40%. epidote 10%. biotite 25%., calcite 5%. In hand specimens a dense grey-green schistose rock, with occasional
yellow-green streaks (epidote-rich) and white lenses or bands (calcite or calcite-quartz-albite). Black lustrous flakes of biotite, up to 1 mm. across, are prominent.
In thin section the rock is typically schistose, the schistosity being largely due to the hornblende, the biotite flakes being randomly oriented. The hornblende has pleochroism × = pale olive green, Y = olive green, Z = blue-green; refractive indices α = 1.640, β = 1.649, γ = 1.654, (-), 2V = 71°. The epidote has α = 1.735, β = 7.750, γ = 1.770; (-), 2V = 69°, corresponding to about 30% of the Ca2Fe3(SiO4)3OH component; pleochroism × = pale yellow, Y = medium yellow, Z = bright yellow. The biotite has pleochroism × = very pale yellow, Y = Z = olive green; refractive indices β = γ = 1.622 A little chlorite (γ = 1.600, (+), 2V = 0°) is present in some sections.
T14: Quartz 25%, albite 25%. calcite 20%, chlorite 20%, epidote 6%, hornblende 3%, biotite 1%. In hand specimen tins rock has a gneissose structure, due to the predominance of broad (5–10 mm.) lenses of quartz, albite, and calcite These lenses are separated by narrower green bands, which have occasional black lustrous flakes of biotite scattered through them.
The thin section shows the very marked segregation of the ferromagnesian minerals from the quartz-albite-calcite bands. The chlorite is markedly pleochroic, × = pale yellow, Y = Z = green. The hornblende occurs as porphyro-blasts in the chlorite; pleochroism × = pale yellow, Y = pale green, Z = blue-green. The epidote is present as strings of small crystals, mostly within the chlorite layers Biotite, pleochroic × = pale yellow, Y = Z = olive green, is present as an accessory mineral.
T15: Calcite 40%. quartz 20%, albite 10% chlorite 15%, epidote 7%, biotite 7%, hornblende 1% In hand specimen this rock is a green fissile schist, with numerous lenses and bands (up to 10 mm thick) of an aggregate of quartz, albite, and calsite. Occasional accumulations of biotite flakes occur on the schistosity planes. It resembles T14 closely.
In thin section the segregation of the ferromagnesian minerals into bands parallel with the schistosity is clearly seen. In the calcite-quartz-albite bands the calcite is clearly predominant, and albite appears to be rather limited in amount. The ferromagnesian layers are dominantly chlorite, moderately pleochroic, × = very pale yellow, Y = Z = pale green, γ = 1.600, optically positive Biotite is present in lesser amount, pleochroism × = pale yellow, Y = Z = olive green. A little hornblende is present in the chlorite layers, pleochroism × = pale yellow, Z = blue-green.
Petrographic Differences Between Arahura and Pounamu Rocks
The Pounamu rocks can be distinguished from the Arahura rocks by differences in mineralogy. The Pounamu rocks are characterized by the presence of essential amounts of hornblende, by containing olive green biotite in contrast to the brown biotite of the Arahura rocks (T9 is exceptional among Pounamu rocks in having brown biotite), and by the presence of typical epidote in the Pounamu rocks, contrasted with clinozoisite in the Arahura rocks Albite and quartz are common in both groups of rocks; calcite is a common accessory in the Pounamu rocks but is rare in Arahura rocks except in isolated bands in which it is a dominant constituent; muscovite is usually present in Arahura rocks, but has
not been observed in Pounamu rocks, chlorite is an accessory or absent in Arahura rocks, but is usually present in Pounamu rocks, sometimes in significant amounts.
The mineralogy of these rocks indicates that they belong within the biotite zone Biotite is common and abundant in most of them Chlorite is present in those rocks containing insufficient potassium to convert this mineral to biotite On the other hand, physicochemical conditions equivalent to the garnet isograd do not appear to have been reached during metamorphism. Garnet is a rare accessory in a few Arahura rocks. In this connection it may be remarked that almandite, the characteristic species for the garnet zone, may not be formed in rocks with sufficient potassium to combine with all the iron and magnesium with the formation of biotite. Biotite is a stable mineral from the biotite isograd practically up to the highest grades of regional metamorphism. In Arahura rocks iron and magnesium may be completely combined as biotite; in such rocks the composition of the feldspar in equilibrium with clinozoisite or epidote is a more sensitive indication of grade oi metamorphism than is the mere presence of biotite. The feldspar in these rocks is an albite with about 5% of the An component.
A useful confirmation of the position of these rocks within the biotite zone is provided by the mineralogical composition of the Pounamu rocks. In these rocks potassium is generally insufficient to combine with all the iron and magnesium as biotite, chlorite is also present, whereas if the garnet isograd had been reached, almandite would be expected instead. It is interesting to note that the chlorite is comparatively Mg-rich, as indicated by a refractive index of 1.600, which from Orcel's diagram indicates an Fe/Mg ratio of ¼; chlorites from Otago schists, within the chlorite zone, have considerably higher refractive indices (Hutton, 1940). This suggests that as metamorphic grade increases iron-rich chlorites become less stable and the iron tends to enter other minerals, such as hornblende.
Fig. 2.—ACF diagram for non-calcareous rocks of the Arahura series
Fig. 3—ACF diagram for calcareous rocks of the Arahura series.
Fig. 4.—ACF diagram for rocks of the Pounamu series.
Quartz and albite can be present in all these locks.
In terms of the facies classification, the schists described in this paper belong to the albite-epidote-amphibolite facies (Turner and Verhoogen, 1951). Figs. 2–4 illustrate the mineralogy of these schists in the form of ACP diagrams. The
typical Arahura schist has the assemblage quartz-albite-biotite-muscovite-clinozoisite, and its composition would be represented by a point in field I of Fig. 2. The calcareous schists in the Arahura series, such as T22, have the assemblage calcite-quartz-albite-epidote-chlorite, and their composition would be represented by points in field II of Fig. 3. In the absence of potassium, chlorite is present in place of biotite, and the presence of epidote instead of clinozoisite may reflect that chlorite is less able than biotite to accommodate ferric iron in its lattice. The typical Pounamu schist has the assemblage albite-quartz-hornblende-biotite-epidote chlorite and its composition would be represented by a point within field III in Fig. 4. The amphibole in these rocks is a true hornblende, as shown by spectrographic analysis of the Al2O3 content. The relative amounts of chlorite and biotite are determined by the amount of potassium in the rock. Calcite is present in small amounts in many of the Pounamu rocks. A glance at Fig. 4 suggests that chlorite and calcite should be incompatible, combining to give hornblende However, in the rocks the chlorite and the calcite are segregated from each other, the calcite being in the quartz-albite lenses, the chlorite in the lenses of the ferromagnesian minerals. Another point to be borne in mind, as emphasized by Turner (1935), is that the reaction of chlorite and calcite to give hornblende or vice versa will be strongly affected by the partial pressure of CO2 as well as by the temperature and pressure. The co-existence of chlorite and calcite in T22 probably reflects a partial pressure of CO2 in this rock during metamorphism sufficiently high to prevent reaction to give hornblende.
The field relations, mineralogical composition, and geochemistry of these rocks support the generally held opinion that the Arahura series represents metamorphased sediments, mainly of the greywacke type, and the Pounamu rocks are metamorphosed basic igneous rocks, probably originally interbedded lava flows.
One of us (B.M.) would like to express his thanks to the John Simon Guggenheim Memorial Foundation for the award of a fellowship, during the tenure of which he was able to study these rocks in the field; the other (S. R.T.) is indebted to the Geology Department of Indiana University for the use of its facilities and for financial assistance during the laboratory examination of the rocks.
Bell, J. M., and Fraser, C. 1906. The geology of the Hokitika Sheet, North Westland Quadrangle; New Zealand Geol Surv. Bull. 1.101 pp
Hutton, C. O., 1940. Metamorphism in the Lake Wakatipurregion, Western Otago, New Zealand; New Zealand Dept. Sci. Ind. Research, Memoir 5, 90 pp.
Morgan, P. G. 1908. The geology of the Mikonu [ unclear: ] i Subdivision, North Westland; New Zealand Geol. Surv. Bull, 6, 175 pp.
Orcel, J. 1927. Recherches [ unclear: ] la composition ch [ unclear: ] imique des chlorites, Bull, Soc. Franc. Min., Vol. 50. pp. 73–456.
Turner, F. J., 1935. Contribution to the interpretation of mineral [ unclear: ] in metamorphic rocks; Amer. Jour Sci., Vol 20. pp. 409–421.
—— and Verhoogen, J., 1951. Igneous and metamorphic petrology; McGraw-Hill Book Co. New York.
Winchell, A. N., 1951. Elements of optical mineralogy [ unclear: ] Part 2, (fourth edition); John Wiley & Sons, New York.