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Volume 78, 1950
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An Intrusion of Porphyrite near Waihao Forks, South Canterbury

*

With an Appendix by D. S. Coombs, “Note on the Occurrence of Further Porphyritic Rocks in River Gravels of South Canterbury and North Otago.”

[Read before Otayo Branch, September 14, 1948; received by the Editor, October 21, 1948.]

I. Introduction and Acknowledgments

In a visit to the Waihao basin to examine the Tertiary sediments exposed there the writer noted purplish pebbles of porphyrite in the river gravel about 100 yards below McCullough's bridge. On the suggestion of Dr Benson, a search upstream was undertaken in order to locate the original outcrop.

At Waihao Forks scattered pebbles were found in the river gravel of the North Branch of the Waihao River while none were observed in the South Branch. The absence of porphyrite pebbles in the South Branch was proved, and on a traverse up the North Branch from Waihao Forks an intrusion was found in the greywackes about three miles past Waihao Forks hotel.

This paper is the result of research done under the terms of the Duffus Lubecki Scholarship at the University of Otago. It was carried out under the direction and supervision of Drs W. N. Benson and C. O. Hutton; their generous assistance and helpful criticism is gratefully acknowledged.

II. Field Occurrence

Where the Waihao River cuts across this irregular intrusion, both contacts with the surrounding greywacke can be seen. The marginal facies of the porphyrite is fine grained, but this is due to granulation rather than rapid cooling. The strike of the eastern contact is 37°E., dip 72°W., while at the western contact the strike is 91°E., dip nearly vertical. On the eastern contact a sheared apophysis was observed separated from the main mass by a few feet of shale and greywacke. Contacts are sharp and well defined in the river bed. Above this, however, the intrusion is heavily covered with bush, and the mapping of its margins (Fig. 1) may be a few yards out.

Both in the field and under the microscope the mass showed rapid variation in mineralogical composition and textural features. Jointing was most irregular; the joint planes in some places appearing twisted. No macroscopic schistosity has been developed, though local linear microstructure is often seen.

III. Petrography

(A) Greywackes and Associated Conglomerates

The country rock consists of greywackes and shales showing well-

[Footnote] * Mr Amies was killed in Malaya on July 25, 1949.

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Fig. 1—Locality map showing situation of the Waihao porphyrite and sections through the intrusion.

developed jointing, but little macroscopic schistosity except in inter-laminated shales. The greywackes fall into the subdivision Chl. 1 as defined by C. O. Hutton and F. J. Turner (1936).

On microscopical examination a greywacke of this area (8431*) is found to consist of fragments of quartz and albite set in a finegrained groundmass. The quartz grains are clear and angular and show undulose extinction, while the albite is studded with alteration

[Footnote] * The numbers refer to specimens in the collection of the Geology Department. University of Otago.

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products. Clastic fragments of augite, biotite, muscovite, unaltered hornblende, magnetite and sphene have been noted. There is some development of chlorite and finely acicular actinolite along the cleavages of the augite, while biotite shows undulose extinction and incipient conversion into a closely associated mixture of chlorite and sericite. Fine-grained semi-opaque, but highly birefringent “leucoxene” is formed from ilmenite, a change typical of this low-grade metamorphism, but larger rounded and broken grains of sphene are purely detrital in origin.

The fine-grained groundmass consists of recrystallised quartz, albite, epidote, chlorite, actinolite and sericite. Flakes of chlorite and sericite show some degree of preferred orientation imparting a streaming effect to the rock section. The epidote commonly occurs in the form of granular masses. Chlorite sometimes forms large interstitial patches consisting either of negative chlorite with deep blue anomalous interference tints, or of positive chlorite with brown anomalous tints (cf. C. O. Hutton, 1940). Apatite and zircon are common accessories.

Some boulders of conglomerate belonging to the greywacke series were sectioned (8432–8436). They were found to contain granites, porphyries, trachytes, greywackes, mylonites and reconstituted mud-stones. Granite pebbles, usually much crushed and sheared, are important members of the assemblage. The feldspar of the granites is mostly orthoclase with some associated albite. Biotite (2%) has almost completely changed to chlorite, while the quartz shows undulose extinction. Porphyry, the next most common igneous pebble, has albite and orthoclase phenocrysts in an orthophyric groundmass and appears to contain a little quartz. The quartz mylonite shows two well-marked s-planes at acute angles in the quartz crystals. Speight (1928, p. 8) records a few granite pebbles in the greywacke conglomerates of Malvern Hills.

(B) The Porphyrite Intrusion

In hand specimen the rock varies from olive green, spotted with white feldspar phenocrysts, to purplish with creamy-coloured pseudomorphs of feldspars tinged with green. The rock passes irregularly into an olive green flaser mass with little porphyritic structure visible.

The plagioclase in the main has been converted into albite with the liberation of CaO + Al2O3 which enter such alteration products as minerals of the epidote group, pumpellyite, actinolite and chlorite (cf. Wiseman, 1934). The shearing of the plagioclases has caused the development of undulose extinction and bending of twin lamellae. With increasing stress, usually localised within a few feet, fracture and then granulation have occurred, these changes being accompanied by considerable mineralogical rearrangement. Minerals of the clinozoisite-epidote group are constantly present. In some specimens (e.g. 8453) there is such a wealth of these minerals that the original igneous structure is barely recognizable. The actual composition of the aggregates varies, a core of ferruginous epidote may be surrounded by a mass of clinozoisitic epidote or clinozoisite. Where ferruginous epidote has not been produced in any quantity, clinozoisite is associated with tiny granules of iron ore scattered throughout the rock (8450). As

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noted by Wiseman (1934), epidote is equivalent to clinozoisite and haematite. The latter mineral has been observed in irregular granules and cutting through the rock in branching veins. In some sections (e.g. 8446), large feldspar phenocrysts (3 X 2 mm.) appear opaque in thin section under low power; under high power and intense illumination they are found to consist of radiating prisms of nearly pure clinozoisite. These may have either negative or positive elongation, and have extremely low double refraction. The refractive index is 1·714 ± ·002 as determined by immersion in suitable oils. Zoisite is of rare occurrence. However, in 8438 it was observed in an aggregate associated with chlorite and calcite.

Besides the members of the clinozoisite-epidote group, pumpellyite and prehnite are common secondary minerals produced from the breakdown of the original plagioclase. Pumpellyite is intimately associated with the minerals of the epidote group and chlorite within the altered phenocrysts and occurs in the groundmass usually in the form of irregular aggregates. In 8447 granular masses of pumpellyite are sometimes as much as 5 mm. in diameter. Sometimes, especially in veinlets of quartz, pumpellyite exhibits a prismatic form (fig. 3). Prisms in 8450 have the following optical properties:

  • Pleochroic Scheme:

    • X = pale yellowish green

    • Y = deep bluish green

    • Z = colourless

    • Y > X >Z

  • Refractive Indices:

    • α = 1·682 ± ·002

    • β = 1·685 "

    • γ = 1·690 "

In 8437 the plagioclase both of the phenocrysts and the felted groundmass has been converted into a colourless platey mineral with extinction parallel to the cleavage. This was at first thought to be sericite, but the double refraction was too low. The refractive index parallel to the cleavage as determined by immersion in suitable oils was found to be 1·63, indicating that the mineral is prehnite. As noted by Winchell (1933) it has positive elongation when in the form of prisms, whereas in the form of plates the elongation is negative. Flett and Hill (1912, p. 895) and W. N. Benson (1913) describe similar occurrences where the whole of the feldspar of a dolerite has been replaced by prehnite both in phenocrysts and in the groundmass.

The porphyrite on the western contact of the intrusion (e.g. 8438, 8440, 8443) contains 30% of augite (see fig. 5). The rock has been so shattered here that it exhibits a flaser structure. Mineral changes accompanying the shattering of the augite crystals have led to the development of plates of negative chlorite and needles of actinolite along the edges and cleavages, as well as to distribution of these minerals throughout the fine-grained groundmass. The augite crystals show ophitic relationship to the plagioclase pseudomorphs. The accumulation of augite at the western contact suggests the existence of some degree of differentiation. This is supported by the complete absence of augite throughout the central portion of the rock mass and

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Fig. 2—Sheared almost completely recrystallized section of porphyrite. Igneous structure only recognizable by the presence, of albite laths. Groundmass almost entirely a mass of alteration products and granules of iron ore (black). Lenticle of recrystallized albite associated with some epidote. X 15.
Fig. 3—Aggregate of prismatic and irregular pumpellyite set in a groundmass of recrystallized albite. 8447. X 50.
Fig. 4—Least altered porphyrite. Large albite phenocryst with a little epidote and iron ore. Groundmass with albite laths set in a very fine grained mass of epidote and magnetite. 8450. X 15.
Fig. 5—Fine grained flaser rock with a fairly large percentage of augite in a fine grained mass of alteration products. 8440. X 15.

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the absence of augite in the eastern and presumably upper margin.

Chlorite in varying amount is a constant member of the assemblage of minerals. In 8455 especially the predominate green mineral is negative chlorite in the form of large irregular aggregates and as a mass of flakes streaming throughout the section. It has the pleochroic scheme:

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

X = light yellowish green

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Z = dark green

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Z > X

The refractive index γ determined by the oil immersion method is 1·634 ± ·002. Rare plates of positive chlorite have been observed. Chlorite found lining veins cutting through the rock has a tendency to be idioblastic in outline. This rock, which has a flaser structure with the feldspars elongated into lenticles, was found at the upper margin of the intrusion. Flaser structure occurs particularly at such slickensided contacts.

Titaniferous iron ore is replaced by “leucoxene,” while in 8454 locally complete recrystallisation has taken place with the formation of spherulitic aggregates, apparently of sphene. Grains of similar material are frequently strung out along shearing planes.

Some sections show thin veins cutting through the rock. Their maximum width is about six inches and they have varying compositions, the most common vein minerals being calcite and haematite, with quartz, albite and either epidote, chlorite or pumpellyite quite common. Some veins were found to consist of quartz and pyrite.

IV. Age and Relationships

The greywackes and shales making up the basement rocks of this area are generally considered Mesozoic in age (Allan, 1926; Speight, 1928; Turner, 1938). Pebbles in the greywackes such as were found by Speight in the Mesozoic rocks of Mid-Canterbury occur in this area also. The presence of Ladino-Carnic fossils at Mount St. Mary (Trechmann, 1918), not many miles away, and the report of McKay (1881) of the presence of Mesozoic? fossils in the greywackes of the Hunters Hills all point to a Mesozoic age for the Waihao greywackes.

There has been as yet no other recorded porphyrite intrusions near Waihao Forks, although a porphyrite pebble from the Kakanui beach gravels found by Dr P. Marshall is in the Otago University collection (6764). The author has seen a small porphyrite pebble from the Waitaki river gravel at the Hakataramea bridge. Turner (in Williamson, 1939) has described a flaser gabbro invading the Kakanui semischists of the Naseby subdivision. At Malakoff Hill, near Nightcaps, an augite porphyrite recorded by Park (1921, p. 40) invades Kaihiku beds (Dr A. R. Lillie, pers. comm.). Bell, Clarke, and Marshall (1911, p. 40) record a dyke of feldspar porphyrite cutting (Triassic) beds at Saxton Creek, near Nelson. Intrusions of porphyrites, pyroxene porphyrites, hornblende porphyrites, dacites and andesites at various localities in the North Island have been injected into possibly Mesozoic rocks (McKay and Sollas, 1906; Fraser and Adams, 1907; Bartrum, 1921; Marwick, 1946). These rocks have undergone various stages of alteration and may belong to the same general petrogenetic epoch as the porphyrite here described.

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It is believed that the partially crystallized magma was injected into a thrust zone and consolidated therein during the orogenic movements (Hokonuian orogeny) conditioning the metamorphism of the invaded sediments. The varying distribution of high temperature in the igneous rock, and the presence of some residual magma may have influenced the development of locally varying mineralogical and structural features. There is no indication of contact alteration of the invaded sediments.

Bibliography

Allan, R. S., 1926. The Geology and Palaeontology of the Lower Waihao Basin, South Canterbury, New Zealand. Trans. N.Z. Inst., vol. 57, pp. 265–307.

Bartrum, J., 1921. Notes on the Geology of Great Barrier Island. Trans. N.Z. Inst., vol. 53, pp. 115–127.

Bell, J. M., Clarke, E. de C., and Marshall, P., 1911. The Geology of the Dun Mountain Subdivision. N.Z. Geol. Surv. Bull., no. 12, 71 pp., esp. p. 40.

Benson, W. N., 1913. The Geology and Petrology of the Great Serpentine Belt of N.S.W., Part 1. Proc. Linn. Soc. N.S.W., vol. 38, pp. 490–724.

Flett, J. S., and Hill, J. B., 1912. The Geology of the Lizard and Menage. Mem. Geol. Surv. of England and Wales.

Fraser, C. A., and Adams, H. A., 1907. The Geology of Hauraki Subdivision, Auckland. N.Z. Geol. Surv. Bull., no. 4.

Hutton, C. O., 1937. An Occurrence of the Mineral Pumpellyite in Lake Wakatipu Region, Western Otago, N.Z. Min. Mag., vol. 14, pp. 529–633.

— 1940. Metamorphism in the Lake Wakatipu Region, Western Otago, N.Z. N.Z.D.S.I.R. Geol. Mem., no. 5.

Hutton, C. O., and Turner, F. J., 1936. Metamorphic Zones in North-West Otago. Trans. Roy. Soc., N.Z., vol. 65, pp. 405–406.

Marwick, J., 1946. The Geology of the Te Kuiti Sub-division. N.Z. Geol. Surv. Bull., no. 41.

McKay, A., 1881. Rep. Geol. Explor. N.Z. Geol. Surv., pp. 56–92.

MoKay, A., and Sollas, W. J., 1906. Rocks of Cape Colville Peninsula. Govt. Printer, Wellington.

Park, J., 1921. Geology and Mineral Resources of Western Southland. N.Z. Geol. Surv. Bull., no. 23, 88 pp.

Speight, R., 1928. The Geology of the Malvern Hills. N.Z.D.S.I.R. Geol. Mem., no. 1.

Trechmann, C. H., 1918. The Trias of New Zealand. Q.J.G.S., vol. 73, pp. 163–247.

Turner, F. J., 1938. Progressive Regional Metamorphism in Southern New Zealand. Geol. Mag., vol. 75, pp. 160–174.

Williamson, J. H., 1939. The Geology of the Naseby Subdivision, Central Otago. N.Z. Geol. Surv. Bull., no. 39, pp. 25–45.

Winchell, A. N., 1933. Elements of Optical Mineralogy. J. Wiley & Sons, New York.

Wiseman, J. D. H., 1934. The Central and South-west Highland Epidiorites: a Study in Progressive Metamorphism. Q.J.G.S., vol. 90, pp. 354–417.

Appendix
By D. S. Coombs, University of Otago

Note on the Occurrence of Further Porphyritic Rocks in River Gravels of South Canterbury and North Otago

Just before his recent departure from New Zealand, Mr Amies collected a series of pebbles of altered porphyritic rocks from river gravels at three localities in South Canterbury and North Otago—

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  • (a) from along the Hakataramea River for about seven miles upstream from the junction with the Waitaki,

  • (b) from Kakanui River mouth,

  • (c) from the North Branch of the Waianakarua River, where porphyrite pebbles are particularly large and abundant near Glencoe (pers. comm.).

It seems appropriate to record brief petrographic descriptions of the rock types represented, namely, relatively unsheared metaporphyrites, severely crushed metaporphyrites, and lastly keratophyres. Professor W. N. Benson has earlier collected pebbles of metaporphyrite from both the North (8070a) and South (8064) branches of the Waianakarua River (pers. comm.) and he has generously suggested that his collection also be recorded in this note.

Unsheared metaporphyrites are represented by 8619 and 8622 from Kakanui Beach, and by all the Waianakarua specimens (e.g. 8064, 8070a, 8627, 8628). They contain large white or very pale green feldspar phenocrysts averaging 5 to 10 mm. in diameter, set in a fine-grained purple-tinted groundmass. In thin section the phenocrysts are seen to be thickly studded with granules and prisms up to 0·1 mm. long of epidote (usually an iron-poor type), together with finely divided sericite and sometimes a little pale green positive chlorite. In general, albite twinning is still discernible in the host plagioclase which was determined as albite, an3–4 (determinations by universal stage methods). The twin planes are bent and sometimes broken, and occasionally the feldspar has completely recrystallized as a fine mosaic of albite grains. Some granules of new albite, twinned and water-clear, were found to have the same composition, an3–4, as the phenocrysts. The groundmass contains the minerals already mentioned and in addition much finely divided “leucoxene” and skeletal or dusty reddish iron ore. Chlorite is more common in the groundmass than in the phenocrysts. Although virtually all the present groundmass minerals are metamorphic in origin, their grain size is so small that the igneous structure of the rock can be recognized without difficulty.

8064 (South Branch) is interesting in that it contains several clusters of quartz grains in part surrounded by very crudely radiating skeletal rods of iron ore and rod-like aggregates of minutely granular light brown material. It is possible that these are relics of coronas formed about small quartzose xenoliths caught up in the original magma. 8070a (North Branch) shows some comparable features.

Severely crushed metaporphyrites. Nos. 8624 and 8629 from Hakataramea River represent this group. They are visibly sheared both in hand specimen and in thin section and s-planes are locally thrown into small-scale contortions. There is no trace of relict twinning in the feldspar phenocrysts, which have been replaced by pseudomorphous aggregates of epidote, sericite, positive chlorite and granular albite. In the groundmass, pseudomorphs after small feldspar laths can still be recognized, although with difficulty. Tiny stringers of dusty “leucoxene” and reddish iron ore are abundant and small veinlets and lenticles of quartz are not uncommon, these latter sometimes

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Fig. 6—General view of southern portion of porphyrite across Waihao River.

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Fig. 7—Slightly enlarged photograph of a specimen showing the sharp contact between porphyrite (light coloured) and shale.

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containing water-clear albite and relatively coarse chlorite and sericite as well. Similar veinlets are present in the unsheared metaporphyrites described above, but they are rare. Scattered aggregates of positive chlorite in 8629 may possibly be pseudomorphous after augite.

8626 and 8630, also from Hakataramea, retain recognizable twinning in the phenocrysts, being transitional towards the unsheared type in this respect. They are notable also for containing pumpellyite. 8630a contains many irregular aggregates (0·5–1 mm.) of pale green positive chlorite fringed with dusty reddish iron ore.

Keratophyres. 8620 and 8621 from Kakanui Beach are classified as keratophyres. They are very hard, almost flinty rocks in which the feldspar phenocrysts, less than 3 mm. long, are small and scattered as compared with those of the metaporphyrites. The groundmass has trachytie texture. Both phenocrysts and groundmass feldspars tend to show fine and often blotchy twinning rather reminiscent of anorthoclase, but all grains determined were optically positive, composition albite, an3 4 They contain sparse inclusions of calcite, epidote and rare pumpellyite. In contrast to the metaporphyrites the groundmass contains only very subordinate epidote together with minor interstitial or finely intergrown quartz, a little sericite, and dusty iron ore and lencoxene. The sections are crossed by a few fine joint-filling quartz-veins, but apart from slight distortion and fracturing of feldspar phenocrysts, there is no evidence of severe crushing.

Comments. Disregarding the keratophyres, the rocks described above differ from the Waihao porphyrites chiefly in their less sheared condition, although the Hakataramea examples are comparable to some of the Waihao ones in this respect. Further, the mineralogical composition seems less varied, and relict augite, so abundant in some of the Waihao rocks, has not been found. The usual mineral assemblage is albite-epidote-sericite-chlorite with iron ore and leucoxene, and occasionally pumpellyite. Evidently ancient bodies of porphyrite, as yet unmapped except at Waihao, are rather widely distributed in the general area considered. It should be possible to locate them by tracing the river pebbles to their sources.