Art. XXXIV.—Some Intrusive Igneous Rocks from the Westport District.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 46, 1913, Page 262

Art. XXXIV.—Some Intrusive Igneous Rocks from the Westport District. By J. A. Babtrum, New Zealand Geological Survey. (By permission of the Director of the Geological Survey.) [Read before the Wellington Philosophical Society, 27th August, 1913.] Introduction. The aim of the present paper is to give a petrographical account of several interesting rock-types encountered during the course of the recent work of the Geological Survey in the Westport district. A statement of consanguinity and absolute classification can only be made when the requisite chemical analyses are available, but it is believed that the petrographical descriptions indicate these sufficiently well in most cases. Since one rock, however—provisionally classed as a syenite-porphyry—has a rare occurrence in New Zealand, it would be more satisfactory were the classification justified also by chemical analysis. Another type—a monchiquite—though known

previously from the river-gravels and old moraines of Central Otago, has not as yet been recorded from the West Coast district. This monchiquite was found by Mr. P. G. Morgan in the boulders of Rider Creek, a small headwaters tributary of the Blackwater River, and, although the outcrop of the dyke shedding these boulders was not located, a similar type and several camptonites were traced by Mr. Morgan to their parent dykes in the vicinity. Locality Map, Westport District. (Scale, 1 in. to 10 miles.) Camptonites have been recorded by Bell and Fraser* Bull. No. 1 (n.s.), N.Z. Geol. Surv., pp. 82–83, 1906. from, the Hokitika district, and Dr. J. Henderson† Personal communication (see also Trans. N.Z. Inst., vol. 46, 1913, p. 315). has determined these and other types of lamprophyres in gravels near Reefton; it is only during the past year, however, that any of these rocks have been discovered in situ around either Reefton or Westport.

General Geology. It is beyond the scope of this paper to deal with the general geology of the district, and a brief sketch will suffice to establish the geological relations of the rocks described. The oldest rocks exposed are argillites and greywackes, classed by McKay*“Papers and Reports relating to Minerals and Mining,” Parliamentary paper C.–13. p. 27, 1895., in the Maitai series of Carboniferous age, which are equivalent to the Greenland formation described by Morgan† Bull. No. 13 (n.s.), N.Z. Geol. Surv., and Bull No. 6 (n.s.), N.Z. Geol. Surv. in the Blackball district and in Westland. These sediments have been intruded by the immense mass of granite that has such an important development along the western flanks of the Southern Alps in Nelson and Westland. The coal-measures and succeeding Tertiary beds were deposited upon a more or less irregular erosion surface of the argillites and greywackes and upon the granite which had been laid bare by denudation. Since the deposition of these Tertiary beds, major uplift, various oscillatory movements of uplift and depression, prolonged denudation, and profound and complex fault-fracturing have all contributed towards the present configuration of the district. At some period following the deposition of the basal beds of the coalmeasures lamprophyres were injected in dykes that transect both the granite and the basal breccias of the bituminous-coal series. The upward limit of these dykes has not yet been determined with certainty, but they have not as yet been found to pass above the horizon of the basal coal breccias. It has not been ascertained to what extent the intrusion of the granite has metamorphosed the older sediments; the prevailing feature of the contact alteration immediately adjacent to the granite is the development nornfels, but widespread gneisses outcrop in some parts of the district, and these probably owe their structure to the pressure and other circumstances attendant on the granitic intrusion. Petrography. The rocks to which attention has been directed in this paper are— (1) biotite-hornblende granite; (2) granitic gneiss with abundant epidote; (3) syenite-porphyry; (4) diorite; (5) lamprophyres; (6) dolerite. Besides the above, many other more widely distributed types occur, but, as these are in most cases not of special interest, they have not been described. Literature. So far as the writer is aware, the only petrographical papers dealing with the igneous rocks of the Westport district are the following:— 1889: Hutton, F. W.—“Description of some Eruptive Rocks from the Neighbourhood of Westport, New Zealand,” Trans. Geol. Soc. of Australia, vol. 1, 1889 (?), pp. 106–10. 1889: Hutton, F. W.—“The Eruptive Rocks, of New Zealand,” Trans. Roy. Soc. N.S.W., vol. 23, 1889, pp. 102–56. 1889: Hutton, F. W.—“Note on the Geology of the Country about Lyell,” Trans. N.Z. Inst., vol. 22, 1889, pp. 387–90. 1906: Sollas, W. J., and McKay, A.—“The Rocks of Cape Colville Peninsula,” vol. 2, p. 159. (Sollas describes a section cut from the gneissoid granite of Cape Foulwind.)

Many descriptions of rocks, based solely, however, on macroscopical examination, appear in various reports dealing with the geology of the district. These cannot be considered to be of sufficient petrographical interest to warrant the inclusion of the reports in which they are embraced in the above list. The writer has been unable to gain access to a copy of Hutton's paper, “Description of some Eruptive Rocks from the Neighbourhood of Westport, New Zealand,” but it is probable that the later paper, “The Eruptive Rocks of New Zealand,” embraces all the rock-types that Hutton discovered near Westport. Amongst these, syenite, diorite, and dolerite—types analogous to those described by the present writer—have been recorded. Description of Rock-types. (1.) Biotite-hornblende Granite. A biotite-hornblende granite outcrops on the beach north of the Mokihinui River, at about 300 yards south of the Grenadier Rocks. In hand-specimen it is a typical even-grained granite, showing dull, chloritized ferro-magnesian minerals. Under the microscope the minerals observed are quartz, orthoclase, oligoclase, brownish-green biotite, deep-green to pale-yellow hornblende, subsidiary apatite, zircon, and muscovite, and secondary chlorite, epidote, pyrite, calcite, sericite, and quartz. Orthoclase and quartz are very abundant, whilst plagioclase also is common; consequently the rock has a leucocratic appearance. The biotite is almost invariably chloritized, or represented by pseudomorphs of chlorite, epidote, and sphene, and is greatly in excess of the amphibole. The structure is not typically granitic, since the feldspar and amphibole are idiomorphic, and the former mineral has preceded the latter in the order of cessation of crystallization. Pressure has caused strain-shadows to be exhibited in the quartz. (2.) Granitic Gneiss with Abundant Epidote. A granitic gneiss containing a great abundance of epidote, and presenting some unusual features, was found near the debouchure of West's, Creek, a small tributary of the Bullar River between Westport and the Nine-mile Ferry. In its present state it is a gneissic rock with irregular mosaic bands of coarse dentated crystals of quartz showing noteworthy undulose extinction. Besides this indication of partial granulation and recrystallization, another frequent and interesting characteristic of the quartz is a prominent fine parallel banding, easily distinguished in ordinary light by reason of slight differences in intensity. It is probable that a replacement of original plagioclase is thus represented, for this striation exactly simulates albite twinning, and in this event a part at least of the quartz must have been introduced during the replacement of the plagioclase. Silicification of plagioclase in schistose and gneissic rocks has been described by Mr. P. G. Morgan from the Mikonui district * Bull. No. 6 (n.s.), N.Z. Geol. Surv., pp. 88 et seq., 1908 (see particularly Nos, 8 And 16).; the writer examined several slides of the rocks in question, but found that the silicification there shown was of the ordinary micro-granular nature. In addition to quartz, the minerals present in the West's Creek gneiss are plagioclase, cryptoperthite and orthoclase, abundant biotite, epidote, and chlorite, accessory zircon and sphene, a few clusters of fine radiating

needles of tourmaline replacing feldspar, a little magnetite, and a moderate amount of sericite derived from feldspar. The plagioclase—an oligoclaseandesine variety—is in excess of the alkali-feldspar. The biotite is partially chloritized, and is probably in part secondary; its lamellae are frequently bent. The epidote occurs abundantly in more or less idiomorphic crystals, which exhibit faint pleochroism and are similar in general characters to hypersthene, but can be distinguished from this mineral by the transverse position of the axial plane compared with the elongation. Apart from the induced gneissic character the structure is typically granitic. A rounded basic segregation or inclusion, of about 1 ft. in diameter, occurring in this gneiss was examined and proved to be a diorite with abundant amphibole. A moderate amount of epidote is again present, with plentiful biotite and sphene. The constituent minerals are markedly equidimensional. On the assumption that this is a segregation in the rock now represented by the above-described gneiss it would appear that this latter rock was originally related to the diorites, and perhaps could have been classed as a monzonite, following Hatch's usage of that name.*“Text-book of Petrology,” 6th ed., London, 1910, p. 153. (3.) Syenite-porphyry. The rock here described as a syenite-porphyry outcrops in a dyke at least 20 ft. wide contained in granite in Slug Creek, a branch of the Pensini Creek, Buller Valley. Macroscopically it is a fine-grained grey granitic rock showing crystals of biotite, feldspar, hornblende, pyrite, and doubtful quartz; under the microscope, however, no quartz phenocrysts were observed. This syenite-porphyry is essentially a fine granular mass of feldspar with a little biotite and quartz enwrapping conspicuous phenocrysts of the minerals noticeable in the hand-specimen. The plagioclase—an oligoclaseandesine variety—is idiomorphic, whilst the orthoclase is commonly corroded. The former feldspar is subsidiary to the orthoclase, particularly in the groundmass. Perthitic intergrowths are occasional. Green, highly pleochroic hornblende, in long prismatic crystals, and dirty-green to yellowish-brown biotite are subequal in amount. Yellowish epidote and chlorite are common alteration-products of the amphibole. Calcite and pyrite are other secondary minerals present. Although the apparent predominance of potash-feldspar warrants the tentative classification of this rock as a syenite-porphyry, such classification cannot be regarded as final until justified by chemical analysis. It must be noted that Hutton†“The Eruptive Rocks of New Zealand,” 1889, p. 123. has described a hornblende-syenite from near the beach 16¾ miles north of Westport. The outcrop has not been located by the present Geological Survey, although a similar type of rock has been found in boulders up Stony Creek, at Waimangaroa. (4.) Diorite. Diorites and quartz-diorites are moderately common in the Westport district, where their usual mode of occurrence is in dykes that transect the general granite. A general description of them has not been attempted, but an interesting type found in boulders in the Fox River has been selected for description. Macroscopically this rock is a holocrystalline dark-green diorite showing prominent coarse amphibole crystals. Under the microscope it is seen to

consist of approximately equal proportions of andesite-labradorite on the one hand, and of hornblende and biotite on the other. The texture is very coarse, and the hornblende presents idiomorphic outlines to the feldspar and biotite. This amphibole is a deeply pleochroic bluish-green to pale yellowish-green variety, and commonly shows separation of iron-ore along its cleavage-traces. Twinning is very common. The form of alteration is to a fine fibrous mineral that resembles actiniolite, with the exception that the strong pleochroism of the unaltered amphibole is retained. Minute strings of sphene are associated with and appear to be derived from the primary amphibole. The feldspar, commonly altered to sericitic aggregates, forms coarse irregular crystals, which enwrap the ferro-magnesian and other earlier minerals. The deep-green pleochroic biotite suggests by its association with the amphibole that it is derived therefrom; it is commonly chloritized. Frequently in association with the biotite and its alteration-products are nests of secondary tremolite. Further minerals present in the diorite are sphene, plentiful coarse apatite, pyrite, iron-ore, and epidote. The sphene is in most curious coarse strings, helping to fill cracks which are partially occupied by feldspar that has been dissected by the original fractures. The origin of the titanium required for the formation of the sphene is obscure; much of it, however, may have been derived from the biotite and amphibole. The epidote forms coarse crystals with very bright yellow tints and strong pleochroism. In the central portions of most of these yellow crystals are well-defined and moderately extensive cores of a strongly pleochroic mineral showing colours varying from sepia-brown to faint grey. The optical character is negative, and the axial plane at right angles to the elongation. The birefringence is high. The writer was unable to obtain more conclusive tests, but considers that the mineral must be classed as an epidote allied to orthite. A similar association of epidote and orthite is fairly well known with granites from America and elsewhere; the primary origin of such epidote has frequently been suggested. (5.) Lamprophyres. As already indicated, no record exists of lamprophyres having previously been found in situ in the Westport district, unless, as is possibly the case, a rock from near Lyell described by Hutton as a dolerite*“Note on the Geology of the Country about Lyell,” 1889. should correctly be termed a camptonite. The lamprophyres are chiefly camptonites, but sections made from stream-boulders and from one dyke reveal the presence also of monchiquite. Camptonites.—The Westport rocks classed as camptonites agree well with the type as defined by Rosen busch.†“Microskopische Physiographie,” Band 2, 1 Hälfte, pp. 684 et seq. Sections from several dykes in the Blackwater River watershed exhibit differences one from another which at first appear important, but, to a large extent, gradations in texture and in weathering account for these. The freshest most typical camptonite section prepared represents a portion of a narrow dyke in the basal brecoias of the coal-measures near the mouth of Haggard Creek, a tributary of Blackwater River, Buller Valley. Sections were cut from other portions of the dyke, and show a passage from the typical camptonite to a monchiquite, in which, in place of the feldspar of the camptonite, there is a moderate amount of colourless isotropic base crowded with crystallites and enclosing minute prisms of augite and amphibole.

In hand-specimen the camptonites are fine-grained blackish rocks showing few, if any, determinable crystals. Under the microscope the most noteworthy features are the precise idiomorphism of the constituent minerals, the great abundance of augite (which forms nearly 50 per cent, of the rock), and the lack of importance of feldspar. The essential minerals are pseudomorphs of iddingsite after olivine, violet titaniferous pleochroic augite, brown hornblende, magnetite, and plagioclase. The only porphyritic mineral is augite, which, with olivine and occasional iron-ore, forms the phenocrysts of the porphyritic types; even in the groundmass, where its stumpy prisms exclude almost all other minerals, it is sharply idiomorphic. In the porphyritic types the pyroxene is usually in “glomeroporphyritic” phenocrysts, the individual crystals of which show good “hour-glass” structure. Olivine is represented solely by pseudomorphs of yellowish-green iddingsite; these are characteristically idiomorphic, moderately coarse, and very plentiful. In one instance a little serpentine showing “mesh” structure is associated with the iddingsite of a pseudomorph. In the rock from one dyke olivine is absent, unless it is represented by rounded pseudomorphs of carbonate, which are particularly abundant, and in some cases possess an outline characteristic of olivine. The amphibole is a deep-brown, rather poorly pleochroic variety, probably barkevicite; it forms characteristic slender cigar-shaped prisms, which, with a moderate amount of fine magnetite, crowd the clear feldspathic base between the numerous stumpy augite crystals of the groundmass. The feldspar—andesine-labradorite—is in long irregular laths usually of small size, or in small crystals embracing all the other minerals and crowded with numerous colourless acicular crystallites. Occasionally it forms most curious arborescent growths. Secondary carbonates are usually abundant. In some sections—namely, those from the dyke in Haggard Creek—the iron-ore is in coarse well-shaped crystals typical of magnetite; zonal intergrowth with ilmenite has taken place, and is represented by a more or less regular internal band of leucoxene parallel to the periphery of the crystal. In the clear portions of the groundmass microscopic pinnate growths of iron-ore are often present. In a section made from one of the Blackwater River dykes several clear glassy rounded patches of isotropic mineral were observed; these probably represent a feldspathoid mineral, but the rock was too decomposed to permit of determinative tests being applied. Monchiquites.—Boulders of typical monchiquite were found by Mr. P. G. Morgan in Rider Creek, a branch of Haggard Creek; and, in addition, as already mentioned, sections from the camptonite dyke in Haggard Creek have a glassy base, and can well be classed as monchiquite. There is a general similarity in mineral constitution between the monchiquite and the porphyritic camptonite already described. Olivine, much of which is fresh and the rest pilitized (talc and tremolite), is in large remarkably idiomorphic phenocrysts, which are often surrounded celyphitically by augite. This latter mineral differs from that in the camptionite in that brown hornblende very frequently forms a parallel terminal outgrowth to its prisms, but otherwise shows the same features. In the groundmass it is in segregations, along with a little iron-ore. These are enwrapped by a clear base containing plentiful laths of feldspar and crowded with crystallites, some of which appear to be green hornblende. Staining tests reveal the presence of a few minute needles of feldspar; and indicate that a moderate amount of gelatinizing isotropic material is also present.

(6.) Dolerite. A dolerite*“Note on the Geology of the Country about Lyell,” 1889. has been described from near Lyell by Hutton. The writer has found no reference other than this to dolerites in the Westport district. Hutton's type is totally different from that herein noted, and, as has already been mentioned, the Lyell rock is possibly a camptonite. The dolerite now described is found in boulders deposited, along with much greywacke debris, by small streams crossing the Karamea–Mokihinui Road not far from the ford of the left branch of the Six-mile Creek. In hand-specimen it is a dark-green basic rock with fine laths of feldspar prominently displayed on partly weathered surfaces. Seen under the microscope, the dolerite is even-grained, holocrystalline, and fairly coarse, and shows most interesting and perfect ophitic structure. The main constituent is andesine in typically long laths, between and around which is abundant augite, its boundaries thus sharply defined by the dominating laths. Ironore, chiefly ilmenite, is in noteworthy amount; it precedes the augite and follows the feldspar. Some of it is secondary. A few zircon prisms, pyrite, secondary calcite, chlorite, and uralite, with a little biotate, were also observed. The augite is widely altered, but frequent cores and crystals show that it is a pale-pink titaniferous variety. Uralitization, the first stage of the alteration, is effected in two steps—first to a fibrous brownish-yellow, faintly pleochroic, and strongly birefringent amphibole, and then to a bluish-green pleochroic product. The final stage is the change of the uralite to chlorite, with a few minute flakes of biotite. In a few cases there is direct chloritization of the augite, with accompanying separation of a little iron-ore. A little of the biotite in the rock seems to be a product, along with iron-ore, of magmatic resorption of the augite. Relations of the Rocks. The granites have already been briefly dealt with in connection with the general geology of the district; from these the gneissose granites cannot be separated. The diorites, syenite-porphyry, and probably the dolerite are directly connected with the granite intrusion, and probably not long posterior to it, for they were subjected to the same erosion as the granites before the deposition of the coal-beds. The lamprophyres must probably be correlated with the similar rocks described by Bell and Fraser†Bull. No. 1 (n.s.), N.Z. Geol. Surv., pp. 82–84, 1906. from the Hokitika district, and furnish reliable data for the determination of the age of the series, since in the Blackwater Valley they intrude the basal beds of the bituminous-coal measures. Descriptions of similar rocks from Mount Tapuaenuka district, Marlborough, and notices of some from the Reefton gravels (per Dr. J. Henderson), have appeared lately in a paper by Dr. J. A. Thomson)‡Trans. N.Z. Inst., vol. 45, pp. 308–15, 1913. “On the Igneous Intrusions of Mount Tapuaenuka, Marlborough.” Acknowledgments. In conclusion, the writer wishes to acknowledge gratefully the kindly help that he has received from Dr. J. A. Thomson, particularly in the determination of some of the minerals, and also to thank Mr. P. G. Morgan, Director of the Geological Survey, for his courtesy in allowing this paper to be presented.