The Rocks of Paritutu.
The rocks outcropping on and around Paritutu are all rather light grey, strongly porphyritic, and with large, prominent, tabular phenocrysts of plagioclase and acicular crystals of hornblende.
In thin section, the porphyritic nature of the rock is emphasised because the feldspar and amphibole crystals are set in a hyalopilitic groundmass of microlites of feldspar and augite with a small residuum of colourless or rarely pale brown glass. In some cases (P. 9336) the phenocrysts increase in number to such an extent that the groundmass is more correctly termed mesośtasis. Crystalline material usually predominates over glass, though in rare cases (P. 6658) glass is predominant.
Plagioclase occurs in idiomorphic tabulae up to 5.0 mm. in length and these are usually flattened parallel to 010. In the groundmass they occur as minute microlites or at the most narrow laths. Zoning is most pronounced and twinning on albite, pericline, and combined Carlsbad-albite laws is usual. The composition of the plagioclase does not vary notably from rock to rock, but owing to zoning the composition in individual crystals varies from An34-An50; usually the composition averages about An45. A rhythmic type of zoning is often developed in the plagioclase phenocrysts, but particulars of this have been discussed earlier in this paper. Inclusions of glass and portions of groundmass are common, and these may be of irregular shape or definitely globular; the arrangement is either haphazard or strictly zonal, but more commonly the latter, although in most cases the marginal zone or periphery of the phenocrystic plagioclase is entirely free of any inclusions. Although inclusions of hornblende, apatite, and glass occur in most of the andesine phenocrysts of a particular rock, some phenocrysts in the same rock may be completely devoid of inclusions; again in some rocks all of the phenocrystic plagioclase is entirely free of inclusions. It is difficult to understand the manner by which some plagioclase crystals in crystallizing during the intratelluric period incorporated some of the magmatic liquid while others did not.
Amphibole, the most important mafic constituent in these rocks, usually occurs in prismatic crystals up to 4.0 mm. in length. It is a strongly pleochroic type of hornblende with colour varying from greenish-brown through yellowish-brown to a warmer brown tint; the latter, however, nowhere approaches the colour of lamprobolite (Rogers, 1940, p. 828). Slight zoning is evident in most cases, and is usually shown by the development of a green peripheral zone surrounding a brown or greenish-brown central portion [ unclear: ] (Text-Fig. 4B); in some cases curious greenish-brown blotches occur in brown crystals. A poor hour-glass structure was noted in some cases. Twinning is uncommon, but if developed, is parallel to the orthopinacoid. Resorption has occurred to a different degree in different specimens, varying from merely a narrow border zone surrounding the hornblende to examples where there has been complete reaction with the magma involving removal of the amphibole. The products of resorption appear to be plagioclase, diopside, and magnetite; as the resorption becomes important, the amount of clinopyroxene in the rock increases considerably.
Usually the products of resorption are closely associated with one another or with the partially resorbed amphibole, but this relationship is not quite so clear in those rocks where the resorption of amphibole is complete or nearly so. In all rocks where resorption is important the groundmass has a minimum amount of glass, but in one glassy type (P. 6658) resorption of the amphibole, or of biotite which is present in this case, has not occurred (Text-Fig. 4B). Larsen and Irving's work (1937) on somewhat similar rocks from the San Juan region, Colorado, has clearly shown comparable features in the resorption of hornblende, and they point out that the absence of resorption in rocks with a glassy groundmass clearly indicates that much of the resorption took place after the lavas had been erupted. The evidence of the Paritutu rocks certainly seems to support this contention. Clinopyroxene is not an abundant constituent and occurs chiefly in close association with resorbed hornblende although to a minor extent dispersed throughout the rock as minute granules or prismatic crystals. Its occurrence as phenocrysts up to 2.0 mm. in length, is rare, and only in a glassy type (P. 6658) are they at all well developed (Text-Fig. 4B). In all cases, the pyroxene is a diopsidic type with very faint green tint and in some examples extremely delicate pleochroism. Slight zoning was noticed in a few cases and lamellar twinning is uncommon. Only rarely was any alteration noticeable, but in one case (P. 9341) a pale green negative chlorite occurring in the form of minute spherules appeared to have developed from the pyroxene. Biotite is an unusual constituent and was observed in only one rock, a glassy type from the cliffs on the north side of Paritutu, where it constitutes less than 5% of the rock. Here the mica occurs in plates of slightly rounded outline up to 2.0 mm. in diameter. Resorption has never proceeded beyond the development of a narrow peripheral zone of fine iron oxide dust and a colourless prismatic or granular mineral, probably pyroxene (Text-Fig. 4B). Resorption is not so extensive as that seen in the San Juan lavas described by Larsen and others (1937, p. 900).
Pleochroism is intense, varying from very pale straw yellow (X) to a very deep rusty-brown (Z).
Cristobalite was recognised in several of these rocks—e.g., P. 9334–9336, both in the groundmass, where it occurs as minute grains, and in what appear to have been steam or fluid cayities; in the latter case the grains rarely exceed 0.6 mm. in diameter. The grains are colourless and irregular and lack the twinning so characteristic of cristobalite. The birefringence is very faint and refractive index determinations exclude the possibility of the mineral being tridymite or opal. Although cristobalite usually crystallises in the vesicles of lavas, it does not seem to be so common as a constituent of the groundmass or mesostasis, but recently, however, Hurlbut (1936) has proved by X-ray methods the existence of cristobalite in the groundmass and as intergrowths with the feldspar of spherulites in some rhyolitic rocks.
The accessory minerals in this group of rocks are apatite, magnetite, and zircon. The apatite generally forms stout idiomorphic hexagonal prisms; not uncommonly the apatite crystals have a central smoky coloured zone, just sensibly pleochroic in some cases. Zircon is rare and was noted in only one case (P. 9341). Iron-ore occurs in rather ragged irregular grains, less commonly in sub-rectangular crystals that rarely exceed 1.5 mm. in diameter. Finely granular iron-ore occurs throughout the groundmass and magnetite dust is important locally in association with clinopyroxene surrounding resorbed amphibole. Microchemical tests on the magnetite dust have shown a rather low percentage of titanium, possibly not in excess of 1 ½–2%.
On account of the amount of visible cristobalite present in a number of these rocks, it seems more appropriate to classify them as dacites. In one specimen (P. 9341), however, with no visible quartz or cristobalite in thin section the norm calculation shows 14.75% quartz. Therefore all of the rocks of the Paritutu group are considered dacites.
At the north-west extremity of Ngataierua Point several specimens were collected that differ somewhat from the Paritutu group, although, of course, very distinctly related thereto. In the hand specimen they are similar to the Paritutu group but contain rather more dark minerals. Microscopically they are also similar but differ in the following points:—(1) Phenocrysts of plagioclase are not so abundant. (2) Hornblende is far less resorbed than in the previous group, the amphibole phenocrysts being surrounded by only narrow zones of finely granular magnetite and a very minor augite. (3) Augite is rare throughout the rocks.
The Hybrid Rocks of Mataora Island (P. 9339, 9342).
These rocks are darker in colour than the Paritutu types and not so coarsely porphyritic.
The base is made up of microlites of feldspar, much fine granular augite, magnetite, cristobalite, and a residuum of almost colourless glass.
The plagioclase occurs in coarse idiomorphic phenocrysts, commonly twinned on several laws, and zoned. The phenocrysts show a curious type of alteration in which most of the centres of the crystals
(A) Basified dacite (P. 9339) from the east side of Mataora Island showing zonally altered bytownite phenocrysts, heavily resorbed hornblende, and clear areas of cristobalite. X 26.
(B) Glassy dacite (P. 6658) from the seaward side of Paritutu. Phenocrysts of andesine are clear and almost unaltered; hornblende, slightly zoned, and biotite, show only faint resorption effects. X 26.
are replaced by a microcrystalline aggregate of pale yellow material with low birefringence and irregular granules of what are believed to be oxidised siderite (Text-Fig. 4A). In many cases, very narrow peripheral zones of the plagioclase crystals are water-clear and devoid of any inclusions (Text-Fig. 4A), and an intermediate clouded zone is seen in some crystals; others are completely free from any inclusions at all. It is considered that much of the pale yellow, poorly birefringent material is glass, perhaps in some cases somewhat devitrified. The cloudy intermediate zone, on the other hand, consists of feldspar, magnetite, shreds of glass and diopsidic pyroxene; other granules are present but the identity of these is not certain. In the main, zoning in the plagioclase is not so pronounced as in the feldspar of previous groups, although the oscillatory types of zoning were also observed here. A few crystals have been noted with corroded margins but lacking zoning.
The unusual feature of these rocks is the composition of the plagioclase, which is in no case less calcic than An73, and examples with up to 80% of the anorthite molecule, particularly in the core of the phenocrysts, have been recorded.
Both hornblende and pyroxene occur as large phenocrysts. The amphibole has more pronounced brown colour in these rocks with a pleochroism that follows the scheme:—
|α = yellow.|
|β = brown.|
|γ = deep olive brown.|
Resorption has occurred extensively, producing broad peripheral zones of finely granular magnetite, and augite (Text-Fig. 4A). In
some instances where resorption has proceeded nearly to completion, a type of sieve-structure is developed in addition to alteration around the peripheral zone; this consists of the skeletal remains of amphibole phenocrysts sieved with rarely twinned, new formed plagioclase, diopsidic pyroxene, and magnetite. Complete resorption of the amphibole has occurred in some instances. Phenocrystic clinopyroxene, in crystals up to 2.0 mm. in length, is a common constituent. Except for its abundance and phenocrystic character, the diopsidic augite is similar to that in the previous groups of rocks. Accessory constituents are similar to those in the dacites—viz., apatite, cristobalite, and iron ores; some celadonic material and ferriferous calcite is also present.
On the west side of Mataora Island an extremely altered type (P. 9340) has been observed. This is a crushed feldspathic rock with the plagioclase averaging about An75. Amphibole and pyroxene are absent, but very abundant carbonate with the γ refractive index varying from 1.658–1.667 is present. Microchemical tests indicate considerable iron in the ferrous state but no manganese or magnesium. From Winchell's table (1933, p. 70) it seems that this carbonate must contain up to approximately 7–10% FeCO3 in solid solution. The refractive index varies considerably and there is marked zonary banding in the ferriferous calcite; this is particularly noticeable when oxidation of the carbonate takes place, the resultant limonite being precipitated in a zonary fashion. A zeolite tentatively referred to as fully hydrated calcium chabazite is an important minor cónstituent of these hydrothermally altered hybrid rocks. It is intimately associated with carbonate, and the idiomorphic form of the mineral suggests a late date for the period of crystallization.
Scraps of intensely pleochroic biotite are scattered throughout the rock: these are occasionally much altered to limonite but show no sign of resorption or chloritization. Rare patches of micaceous quartzo-feldspathic schist with intensely pleochroic biotite were also observed; these patches are interpreted as xenoliths of pelitic sediment that have been caught up and recrystallized by hot andesitic magma in the deeper zones of the magma chamber where temperatures have been higher than appear to have existed in the porphyrite.
Cristobalite, coarse crystals of apatite (up to 0.3 mm. in diameter) and iron-ore are accessory constituents.