Art. XXXII. — Additional Facts concerning the Distribution of Igneous Rocks in New Zealand.
[Read before the Auckland Institute, ‘13th December, 1916; received by Editors, 30th December, 1916; issued separately, 30th November, 1917.]
The following notes record a few facts which have come under the notice of the writer, and which may be of use to other workers in New Zealand geology. Some of the facts doubtless are known to other New Zealand, geologists, but the author believes that the majority have not yet been published. If they have, he pleads the lack of time for full perusal of literature, and the inaccessibility of some of the papers bearing on New Zealand petrography, as his excuse for including them in this paper.
In some cases considerable work, particularly with regard to the chemical characters of the rocks described, is required before the knowledge of them can be regarded as at all satisfactory, and the writer hopes to be able to continue this work later.
Hypersthene Basalt, Ruatangata, Near Whangarei
In slides made from a flow basalt of Pleistocene or Recent age, which, macroscopically, is similar exactly to the general ophitic basalts of the Whangarei district, the mineral hypersthene has been detected in interesting circumstances.
It is believed that this is the first olivine-hypersthene basalt described from New Zealand, though it must be remarked that some of the hypersthene and other andesites approach closely to basalts. Professor Thomas has pointed out, for example, that some of the andesites of the Tarawera district approximate basalts, some carrying scanty olivine.* The majority, however, are very evidently andesites. Professor Thomas notes also, in connection with lavas from Tongariro, that “Most of them are free from olivine; but others are rich in this mineral, and must be termed basalts.”†
In the Auckland University College collection is a light-grey rock, with well-marked fluxion banding, showing numerous phenocrysts (up to 5 mm. in diameter) of olivine, with smaller ones of hypersthene, a few fragments of xenolitic quartz, and somewhat inconspicuous feldspar. It is labelled “Otouku, near Tongariro,” and evidently represents one of Professor Thomas's basalts. Small partings between plates parallel with the fluxion bands show very minute crystals of some zeolite, apparently cubic, though in the absence of an exact goniometer this could not be established.
In section, besides the coarse olivine phenocrysts there are numerous smaller ones showing a corrosion border of iron-ore. Many highly corroded coarse plagioclase phenocrysts occur, and ldiomorphic hypersthene is abundant. Augite frequently forms a fringe to the hypersthene, and there are one or two small phenocrysts apparently of augite. The groundmass is pilotaxitic, and the feldspar mesh is crowded by hypersthene prisms and a very little iron-ore.
[Footnote] * A P W Thomas, Report on the Eruption of Tarawera and Rotomahana, N Z, Wellington, Government Printer, 1888, p 58.
[Footnote] † Loc cit, p. 21.
The abundance of olivine and hypersthene appear to justify the classification of the rock as a basalt, though only chemical analysis can establish this.
The Ruatangata rock is similar to the more feldspathic of the Auckland basalts, but besides phenocrysts of olivine, augite, and plentiful coarse labradorite there is also subsidiary hypersthene. The groundmass is pilotaxitic, and plagioclase predominates slightly over pyroxene. Iron-ore is unimportant. There are occasional xenolites of andesitic nature.
In several instances the augite has formed a parallel growth about a central crystal of hypersthene; the latter mineral apparently became unstable during the last stage of consolidation, although sharply idio-morphic unattacked hypersthene is also common. The photomicrograph (Plate XXVIII, fig. 2) illustrates this feature satisfactorily, the hypersthene being extinguished in the centre of the field.
Abundance of feldspar is more marked than in most Auckland basalts, whilst olivine is less noteworthy; but field relations and microscopic characters alike show that the rock is one of the series of the latest extrusives, which, in the Auckland and Whangarei districts, are universally basalts.
Troctolite, Wade, Auckland.
In hand-specimens this is a blotched dark-green and white coarse gabbroid rock, showing serpentine, some evident fresh plagioclase with albite striations, and much altered feldspathic material.
In section serpentine shows up as almost 50 per cent, of the rock. It presents the usual characters of serpentine derived from olivine, and encloses here and there large kernels of unaltered olivine. A little pale bladed amphibole (tremolite) is associated with the alteration products both of the olivine and of the plagioclase. This latter mineral where unaltered has extinction angles on albite twin lamellae of from 30° up to 44° (approximately), and is therefore labradorite. Its alteration presents difficulties. Often the result of the alteration is a greyish product, with very high refractive index and very low polarization tints, which is apparently a saussurite consisting mainly of some mineral of the epidote group. Again the feldspar is largely converted first into strings and then into larger patches of a densely fibrous transparent mineral with approximately the same index of refraction and the same birefringence as the feldspar itself. This is probably a zeolite.
Granodiorite with Primary Epidote, Reefton.
The field relations of this rock are unknown. It is represented by a specimen collected by Professor A. P. W. Thomas (apparently from river-gravels), and now in the collections of the Auckland University College.
Macroscopically it is a moderately coarse light-coloured granitic rock, with abundant biotite (occasionally aggregated in small basic secretions), and showing other pale yellowish-green crystals along with brownish rather resinous ones. The microscope shows that these latter are epidote and sphene respectively.
Microscopically the rock is readily classed as a biotite-rich granodiorite. The feldspars include microcline, orthoclase, microcline perthite, perthite, and plagioclase which is probably acid andesine, though this latter determination is not exact. The lime-soda feldspar predominates slightly over the alkali variety.
Muscovite is present in small amount with the biotite, and epidote is very plentiful and moderately coarse. Sphene is abundant in coarse characteristic wedge-shaped crystals. Magnetite occurs sparingly in coarse crystals, and apatite and zircon are accessories.
The chief interest lies in the epidote. This is a pleochroic pale-yellow to colourless variety, with the usual optical characters. It is very frequently enclosed as sharply euhedral crystals in feldspar; sometimes nests of sharply bordered rounded grains are enclosed pseudo-poecilitically by the feldspar. Very commonly it is enwrapped or enclosed, sometimes as idiomorphic crystals, by the brown biotite. This last mineral is thus important in its relations to the epidote; there can be little doubt that it is primary: all indications point to this conclusion.
Representation of the relations of epidote to hornblende, plagioclase, and biotite in a granodiorite from Reefton. Crossed nicols. × 60 approx. Qu = quartz; Pl = plagioclase; Bi = biotite; Ep = epidote; Ap = apatite; Ho = hornblende; Mi = microcline.
An important fact bearing on the question of the primary character of the epidote is that, in no less than seven cases in the two sections cut of the rock under consideration, the mineral surrounds a central crystal of brown hornblende; only one or two hornblende crystals were observed that were not in association with the epidote. In all cases the amphibole is in small crystals.
At first thought the above facts would suggest that the epidote is an alteration product of the amphibole; but against this view it is to be observed that the idiomorphic epidote enclosed in feldspar or biotite does not present the outlines of hornblende crystals. Possibly some of the epidote has been derived thus, but it appears more probable that the epidote, in great part
Fig. 1—Photomicrograph of gneıssıc dio
te, Albany, near Auckland. The bent town lamellae of the plagioclase and the g
lation are well shown. Crossed nicols. × 22.
Fig. 2—Photomicrograph of hypersthene basalt, Ruatangata, showing a central crystal of hypersthene (oxtinguished in the centre of the field) surrounded by a parallel growth of augite. Crossed nicols. × 22.
at all events, crystallized out during the solidification of the magma, probably largely at the expense of unstable hornblende. It is shown by the figure, which represents one of several similar examples, that, on occasion at all events, the epidote is one of the earlier products of crystallization. A twin crystal of it here surrounds a central twin crystal of idiomorphic hornblende; it encloses a few flakes of biotite, and is itself enwrapped by biotite and plagioclase. A very narrow partial border to the hornblende (exaggerated in the figure) is possibly also epidote.
It seems impossible to escape from the conclusion that the relations of mineral to mineral establish the primary character of at least some of the epidote. Such epidote is not unknown, but it does not seem commonly to have been described.*
Basalt with Biotite (Var. ? Anomite), South of Tokatoka Swamp, Wairoa River.
This is the stone often used in Auckland for monumental work, and called locally the “Kaipara granite.” The writer has examined this and several other of the Kaipara extrusive and intrusive rocks and has found them mainly normal basalts, but there are also andesites such as that at Tokatoka, near Dargaville, which carries a deep-green hornblende.
The “Kaipara granite” is a very coarse-grained basalt with abundant coarse olivine, enclosed with plentiful pale-green augite and coarse ilmenite in a coarsely pilotaxitic holocrystalline feldspathic groundmass.
The point of interest about the rock is that it contains a highly pleochroic (rich reddish-brown to pale canary-yellow) biotite in abundant flakes which are often sharply euhedral. In one or two instances these enclose ophitically the feldspar of the groundmass.
The optic axial angle in the few favourable sections obtained varies from zero to a few degrees, the biaxial character being distinct in only one instance. The dispersion for red is greater than that for the blue rays. The pleochroism is Y = Z = rich reddish brown, × = pale canary-yellow or golden.
This mineral is similar to a biotite in basanites near Dunedin which Professor P. Marshall first showed me in 1907, and which he considered anomite. The characters of the biotite in the Kaipara rock, so far as they are determinable, agree with those given by Rosenbusch for anomite.†
Hornblende Basalt, Sumner-Lyttelton Road, Near Sumner.
Marshall has mentioned the occurrence of hornblende basalts at Dunedin,∥ but, apart from these, few hornblende basalts seem to have been noted from New Zealand.
[Footnote] * See, for example, H Rosenbusch, Mikroskopische Physiographie, 4th ed, i, 2, 1905, p 284; and E Weinschenk (trans by R W Clark), Petrographic Methods, 1912, pp 264–65.
[Footnote] † H Rosenbusch, loc cit, pp 259–60.
[Footnote] ‡ W J Sollas and A Mckay, Rocks of Cape Colville Peninsula, Wellington Government Printer, vol 2, 1906, p 176.
[Footnote] § A R Andrew, On the Geology of the Clarendon Phosphate-deposits, Otago, New Zealand, Trans N Z Inst, vol 38, 1906, p 459
[Footnote] ∥ P Marshall, The Geology of Dunedin (New Zealand), Quart Journ Geol Soc vol 62, 1906, p 412.
The Sumner rock is known to other geologists, but has not been reported in literature so far as the writer is aware. It contains abundant iron-stained idiomorphic olivine crystals, which are frequently enclosed in the coarser pale-green augite. The hornblende is quite fresh and unresorbed; it is greenish-brown, and lacks the deep absorption tints of the usual basaltic hornblende.
Andesites in Mid-Mesozoic Rocks at Port Waikato.
At a point on the south bank of the Waikato River about a mile and a half up-stream from Port Waikato a well-marked conglomerate with rounded pebbles up to 5 in. in diameter outcrops amongst finer plant-bearing mid-Mesozoic sediments. The pebbles include many sedimentary rocks, such as quartzite, shale, and greywacke, and several varieties of andesite. Similar pebbles are frequent in irregular bands along the “strike” coast south of the mouth of the river.
There are several highly feldspathic types of andesite, one with perfect trachytic groundmass and practically no fer [ unclear: ] o-magnesian mineral, though magnetite is fairly abundant in specks in the groundmass. There are also augite andesites and one hornblende type rich in fresh greenish-brown horn-blende and with very coarse plagioclase.
Andesitic, rhyolitic, and other pebbles are found in the basement sediments of Coromandel Peninsula,* and in the “Maitai” shales on the east shore of Palliser Bay, Wellington,† and thus in rocks, so far as we know, roughly coeval with those at Port Waikato.
Another well-known occurrence of pebble-beds of igneous rocks in sediments belonging to the same. Trias-Jura succession is in the hills near Nelson; the included pebbles there are largely derived from plutonic rocks.∥
Gneissic Diorites, Albany, Near Auckland.
An interesting occurrence of gneissic diorites or dioritic gneisses was made known recently to the writer by some specimens collected at Albany by Mr. G B. Battersby from boulders up to several feet in diameter which were unearthed by farming operations. As they are located a moderate distance from the water's edge it is very unlikely that they have been brought thither by vessels trading up Lucas Creek, and they more probably come from a boulder-bed, belonging to the Miocene Waitemata series, known
[Footnote] * W J Sollas and A Mckay, Rocks of Cape Colville Peninsula, vol 1, 1905, p 52: C. Fraser and J Adams, The Geology of the Coromandel Subdivision, N Z Geol Surv Bull, No 4, 1907, pp 45 and 52.
[Footnote] † W. J Sollas and A. Mokay, Rocks of Cape Colville Peninsula, vol. 2, 1906, pp. 178 et seq.
[Footnote] ‡ P. Marshall, Distribution of the Igneous Rocks of New Zealand, Rep Austral. Assoc. Adv. Sci., vol. 11, 1907, p. 6 of reprint
[Footnote] § Cf J. Park, Geology of New Zealand, Wellington, 1910, pp 82–83.
[Footnote] ∥ P Marshall, Boulders in a Trıassıc Conglomerate, Nelson, Trans N Z Inst., vol. 36, 1904, pp. 467–71. Since the above was written Professor Marshall has informed me that a thick conglomerate with similar pebbles of even greater variety outcrops in the Mesozoic rocks at Kawhia.
to outcrop in that creek, which has not yet been investigated by the writer.
One specimen is a quartz diorite with abundant greenish-brown horn-blende. The plagioclase shows bent twinning lamellae, and its margins have been markedly granulated, as the photomicrograph (Plate XXVIII, fig 1) shows. The margins of the hornblende are less perfectly granulated, but the fact that granulation is present indicates the probable primary character of the amphibole. Typical fine-grained dioritic gneisses are represented by a rock consisting of ragged green or bluish-green horn-blende and plagioclase with much granulated material, and possessing a banded structure.
A most interesting discovery was made later when small pebbles of several diorites were found in the alluvium laid bare by wave-attack in a small flood-plain on the west shore of Shoal Bay, Auckland Harbour. They were greatly weathered, but their character was readily decipherable. One diorite shows abundant greenish-yellow hornblende which has been largely chloritized, and very prominent and typical twinned ilmenite. The presumption is that these pebbles, like the Albany boulders, come from the Tertiary beds.
In June, 1916, Professor Marshall showed the writer a dioritic rock obtained from the Tertiary sequence near Komiti Point, Kaipara Harbour, so that diorites seem to have formed an important element in the pre-Tertiary terrain of the Auckland district.
It is interesting to note that similar gneissic rocks (hornblende gneiss, &c.) are present in a mid-Tertiary conglomerate in the gorge of the Waipaoa River, Poverty Bay.*
Gabbroitic and Dioritic Rocks, Baton and Graham Rivers, Nelson.
There are several specimens of coarse basic rocks collected some years ago by Professor A. P. W. Thomas from the gravels of the Baton and Graham Rivers, Nelson, in the collections of Auckland University College, and a note upon them may be of interest, though full descriptions can serve no purpose.
They are essentially very coarse-grained hornblende rocks, with abundant coarse iron-ore (mainly ilmenite), usually secondary epidote, quartz in varying amounts, coarse apatite, a few flakes of biotite, and finally sphene. Pale pyroxene is generally present in large amount, and perhaps gives the clue to the origin of much of the hornblende.
There are several types of rock represented. In the majority, deeply pleochroic brown and often strongly schillerized hornblende is the predominant mineral, feldspar being comparatively unimportant. The amphibole usually encloses pale augite poecilitically, and the sharp outlines of this latter often show that much of the brown hornblende is not, as was suspected, uralite. There is, however, undoubted pale-green uralite present in most sections, which sometimes shows distinctly a transition or alteration into the deep-brown hornblende.
In two sections with very coarse, sharply idiomorphic hornblende a pleochroic deep-green to brownish-yellow hornblende borders the deep-brown variety, and is occasionally intergrown with plagioclase. In one of these
[Footnote] * W. J Sollas and A. McKAy, Rocks of Cape Colville Peninsula, vol. 2, 1906, pp. 175 et seq.
sections—a very distinct type, perhaps outside the series—there are abundant idiomorphic crystals of the green hornblende, and a large proportion of cryptoperthite, whilst quartz is abundant.
It is evident from the foregoing description that it is difficult to classify these rocks: there is an element of doubt about the origin of much or most of the hornblende; but, from several considerations, it appears that they may provisionally be classed as quartz-hornblende gabbros.
Finally, the sections cut represent normal dioritic rocks, unlike the former in that the plagioclase is now predominant and the structure is hypidiomorphic. The hornblende in unaltered rocks is a deep-bluish-green to golden-brown variety. Quartz and augite are absent. Sections of another diorite show that it has been subjected to pressure which has caused partial granulation. The hornblende has been a strongly schillerized deep-brown variety, but is now converted in great measure to an opaque whitish-grey product
Trachyte, Miocene Breccia-Conglomerate, Wairau Creek, Milford, Auckland.
Mr. C. E. Fox, in his admirable paper on the volcanic beds of the Waitemata series, Auckland, noted in the coarser fragmental beds that “besides the more basic fragments there others present the appearance of true trachytes, pale-grey in colour, with a specific gravity of 2.54”*. From his statement in connection with the fragments, “On the whole, then, they may be taken as typical andesites, while a small percentage are basalts without olivine,”† one judges, perhaps wrongly, that he regards these trachytic rocks as a phase of the andesites
There is an interesting assortment of andesites, some with a little olivine,‡ in the Wairau Creek volcanic breccia-conglomerates at Takapuna, but the trachyte that has been found there by the writer shows but little mineralogical resemblance to any of these. It was discovered in several boulders up to 10 in. in diameter, all most perfectly rounded, and contrasting sharply with the general subangular and rough material of the bed in which they were found
Microscopically the trachyte consists of idiomorphic phenocrysts of clear orthoclase, slightly subordinate plagioclase, and a little pale augite, in a dense finely trachytic groundmass showing perfect fluxional arrangement. The feldspar laths enclose between them very fine granules of augite and magnetite. The proportion of plagioclase laths to those of alkali feldspar is somewhat higher than in typical trachytes, a feature to be noticed in most of our New Zealand trachytes
Trachytes are rare in the North Island of New Zealand the only occurrence in situ being at Pukēkaroro, near Maungaturoto, and the presence of the rock in the Wairau Creek volcanic fragmental bed is therefore interesting.
[Footnote] * C. E. Fox, The Volcanic Beds of the Waitemata Series, Trans N Z Inst, vol 34, 1902, p. 426
[Footnote] † C E. Fox, loc cit
[Footnote] ‡ See also E K Mulgan, On the Volcanic Grits and Ash-beds in the Waitemata Series, Trans. N Z Inst., vol 34, 1902, pp. 420, 423, 424, &c.