Art. XV.—A Conglomerate at Onerahi, near Whangarei, Auckland, New Zealand.
[Read before the New Zealand Science Congress, Palmerston North, 27th January, 1921: received by Editor, 21st February, 1921; issued -separately, 27th June, 1921.]
There are certain conglomerates intercalated in Tertiary and earlier strata in various parts of the North Island of New Zealand, amongst the constituent pebbles and boulders of many of which there is material showing evidence of having been subjected to far more intense pressures than have any of the rocks constituting the basement of that Island, with the exception of some at Whangaroa (Bell and Clarke, 1909, p. 44). At this place schistosity is locally developed upon rocks described by Bell and Clarke as altered igneous types, but the cause is ascribed by them to shearing-stresses along a fault-zone.
In a recent paper the writer summarized the recorded occurrences in the North Island of conglomerates containing granitic or dioritic material showing acute pressure-effects, and described a variety of rocks from an occurrence near Albany, in the vicinity of Auckland (Bartrum, 1920). In a second paper he described granitic and granulitic boulders from a conglomerate in the basement rocks of Great Barrier Island.*
The present note, published here by permission of the Director of the New Zealand Geological Survey, describes interesting pressure-affected and other rock-types from a conglomerate at Onerahi, near Whangarei, which furnish additional evidence of the widespread nature of this pre-Mesozoic land. It is believed that it was not wholly destroyed until after mid-Tertiary times, for the boulders of the Tertiary conglomerates seem too free from decomposition to be merely a rewash of Mesozoic conglomerates. These latter, in any case, are scarce in relation to the wide extent of basement rocks now uncovered for examination.
In a paper read before the Melbourne meeting of the Australasian Association for the Advancement of Science in January, 1921, the present writer adduced arguments in favour of the correlation of the rocks of the pre-Mesozoic land-mass of the North Island with those of the Aorere system, so well developed in the South Island.
General and Petrographic Description of Onerahi Conglomerate.
In December, 1919, in company with Mr. H. T. Ferrar, of the New Zealand Geological Survey, the writer discovered a conglomerate intercalated in greensands exposed between tide-marks a short distance east of the wharf at Onerahi, near Whangarei. The series of beds of which it is part is probably Tertiary in age, and is considerably disturbed by folding. The beds below the conglomerate are not visible, but above it come about 20 ft. of greensands, and then a gradual passage to an argillaceous limestone
Fig. 1.—Granulated (?) quartzite from conglomerate at Onerahi. Crossed nicols. × 25.
Fig. 2—Perthitic feldspar from gneissic granite from conglomerate at Onerahi. Crossed nicols. × 25.
Fig. 3—Plagioclase phenocrysts in granophyre from conglomerate at Onerahi. Crossed nicols. × 25.
Fig. 4—Granophyric matrix of rock illustrated by fig. 3 above. Crossed nicols × 133.
Fig. 5.—Spheruhtic structures in matrix of acidic (?) tuff from Onerahi conglomerate. Crossed mcols. × 133.
Fig. 6.—Quaitz phenocryst from acidic (?) tuff from conglomerate at Onerahi. It contains rounded fragments of finely granophyric hornblende-feldspar rock. Crossed nicols. × 133.
similar to the very extensive limestone of the North Auckland area usually spoken of as the “hydraulic limestone.” The question of its age is still unsettled.
The band of conglomerate is lensoid, ranging up to about 5 ft. in depth, and is exposed standing steeply on edge for about 15 yards. Its pebbles and boulders are well rounded, and, though mainly small, vary upwards in size to 3 in. or more in diameter. The material is chiefly greywacke and shale, with other types of sedimentary rocks, but interesting igneous rocks are also frequent. One other type was also found: it is a quartz-rich granulite, which probably represents a granulated and partially recrystallized quartzite. The rocks of igneous origin include :—
(?) Tufaceous acidic rock.
Granulated (?) Quartzite. (Plate XXVIII, fig. 1.)
Macroscopically this rock resembles hornfels. In section it is seen to be almost wholly quartz showing highly prominent shadow-extinction. with a little untwinned feldspar and perthite. Granulation has been intense, and the minerals form a mass of somewhat interlocking, coarse, partially recrystallized grains with granulated borders.
There are occasional grains of epidote, whilst minute shreds of muscovite are present, mainly in the granulation-products.
Gneissic Granite. (Plate XXVIII, fig. 2.)
This is a very curious granitic rock showing gneissic structure distinctly in hand-specimen. Quartz makes up about half the bulk of the rock, the rest consisting of a very little acid plagioclase and of the coarse perthite so prominently displayed by the photomicrograph.
The only other minerals are rare sphene, a few tiny flakes of muscovite, and some irregular patches representing alteration-products, probably of biotite.
The perthite is on a very coarse scale, and at first glance resembles graphically intergrown quartz and orthoclase; occasionally the crystals may actually be such intergrowths, but in the majority of cases there is very minute albite-lamination crossing the coarse perthitic striping of the mineral.
The gneissic structure visible macroscopically appears in section as zones of shearing.
These are conspicuously porphyritic rocks, somewhat dark in colour, showing feldspar and occasionally quartz phenocrysts in hand-specimen.
Nearly all the sections show a fairly finely holocrystalline granular groundmass, in which are abundant large acid plagioclase phenocrysts. Occasionally there are plentiful orthoclase and quartz crystals. Biotite is present in greatly inferior amount; it may be fresh or represented by alteration pseudomorphs. The groundmass typically consists of quartz and orthoclase in subequal amounts, but where quartz occurs as a
phenocryst the groundmass shows correspondingly less of that mineral. It is sometimes so coarse that the rocks then deserve to be classed as granite-porphyry.
In one section there are a few spongy crystals of brown hornblende, which enclose several quartz-grains; in the same section there is a xenolite which is apparently a fine weathered granodiorite.
Granophyre. (Plate XXVIII, fig. 3.)
This rock has occasional fairly coarse idiomorphic phenocrysts of plagioclase in a groundmass which contains a moderate amount of plagioclase in laths enwrapped pseudo-poecilitically by quartz crystals which sometimes are large. A very great portion of the matrix, however, consists of micro-pegmatite, as is well exemplified by fig. 4, Plate XXVIII. There are a few minute flakes of greenish-brown biotite.
The occurrence of granophyres in New Zealand is rather limited. Sollas and McKay (1906, vol. 2, p. 182) describe them from a conglomerate outcropping on the east shore of Palliser Bay, near Wellington. A rock collected by Smith (1908) from river-gravels in Westland, and described by him as granite-porphyry, seems equally to merit the name “spherulitic granophyre.”
(?) Tufaceous Acidic Rock.
A difficult rock to classify. It is decidedly fragmental in general character, but appears to have the fragments enclosed in a matrix which is unlike that of a tuff. It is a finely granular mixture of feldspar, green hornblende, and probably quartz, with frequent spherulitic and micropegmatitic intergrowths (see photomicrograph, Plate XXVIII, fig. 5). In it there are large broken crystals of plagioclase, a few of orthoclase, many coarse ones of quartz, and some biotites which are generally chloritized. There are also very small fragments of finely micropegmatitic material, of very fine-grained andesites (some with a little green hornblende), and of an exceedingly fine-grained rock made up of green hornblende and feldspar along with a little micropegmatite and probably quartz
An interesting phenomenon is the enclosure of micropegmatite and of some of this last-mentioned rock by quartz crystals. In some instances (see Plate XXVIII, fig. 6) there is a very definite band-like margin to the inclusions.
These inclusions, and the nature of the matrix, furnish grounds for suspecting that the rock is not a tuff, but an intrusive in which fragments of intruded rocks have been entangled. Marginal resorption could readily explain the rounded forms of the rock-fragments.
This is a compact fine-grained rock having an andesitic matrix in which are enclosed small particles of very fine-grained trachytic rock, either trachyte or trachyandesite.
Bartrum, J. A., 1920. The Conglomerate at Albany, Lucas Creek, Waitemata Harbour, Trans. N.Z. Inst., vol. 52, pp. 422–30.
Bell, J. M., and Clarke, E. de C., 1909. The Geology of the Whangaroa Subdivision, N.Z. Geol. Surv. Bull. No. 8 (n.s.).
Smith, J. P., 1908. Some Alkaline and Nepheline Rocks from Westland, Trans. N.Z. Inst., vol. 40, pp. 122–37.
Sollas, W. J., and McKay, A., 1906. The Rocks of Cape Colville Peninsula.