Ultrabasic And Associated Intrusive Rocks.
As previously shewn, doleritic rocks with serpentines and associated ultrabasic types invade the Upper Cretaceous Onerahi strata. Bartrum (1924, pp. 150-152) has recognised and described from the adjoining Riverhead-Kaukapakapa district most of the types discovered in the present area, so that little detail is here necessary.
Serpentines.—These are, the prevailing type of rock, and have been derived from peridotites ranging from harzburgite to dunite. No unserpentinized remnants of the original minerals exist in any of the rocks encountered, though at Parakakau and elsewhere just north of the present district it is otherwise.
Harzburgite-serpentine is the prevailing variety (Fig. 23), and by reduction in the quantity of bastite present grades into dunite-
serpentine. Picotite is by no means an infrequent accessory constituent, though grains of chromite are rare; the former is especially, plentiful in a coarse bastite-rich serpentine from Matthews' quarry, 3 miles south of Silverdale. Many of the serpentines have veinlets of secondary chrysotile and sometimes of carbonates.
A high degree of variation in facies may be displayed in the one relatively small intrusive mass, as has already been shewn by Bartrum (loc. cit.) for that at Parakakau. In the vicinity of Wainui Cemetery in Upper Orewa Valley, for example, whilst serpentinized dunite is the prevailing type, one specimen collected contains about 10 per cent, of partially-altered large crystals of diallage and enstatite set in the usual felted mass of serpentine, shewing that the rock of the intrusion is locally an olivine-rich lherzolite. Other specimens from this intrusion include veinlets composed of finely-fibrous serpentine in very coarse-bladed crystals, and a rock described by Bartrum (loc. cit.) as consisting of chlorite with abundant sphene and epidote. The chlorite has the lattice-structure usually characteristic of antigorite, but is not this latter mineral, for it is optically positive and has negative elongation. Masses of chrysotile in bladed aggregates similar to those at Wainui Cemetery occur also in the bastite-serpentine of Matthews' quarry, whilst, from this same type of rock at Major Jolly's quarry 3½ miles west of Silverdale, two small nodules were collected, one of coarse-grained bastite, with a little serpentine pseudomorphous after olivine, and the other of a fibrous chlorite in fairly large blade-like aggregates which appears to be clinochlore.
An interesting rock was unearthed in “serpentine” in the valley of Kaitoke Creek east of Plat Top Mountain, 8 miles northwest of Silverdale, in the area already described by Bartrum (1924). Some unusual alteration has affected the chief constituent, of the rock, causing it to be so reduced in translucency that its polarization-tints are almost obscured in most parts of the slice examined. This constituent appears to be olivine in long prisms arranged in coarse radial aggregates, though the application of decisive optical tests proved impracticable. When not in radial prisms the mineral shews its normal alteration to serpentine with the usual mesh-structure, whilst a felt of small tufts of serpentine invades the radiate material and appears to be its alteration-product. Bastite is present in moderate quantity amidst such serpentine, though not in intimate association with the radial olivine.
An unusual rock was given the authors by Mr. H. T. Ferrar of the New Zealand Geological Survey, and is said to outcrop outside the present area near the junction of Kaukapakapa-Silverdale and Kaukapakapa-Parakakau roads. This is a pyroxenite composed dominantly of enstatite in crystals about 4 mm. to 5 mm. in average diameter, and in common with other constituents almost unaffected by alteration. Besides the enstatite there is a minutely lamellar fibrous pyroxene which does not lend itself to exact microscopic determination, but appears to be diallage rather than an intergrowth of orthorhombic and monoclinic varieties. Slightly serpentinized subsidiary olivine is entangled in the interstices between the larger crystals; it is rarely enclosed in the latter. Narrow strings of serpentine penetrate the pyroxene, whilst a little secondary pyrite is also present.
The effect of pressure is seen in the granulation of the margins of some of the crystals and bending of the lamellae of the monoclinic pyroxene.
Anorthosite.—The writers are again indebted to Mr. H. T. Ferrar for a chip of an anorthosite which was collected from, boulders at Davidson's farm, a few miles west of Silverdale, and consists of predominant anorthite enwrapped ophitically by a small quantity of interstitial diallage, but contains also large crystals of what appears to be enstatite deeply oxidized and stained by haematite.
Dolerites and Associated Rocks.—Bartrum (1920a, p. 420; 1924, p. 152) has already described a dolerite from the swinging basin at Silverdale and an epidiorite and a dolerite from Upper Orewa. Additional occurrences include boulders of epidiorite, similar to that from Upper Orewa, at Davidson's farm west of Silverdale, and of dolerites and other somewhat less basic intrusive rocks, along with lavas, at Orewa Heads.
One coarse holocryatalline dolerite is very different in facies from any other rock of the area. It is not ophitic, but contains very abundant idiomorphic pyroxene and brown barkevicitic amphibole, which are enclosed by coarse plates of plagioclase, along with a moderate quantity of very coarse ilmenite, often in typical skeleton crystals, a little more or less chloritized intensely absorptive brown biotite, and very numerous long needles of apatite. Chlorite is moderately plentiful. The pyroxene is a strongly titaniferous variety, and occurs in stout crystals usually about 1.5 mm. in length, but as much as 6 mm. The barkevicite is sometimes enclosed by the augite, but more often enwraps this latter, occasionally being outgrown upon it. Its pleochroism varies from brownish-yellow to deep reddish-brown, and its extinction angle is 17°. The variety of plagioclase was not determined with certainty, though it is at least as basic as acid labradorite. It is considerably weathered, and in places is replaced by calcite and an isotropic mineral which is identified as analcite. The latter was at first suspected to be opal, but was not dissolved upon treatment with caustic potash, whilst it gelatinized with hydrochloric and nitric acids, yielding cubes of salt only with the former. It generally occurs as small irregular patches in the feldspar, but larger masses as much as 3 mm. in diameter were also observed.
Others of the Orewa boulders furnish, a considerably crushed coarsely crystalline quartz-augite porphyrite, or possibly dolerite, for the variety of plagioclase was not precisely determinable, and in uncrushed remnants the long laths of this latter mineral are ophitically related to pseudomorphs after augite. Quartz is not infrequent in fairly large crystals, whilst there are a few unaltered flakes of brown biotite. Generally, however, this last mineral has been converted to scales of chlorite, which forms some conspicuous irregular foliae. Apatite is in moderate quantity in very long needles, whilst ilmenite is common usually in string-like aggregates. The augite is an almost colourless variety, and now is largely represented by calcite with a little chlorite. It is somewhat subordinate in amount to the plagioclase, which is not less basic than medium labradorite.
A further rock represented in the Orewa Heads boulders appears to be a feldspathic porphyrite, with not more than 15 per cent, of partially-chloritized almost colourless augite, plentiful coarse ilmenite and numerous tiny prisms of apatite. Secondary calcite is very abundunt, usually in fairly large nests. The plagioclase is weathered and could not be determined with exactness. It seems to be a variety approximating acid andesine, and is in large unoriented laths about 0.7 mm. in length.
Several andesitic lavas were recognized in the Orewa débris. They are coarsely amygdaloldal, some of the amygdules exceeding 6 mm. in diameter. Advanced weathering again militates against their precise determination.
One type is an aphyric mass of irregularly-disposed laths of plagioclase (? basic andesine), generally less than 0.1 mm. in length, sub-ophitically entangling about 10 per cent, to 15 per cent, of pale greyish-green augite and enclosing about 7 per cent, of iron-ore (largely magnetite). The plagioclase is generally replaced internally by an isotropic material resembling opal, and by a little calcite.
Another amygdaloidal lava has a very open-textured lattice of laths of acid labradorite about 0.2 mm. in length, with abundant chlorite between the laths, and a considerable number of small pseudomorphs of haematite, chlorite and sometimes calcite, apparently after olivine. The laths of plagioclase have been considerably bent by pressure. In this, as in the other lava, the amygdules are filled by calcite crystallized upon an early thin lining of chlorite.
Inclusions in the Serpentine of Fisherman Creek.—In an outcrop of serpentine exposed in a small quarry at the head of Fisherman Creek, on the east side of the main south road, about 1½ miles south of Silverdale, there are several interesting inclusions of greywacke, schist, dolerite, and probably other types of igneous rocks.
The only igneous rock sectioned is a moderately fine-textured dolerite (Fig. 21) with somewhat lath-shaped crystals of plagioclase (basic labradorite) ophitically enwrapped by about 40 per cent. of pale-grey augite, which occasionally shows incipient uralitization. Iron-ore (ilmenite) is in small amount.
The schist (Fig. 22) is composed essentially of narrow bands rich alternately in plagioclase and diopside, along with foliae consisting of streaked-out porphyroblasts of brown hornblende. The plagioclase is andesine-labradorite, and usually shows albite twin lamellae; it forms a mosaic with grains of diopside and is considerably in excess of this latter. The grain-size is variable laterally, but in coarser portions is about 0.04 mm. in diameter. There are some serpentine and carbonate pseudomorphs after olivine, along with ramifying strings of serpentine, which probably represent injected peridotitic material. In addition, a single porphyroblast of almost colourless diopside was noted; it appears as if it is intergrown with tmy flakes of brown hornblende, but the true relationships between the two minerals could not be determined. Pyrite is plentiful in association with the amphibole; this latter is only sparsely present in the general mosaic of the schist. Sieve-structure, which is so general in similar metamorphic rocks, is absent.
Albany Conglomerates.—The petrography of the Albany conglomerates has already been described by Bartrum (1920; 1924); no fresh types have been added during the present investigation.
Basalt from Whangaparaoa.—As noted in an earlier section, a basalt was found as a large mass involved in acute dislocations of Tertiary (Waitemata) strata at Whangaparaoa. It is possibly a considerably altered representative of a basalt, said to be from. Silver-dale, described by Sollas and McKay (1906, p. 158).
The basalt (Fig. 24) is characterized by a moderate number of rather small phepocrysts of faintly pleochroic titaniferous augite, which are enclosed by a highly-feldspathic matrix of basic plagioclase (labradorite-bytownite), pinkish titaniferous augite and a moderate quantity of magnetite. The augite is generally in idiomorphic crystals shewing hour-glass structure. Calcite and chlorite and abundant secondary products. The olivine is usually represented by pseudomorphs of pleochroic green to almost colourless fibres or cleaved plates of tddingsite, with a little calcite.
Igneous Rocks of the Parnell Grit.—As has previously been noted by Fox (l902) and Mulgan (1902), the fragments of volcanic rock which constitutes a large portion of the Parnell Grit are nearly all andesites, though trachyte has been described by Bartrum (1917).
The andesites are typically porphyritic with large phenocrysts of plagioclase and often of augite or hypersthene set in a groundmass of laths of plagioclase, granules of pyroxene and grains of magnetite. A little olivine has been noted in a type described by Bartrum (1917).
Pleistocene Basalts.—The basalts from Lake Pupuke (Fig. 26) are holocrystallme rocks with numerous phenocrysts of olivine set in a groundmass consisting of plagioclase, titaniferous augite and about 4 per cent. of ilmenite, the plagioclase (medium labradorite) being in decided excess of the other constituents. The grain-size is a little variable, though relatively coarse, for the laths of feldspar generally average about 0.2 mm. in length. Fluxional arrangement of these latter is not infrequent. The olivine is usually in small sub-idiomorphic crystals, though in occasional flows crystals as much as 3 mm. and more in diameter ate prominent.
On the west side of the lake the lava sometimes has drusy nests of coarsely holocrystalline material (Fig. 25), perhaps owing its coarseness of grain to accumulated volatile constituents. Sharp, elongated though small prisms of pyroxene, and occasionally olivine, project into the tiny druses. The pyroxene is a titaniferous variety with the usual hour-glass structure, and is subordinate to the plagioclase. It is sometimes bordered by aegirine-augite, of which there are also occasional isolated crystals. Olivine is not plentiful, but ilmenite is prominent in characteristic skeleton crystals along with long needles of apatite. Radial aragonite is sometimes found us a filling of vesicles of the associated lavas.
A slide of vesicular material from the southern of the two Shoal Bay craters shows a dense, largely glassy matrix crowded with rod-like crystallites, laths and prisms of plagioclase and pyroxene in small number and a moderate quantity of iron-ore (? magnetite). Irregularly-shaped phenocrysts of olivine of varied size are common,
whilst titaniferous augite is fairly plentiful, usually in small prismatic phenocrysts, though sometimes in large irregular crystals. The olivine has frequently been resorbed at the margins, yielding place to granular pyroxene, which occasionally forma definite outgrowths upon the enclosed mineral. One large crystal of augite about ½ in. in diameter was picked out of the debris of the cone, and on sectioning showed similar reaction-zones developed from enclosed divine.
In quarries on the west margin of Lake Pupuke nodules of olivine up to 1½ in. in diameter are plentiful amidst the lapilli. An analysis of the mineral is published below by kind permission of Dr. J. Henderson, Director of N.Z. Geological Survey.
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(Analysis of olivine from nodules in Takapuna Borough quarry near the junction of Tauroto and Northcote roads, west margin of Pupuke Lake, Takapuna. Analyst, F. T. Seelye.)
Bartrum, J. A., 1917. Additional Facts concerning the Distribution of Igneous Rocks in New Zealand, Trans. N.Z. Inst., vol. 49, pp. 418-424.
—— 1920. The Conglomerate at Albany, Lucas Creek, Waitemata Harbour, Ibid., vol. 52, pp. 422-430.
—— 1920a. Additional Facts concerning the Distribution of Igneous Rocks in New Zealand: No. 2. Ibid., vol. 52, pp. 416-422.
—— 1921. Notes on the Geology of Great Barrier Island, Ibid., Vol. 53, pp. 115-127.
Bartrum, J. A., 1921a. A Conglomerate at Onerahi, near Whangarei, Auckland, New Zealand, Ibid., vol. 53, pp. 128-130.
—— 1922. Physiographic Notes on Auckland, New Zealand Nature Notes, Austr. Ass. Adv. Sc., Wellington, 1923.
—— 1924. The Geology of the Riverhead-Kaukapakapa District, Waitemata County, Auckland, Trans. N.Z. Inst., vol. 55, pp. 139-153.
—— 1924a. The Igneous Rocks of a (?) Tertiary Conglomerate near Mangapai, Whangarei Harbour, New Zealand, proc. Austr. Ass. Adv. Sc., vol. 16, Wellington, 1923, p. 345 (abstract).
—— 1924b. An Account of the Stratigraphy of the Riverhead-Kaukapakapa District, Waitemata County, Auckland, New Zealand, Ibid., p. 346 (abstract).
Bartrum, J. A., and Turner, F. J., 1928. Pillow-Lavas, Peridotites and Associated Rocks of Northernmost New Zealand, Trans. N.Z. Inst., vol. 59, pp. 98-138.
Bell, J. M. and Clarke, E. de C. 1909. The Geology of the Whangaroa Subdivision, N.Z. Geol. Surv. Bull. No. 8 (n.s.).
—— 1910. A Geological Reconnaissance of Northernmost New Zealand, Trans. N.Z. Inst., vol. 42, pp. 613-624.
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—— 1923. Palaeozoic and Mesozoic Seas in Australasia, Trans. N.Z. Inst., vol. 54, pp. 1-62.
—— 1924. The Structural Features of the Margin, of Australasia, Ibid., vol. 55, pp. 99-137.
—— 1926. The Tectonic Conditions accompanying the Intrusion of Basic and Ultrabasic Rocks, Mem. Nat. Acad. Sc., vol. 19, Mem. 1.
Cotton, C. A., 1916. The Structure and Later Geological History of New Zealand, Geol. Mag., dec. 6, vol. 3, pp. 243-249, 314-320.
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—— 1882. North Auckland District, Rep. Geol. Explor. during 1881, pp. 17-41.
Ferrar, H. T., 1920. Whangarei Subdivision, 14th Ann. Rep. (n.s.) N.Z. Geol. Surv., pp. 4-5.
—— 1924. Rodney Subdivision, 18th Ann. Rep. (n.s.) N.Z. Geol. Surv., pp. 6-7.
—— 1925. The Geology of the Whangarei-Bay of Islands Subdivision, N.Z. Geol, Surv. Bull. No. 27 (n.s.).
—— 1925a. Rodney Subdivision, 19th Ann. Rep. (n.s.) N.Z. Geol. Surv., PP. 6-8.
Fox, C. E., 1902. The Volcanic Beds of the Waitemata Series, Trans. N.Z. Inst., vol. 34, pp. 452-493.
Grange, L. I., 1927. The Geology Of the Tongaporutu-Ohura Subdivision, N.Z. Geol. Surv. Bull. No. 31 (n.s.).
Hector, J. 1884. Progress Report, Rep. Geol. Explor. during 1883-84.
—— 1886. Progress Report, Rep. Geol. Explor. during 1885.
—— 1888. Progress Report, Rep. Geol. Explor. during 1887-88.
Henderson, J., 1914. Coal Possibilities or the Warkworth District, 8th Ann. Rep. (n.s.) N.Z. Geol. Surv., pp. 167-158.
—— 1924. The Post-Tertiary History of New Zealand, Trans. N.Z. Inst., vol. 55, pp. 580-599.
—— 1924a. The Structure of the Taupo-Rotorua Region, N.Z. Journ. Sc. and Techn., vol. 6, pp. 270-274.
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McKay, A., 1884a. On the Relations or the Tertiary and Cretaceo-Tortiary Strata on the Coast-line between Auckland and Mahurangi, Ibid., pp. 101-106.
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—— 1884c. On the Geology of the Kawhia District, Ibid., pp. 140-148.
—— 1887. On the Geology of East Auckland and the Northern District of Hawkes Bay, Rep. Geol. Explor. during 1886-87, pp. 182-219.
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—— 1916. The Younger Limestones of New Zealand, Ibid., vol. 48, pp. 87-99.
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—— 1917. The Geology of the Central Kaipara, Ibid., vol. 49, 433-450.
—— 1924. The “Hydraulic Limestones” of North Auckland, Ibid., vol. 55, pp. 617-618.
—— 1926. The Upper Cretaceous Ammonites of New Zealand, ibid., vol. 56, pp. 129-210.
Mulgan, E. K., 1902. The Volcanic Grits and Ash Beds la the Waitemanta Series, Ibid., vol. 84, pp. 414-435.
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