Granite Enclosures in a Quartz-biotite-diorite at Green Islets, Southland.
[Read before the Otago Institute, 11th November, 1924; received by Editor, 19th December, 1924, issued separately, 31st March, 1926.]
In February, 1923, I visited Preservation Inlet and Chalky Sound. [On the passage from the Bluff the boat, a small steam trawler, called at the Green Islets, a group of rocks which lie close to the mainland about twelve miles south-east of Preservation Inlet. At one place they enclose a small cove with a precarious boat-landing. Here the shore of the mainland is fringed between low-water and high-water marks by an uneven shelving water-worn platform of diorite.
A hundred yards to the east the diorite rises into a narrow ridge, the seaward end of which is detached from the mainland, forming a great natural arch, pierced by an archway 70 ft. high and 75 ft. wide at the floor-level. To the west the diorite is overlain by a coarse dioritic breccia, which here is the lowermost member of the Tertiary coal-bearing series.
A few yards to the right of the boat-landing the diorite encloses two masses of grey granite. The smaller is roughly circular in form and 4 ft. in diameter, while the larger, lying 4 yards to the east, is elliptical, and measures 10 ft. and 20 ft. in least and greatest diameters. These two occur on the crown of a flat hummock of rock, and are entirely surrounded by the diorite. In the larger mass the granite in parts grades into aplite.
Gneisses, crystalline schists, and limestones, followed by altered grey-wackes and slaty argillites that contain Cambrian and Ordovician grapto-lites, at one time occupied the whole of what is now the fiordland of Otago and Southland. In the late Palaeozoic these rocks were intruded by a batholith of diorite, which differentiates into granite in the south and into norite or gabbro and even serpentine around its northern periphery. This dioritic complex constitutes the Clinton River Intrusive Series described by me in 1921. *
As a consequence of subsequent uplift followed by subaerial denudation, the crystalline rocks have become broken up into many detached blocks, for the most part deeply involved among the intrusives. The largest remnant of the Cambrian and Ordovician crystalline rocks occurs between Preservation Inlet and Dusky Sound. The areas occupied by these rocks in the Princess Mountains and at Caswell and Doubtful Sounds are relatively small. All the great fiords have been carved in rocks belonging to the Clinton River dioritic intrusives, and not in the pre-Cambrian gneisses
[Footnote] * J. Park, Geology and Mineral Resources of Western Southland, N.Z. Geol. Surv. Bull. 23 (n.s.), pp. 42–48; Wellington, 1921.
and crystalline schists as supposed by Hector and Hutton. The plutonic intrusion is believed to have taken place in the late Palaeozoic, and the general uplift towards the close of the Mesozoic.
Green Islets Diorite.
The diorite is coarsely granular and typically hypidiomorphic. The dominant constituents of which it is composed are feldspars, quartz, and biotite; there is also some hornblende present. Petrologically the rock may be called quartz-biotite-diorite.
Well-twinned plagioclases are plentiful, and most of them are partially altered to sericite. Some large phenocrysts of anorthoclase with zonal structure are also present. Enclosed in the feldspars occur a few columns of zoisite.
Quartz is very abundant, filling the interstices. In the quartz occur minute microlites that may possibly be diopside. Some apatite and zircons are also present. There is much titanite between the biotite and hornblende, apparently secondary.
The biotite occurs in rounded piles and in six-sided forms. With the biotite is often associated a common green hornblende with vivid pleo-chroism—blue-green, brownish-olive, and greenish-brown.
The examples of granite examined in thin slice and submitted for analysis were collected from the larger of the two masses. The structure may be described as hypidiomorphic-granular. The rock is not fresh. The quartz and feldspars exhibit undulating extinction originating from intense pressure, but notwithstanding this the typical granite structure is perfectly preserved.
The constituent feldspars are orthoclase, often with traces of microcline structure, but seldom exhibiting the typical microcline cross-banding, and albite-oligoclase with basic oligoclase occurring as inclusions. There are also present some plagioclases with normal zoning and broad margins of albite. Albite as patches and in long bands occurs in the orthoclase. Sericite has developed in the feldspars most often in the middle of the plagioclases.
Biotite occurs as aggregates of small plates with irregular outlines. The margins are in places altered to chlorite. Rutile, titanite, and iron-ore are also present but not abundant. The mode of occurrence of the biotite shows that it developed from hornblende or some other mafic mineral.
The quartz is abundant, and, like the feldspars, shows strain effects arising from pressure. The rock is a biotite-granite.
The aplite possesses an evenly granular texture and an allotriomorphic structure. The dominant feldspar is albite. The orthoclase shows prominent microclinic cross-banding. There is also present a little feldspar with a micropegmatitic structure, and grains which show an intergrowth of plagioclase and vermicular quartz apparently replacing potash feldspar. Muscovite is plentiful. Both the quartz and feldspars show the strain effects usually associated with pressure. The rock is a phase of the granite, and may be called a granite-aplite.
As a consequence of the concentration of the basic feldspars and mafic minerals, large bodies of granite are known to grade into quartz-diorite
or even into norite and gabbro, but the occurrence of enclosures of granite in diorite is not so common. The transition from a quartz-biotite-diorite to a biotite-granite does not, after all, involve a violent amount of magmatic differentiation.
The dominance of the alcalic feldspars is what might be looked for in igneous rocks of a quartzose character. All three show a pyrogenetic relationship arising from progressive differentiation. The granite is clearly a phase of the diorite, and the aplite of the granite.
Through the courtesy of Mr. P. G. Morgan, M.A., F.G.S., Director of the Geological Survey, I obtained the following very complete analyses of the diorite, granite, and aplite, made in the Dominion Laboratory by Mr. F. T. Seelye, B.Sc. The molecular proportions are also given.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Potash||K2O||4.69||0.050||5.03||0 053||4.02||0 043|
|Water lost above 105° C.||H2O||1.07||0.46||0.23|
|Water lost below 105° C.||H2O||0.39||0.39||0.21|
|Titanium dioxide||T1O2||0.80||0 010||0.23||0.003||0.03|
The carbon dioxide in the diorite is not present as calcite, as it is liberated only on heating with dilute hydrochloric acid.
Although in the course of the analysis no nickel was detected in the aplite, the joint-planes of the original specimen were coated with green stains which were found to contain nickel. Such surfaces were not included in the sample taken for analysis.