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Art. XL.—Note on a Dyke at Nugget Point.
[Read before the Philosophical Institute of, Canterbury, 4th November, 1903.]
This dyke is mentioned by Captain Hutton in the “Geology, of Otago,” pages 41 and 42, and also by the Geological Survey in several reports on the district. My attention was drawn to it during a visit to Nugget Point in January, 1903. On examining hand-specimens subsequently one or two features of the rock attracted my notice. It belongs to a division of the syenite group not common in New Zealand—viz., feldspar porphyry. This is evident from the inspection of hand-specimens. The porphyritic feldspars constitute the bulk of the rock, the size of the crystals ranging up to 5 mm. in length. They show the characteristic shapes due to pinacoidal cleavages, and from the absence of twin striation they are most probably to be classed as orthoclase. This is confirmed by flame tests. No other porphyritic mineral could be observed in a macroscopic examination, but a number of inclusions of foreign material occur. These I took at first for altered feldspars, but I believe they have been caught up
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in the rock as it passed through underlying beds. They are usually small, but I observed one a centimetre in length.
The specific gravity of the rock is 2.65. This is rather low for a syenite, but the small amount of basic mineral present and the large predominance of orthoclase feldspar accounts for it.
A partial chemical analysis gave the silica-percentage as 58. This is also low for a syenite. This maybe explained by the small amount of free silica and the large amount of altered feldspar present in spite of the comparatively small amount of ferro-magnesian mineral that occurs.
A microscopic examination shows that the rock is principally composed of idiomorphic feldspars in a crystalline groundmass. The feldspars are much clouded by alteration, and occasionally occur as binary twins; others show undulose extinction and zonal structure. They are mainly orthoclase, but plagioclase is also present. In sections from the middle of the dyke quartz is rare, but in sections from the edge it was observed in small individual crystals, and also replacing feldspar as a mosaic of small grains. This is perhaps due to pressure as well as to the alteration of the feldspar.
The ferro-magnesian mineral which, no doubt, occurred originally in the rock has been replaced by chloritic material which shows at times a doubtful pleochroism. The small percentage of ferro-magnesian mineral is characteristic of feldspar porphyries. However, on digesting the powdered rock with hydrochloric acid for twenty-four hours the solution gave, with ammonia, a decided precipitate of ferric hydrate. This must have come partly from the alteration products and from small crystals of iron-ore forming an original constituent of the rock. The groundmass is at times hard to determine, owing to alteration, but it appears to consist of small feldspars, which are so completely clouded as to be opaque between crossed nicols. In the finer portions of the rock the groundmass is clearly holocrystalline. A larger quantity of free silica is also present here. The porphyritic crystals, too, do not exhibit a distinct crystalline outline, but have the form of allotriomorphic grains.
The inclusions in the rock, which I took at first for altered feldspars, are most probably fragments of a trachytic or andesitic lava. They consist of a semi-crystalline groundmass, and in one I noticed small porphyritic crystals of plagioclase which had undergone a certain amount of alteration, but not sufficient to render their determination at all doubtful. No ferro-magnesian minerals occur in them, but I should say they are most probably fragments of andesitic lava. It is possible that they have been produced from feldspars by metasomatic processes, but I think it hardly likely. As vol-
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canic rocks of a greater age are known to occur in other parts of Southland at no great distance, it is not unlikely that they underlie the rocks of the Hokonui system at Nugget Point, and the presence of derived fragments in a dyke penetrating them would not be at all improbable.
The date of the intrusion of the dyke can be fixed with tolerable certainty. It is found in Triassic rocks, so that it cannot be earlier than that period. Further, I found near it a derived sedimentary rock which, contained a small Monotis. If the rock was really formed from the dyke it fixes the date for the close of the Triassic period, or not later than early Jurassic. There is some difficulty, however, in determining accurately the origin of the derived sandstone, as the sandstones in the immediate neighbourhood are highly feldspathic, but contain a large proportion of plagioclase. However, I did not notice this in the derived volcanic sedimentary rock. A granite intrusion occurs in the neighbourhood at Omaru Creek (see paper by Captain Hutton, read before the Royal Society of New South Wales; 1889). This is on the same line of country, and from the description of the rock there are marked affinities to the one at Nugget Point. They may therefore be of the same age.