Art. XXVII.—Note on the Gabbro of the Dun Mountain.
[Read before the Philosophical Institute of Canterbury, 11th December, 1907.]
Captain Hutton first called attention to this rock,* and correctly described some of its peculiarities. Its coarse structure and its simple composition (for it contains only two minerals) were both noted. The specimen was given to him by Sir J. von Haast, and the field relations of the rocks were unknown.
The two minerals were called by Hutton anthophyllite and saussurite, which he supposed to be derived from anorthite or labradorite. In a later paper †† he again classed the rock as saussurite-gabbro, but classed the ferro-magnesian mineral as diallage, enstatite, and hornblende in different portions.
A geological report of the district by E. H. Davis ** does not refer to gabbro rocks specifically, though it is probably in-
[Footnote] * Trans. N.Z. Inst., vol. xix, p. 412.
[Footnote] † Journ. Roy. Soc. N.S.W., vol. xxiii, p. 154.
[Footnote] ** Geological Reports, 1870–71, p. 103.
cluded in the confused mass of feldstone, bronzite, anthophyllite, &c. referred to on page 118 of that report.
Hochstetter* refers to dykes of diallage rock in the dunite of this region.
A visit to the district in December, 1906, enabled me to collect specimens, though in the many localities in which the rock was found I was unable to do anything in the way of determining its field relations, for I found it nowhere in situ, though blocks were abundant on the north-east slopes of the Dun Mountain and in the valleys of the Maitai Stream and Roding River, especially where they issue from the magnesian country. Microscopic examination showed that the so-called saussurite was entirely isotropic, as previously mentioned by Hutton. In all my specimens the ferro-magnesian mineral was diallage.
In the fresh specimens the white mineral was absolutely clear and colourles in section, but in weathered specimens it was somewhat cloudy. A specimen was obtained sbsolutely pure for analysis, and it gave the following result:—
1. Grossularite-gabbro, Dun Mountain, Nelson, New Zealand.
2. Grossularite, River Iser, Peru.
From a comparison with the adjacent analysis of typical grossularite it will be seen that the chemical composition of the mineral shows clearly enough that it should be referred to grossularite. The percentage of magnesia is high, though when the nature of the adjoining dunite magma is considered such a percentage is to be expected.
The specific gravity of the mineral confirms this result. From a specimen of absolute purity the following result was obtained: G. 3.502. The average specific gravity for the examples quoted (“Dana's System of Mineralogy”) is rather over 3.5. The refractive index has not yet been measured, though from the aspect of the surface in section the value appears below that usual for grossularite.
[Footnote] * “New Zealand,” p. 475.
The saussurite-gabbro therefore becomes a grossularitegabbro. I can find no reference to a similar rock, though in Rosenbusch's “Physiographie der Mossigen Gesteine,” 1906, 3rd edition, p. 338, it is stated that garnet occurs as an accessory constituent of gabbro, though it is not said whether grossularite is the type referred to.
The only field relation that was determined for this gabbro was its proximity to the Maitai limestone which fringes the peridotite intrusion on the north-west. This fact causes the author to offer the suggestion that the peculiar rock type has resulted from the digestion of some of the limestone at the periphery of the magma. Captain Hutton has already described a pyroxenite from the district. It contains a large amount of bastite, often in large plates, but otherwise consists entirely of diallage. In addition, peculiar white rock masses project from the surface of the hill in various places. These project from the surface of the hill in various places. These have been called felsite and felstone by Davis, but they are probably the material which was afterwards found by Skey to be wollastonite—a conslusion with which the author agrees.
The presence of masses of wollastonite appears to offer confirmation of the suggestion offered that digestion of masses of the Maitai limestone has taken place.