MINOR ELEMENT DISTRIBUTION IN SOME PYROXENES OF METAMORPHIC ORIGIN

Abstract

Thirty-six samples of pyroxene and four samples of amphiboles were extracted from a suite of hybrid igneous rocks, skarn rocks and pyroxenites collected from various localities in the Grenville rocks of Western Quebec. Two samples of pyroxene and one of amphibole were obtained from a skarn in Monmouth township, Ontario, making a total of forty- three specimens. Spectrographic analyses were made for B, Be, C∙a, Li, Ti, Ni, Mo, Co, Cu, Cr, V, Zr, Mn, Sc, Y, Sn, Sr, and Ba, using an air-jet method, and Li, Ga, Cu, and Pb, using a method employing an inert atmosphere of argon and oxygen. The location of minor elements is discussed in terns of the crystal chemistry of the pyroxene group, and in terms of the known behaviour patterns of the elements in rocks and minerals. On the basis of Fairbairn’s packing index and other considerations, it is concluded that the minor components are, for the most part, occupying structural sites in the mineral. When compared with pyroxenes of igneous genesis, the pyroxenes studied are conspicuously poorer in most elements except Be, Ga, Sr, and Ba, which are at about the same level, B is richer than in igneous pyroxenes. This is explained in terms of the abundances of minor elements in the Grenville sediments. It is concluded that, just as in igneous crystal lization the bulk composition of the magma influences the minor element composition of the individual minerals, so in meta morphic recrystallization the composition is determined by the country rock. Calculations were made which show that the total minor element content of the pyroxenes studied was much less than that of igneous pyroxenes in the literature, and less than that of amphiboles of the same rocks. The method of calculation was devised so that the elements were weighted with respect to their natural abundance. Correlation of rock-type with minor clement values was attempted, with negative results. Some elements, particularly the ferrides, show a diminution in the lighter-coloured, i.e. Fe-poor specimens. Inter-element trends were investigated, with results showing some sympathetic variations among Zr, Mn, V, Ni, and Co.