The crystalline rocks of Yorkes Peninsula, S.A.

Abstract

This report deals with the crystalline rocks of Southern Yorkes Peninsula. The Precambrian age of these is considered and the petrography discussed. The rocks are divided into the main varieties, which are as follows:- (i) Gneisses: (a) "Grey banded gneiss". (b) "Red gneissic granite". (c) "Augen gneisses". (d) Cape Spencer and Royston Head gneisses. (e) "Dark biotite granite". (ii) Amphibolites: (a) Conformable amphibolite bodies. (b) Altered dyke rocks. (iii) Pegmatites. The mineralogy and texture are described where necessary, and structural relations are considered. The older rocks in the Point Yorke area are described as possible metasediments containing the conformable amphibolite bodies which may represent metamorphosed basic flows or sills. The "red gneissic granite" is shown to have subsequently engulfed these rocks forming migmatites and injection gneisses, and many of the pegmatites of the area are tentatively related on structural evidence to this later activity. Rapakivi type porphyritic granites are common, usually as narrow zones related to intrusive contacts. The paper is concluded with a discussion concerning similarities between those rocks and rocks of other Precambrian terrains of the world.