Mineral Chemistry of Coexisting Phases from Shonkinitic Rocks, Salem, Tamil Nadu, India

C. R. L. Friend and A. S. Janardhan
Department of Geology and Physical Sciences, Oxford Polytechnic, Oxford, OX3 0BP, UK
Department of Geology, University of Mysore, Manasagangotri, Mysore 570006, India

Abstract: Gabbroic-textured shonkinitic rocks of unknown age cut deformed Archaean dunites and peridotites in the area immediately north of Salem. Magnesite formation post-dates their intrusion. The location of these rocks and the magnesite is thought to be controlled by the intersection of two major basement lineaments. A sequence of crystallization of the minerals and rocks from early undersaturated to late oversaturated rocks is established. Compositions of the coexisting mineral phases are reported here for the first time. Clinopyroxenes, which are relatively unevolved, commonly coexist with olivine (chrysolite) and sanidine. Bronzite, often intergrown with magnetite and green spinel, is occasionally present. In the most undersaturated rocks nepheline-sanidine fingerprint intergrowths occur, whilst perthitic sanidine, albitic plagioclase, and quartz are present in over-saturated rocks. Late in the crystallization sequence, in over-saturated rocks, PH2O appears to have increased allowing the formation of amphibole as well as clinopyroxene.

Mineralogical Magazine; June 1984 v. 48; no. 347; p. 181-193; DOI: 10.1180/minmag.1984.048.347.03
© 1984, The Mineralogical Society
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