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Pyroxene and biotite from the charnockitic rocks of Garbham area, Srikakulam district, Andhra Pradesh, India

Published online by Cambridge University Press:  05 July 2018

C. Bhattacharyya
C. Bhattacharyya*
Affiliation:
Department of Geology, Presidency College, Calcutta 12, India
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Summary

Chemical analyses and optical properties of ten orthopyroxenes, two clinopyroxenes, and six biotites (partial analyses) are presented from basic to intermediate charnockites of the Garbham area. The orthopyroxene is hypersthene to ferrohypersthene and the clinopyroxene is sahlite. Biotite has secondarily formed from pyroxene, particularly orthopyroxene, as a result of injection of potassic fluid in the basic charnockite. By combining the data of Srikakulam with those from other areas of the world it is shown that during the transformation of pyroxene to biotite in charnockitic rocks, in general, Mg and Fe 2+ attain near-equilibrium distribution between orthopyroxene and biotite as well as between clinopyroxene and biotite. The magnesium distribution coefficient (KD) of coexisting orthopyroxene and biotite is found to increase with the increase of temperature of biotite formation. A tentative geothermometric scale has been proposed plotting temperature of biotite formation against magnesium distribution coefficient of coexisting orthopyroxene and biotite. According to this scale, biotite in basic and semibasic charnockites from Garbham has formed between 285 and 465 °C.

Type
Research Article
Information
Mineralogical Magazine , Volume 37 , Issue 290 , June 1970 , pp. 682 - 692
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1970

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