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Mineralogy and metamorphic history of norite dykes within granulite facies gneisses from Sand Mata, Rajasthan, NW India

Published online by Cambridge University Press:  05 July 2018

Ram S. Sharma ,
Jane D. Sills  and
M. Joshi
Ram S. Sharma
Affiliation:
Department of Geology, Banaras Hindu University, Varanasi 22100, India
Jane D. Sills
Affiliation:
Department of Geology, Leicester University, Leicester LE1 7RH, England
M. Joshi
Affiliation:
Department of Geology, Banaras Hindu University, Varanasi 22100, India
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Abstract

Metanorite dykes intrude the Banded Gneiss Complex at various places in Rajasthan, N.W. India. They show neither chilled margins nor gradational contacts with the country rock amphibolite or granulite facies gneisses. They have ophitic to subophitic texture with strongly zoned subcalcic clinopyroxene and orthopyroxene, olivine and plagioclase, with subsidiary biotite. During slow cooling a series of reaction coronas developed with garnet forming round biotite, ilmenite and orthopyroxene; hornblende round pyroxenes and orthopyroxene, hornblende ± spinel round olivine, which may be totally replaced. It is inferred that the dykes crystallised from a tholeiitic magma at about 1100-1150 °C and were intruded during the waning stages of granulite facies metamorphism. The corona minerals grew at about 650–700 °C. A series of reactions to account for the development of the coronas is proposed using measured mineral compositions. Although these reactions do not balance for individual corona formation, metamorphism was probably isochemical with Ca, Na, K, Ti, Si and H2O only mobile on the scale of a thin section. Si and H2O were possibly mobile on a larger scale.

Keywords

noritesdykesgneissesgranulite faciesRajasthanIndia
Type
Petrology
Information
Mineralogical Magazine , Volume 51 , Issue 360 , June 1987 , pp. 207 - 215
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1987

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