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Garnets as indices of progressive regional metamorphism

Published online by Cambridge University Press:  14 March 2018

Kamal Nandi
Kamal Nandi*
Affiliation:
Department of Geology, Presidency College, Calcutta, India
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Summary

Garnets undergo readjustment of ion-assemblages under external mechanical and thermal stresses; this adjustment favours those paths that annul the very cause to which it is due. Metal ions in garnets being replaceable, with rising metamorphism garnets become reluctant to accommodate ions of larger ionic radii.

Data from recent literature on chemical analyses of garnets from rocks belonging to different grades of regional metamorphism reveal that the (FeO + MgO) content of garnet varies linearly with (CaO + MnO) content. As the Fe/Mg ratio does not vary appreciably, a fair estimate of metamorphic grade can be made from a knowledge of (CaO + MnO) or of (FeO +MgO) individually or even, with less confidence, of FeO or MgO alone. From the study of the available data it is found that (CaO + MnO) and (FeO + MgO)/(CaO + MnO) provide the best indicators of progressive metamorphism. Cell edges of garnets decrease progressively with increasing metamorphism and this may also be used as a fairly good index of metamorphism. When cell edges are plotted against (FeO + MgO)/(CaO + MnO), a hyperbolic curve is obtained.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 36 , Issue 277 , March 1967 , pp. 89 - 93
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1967

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