Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Salil Agrawal ¹, R. K. Srivastava ²

Affiliations
1 Department of Geology, University of Rajasthan, Jaipur 302 004, IN
2 Department of Geology, M. L. Sukhadia University, Udaipur - 313 001, IN

Abstract

Cluster analysis based on major elements demonstrates existence of granite, granodiorite and tonalites as distinct natural sub-population in the Sendra granitoid suite. The peraluminous granite with limited variation in mineralogy and chemistry is characterised by higher total iron, K₂O, mol. A/CNK, K₂O/Na₂O, Rb, Ba, Li and lower Sr as compared to the granodiorites and tonalites of the suite. Different trends for tonalites. granodiorites and granites in variation diagrams coupled with lower DI values of the granites in comparison to some of the tonalites and granodiorites, disallow interpretation of their relationship in terms of closed system equilibrium crystal fractionation or rcstite unmixing.

Principal component analysis of the data negates the possibility of the geochemical variations being controlled by a single process. The presence of mafic microgranular enclaves (MME) in the granodiorite and tonalite suggest mixing of the felsic and mafic magmas. The homogeneous nature of the granite and the absence of tonalitic and granodioritic components in the Chang pluton are due to emplacement of unadulterated granitic melt generated in the lower crustal region by basaltic underplating. The granodiorite and tonalite with variable chemical composition, represent subsequent mixing of granitic melt with the underplating basaltic magma and different degrees of thermal and chemical disequilibrium in the hybrid magma.

Keywords

Geochemistry, Proterozoic Granitoid Suite, Delhi Fold Belt, Magma Mixing, Rajasthan.

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Salil Agrawal ¹

Affiliations
1 Department of Geology, University of Rajasthan, Jaipur 302 004, IN

Abstract

The subjective, eye-fitted, field boundaries in geochemical discrimination diagrams constitute one of the possible reasons for their often-noted inability to determine the source regime/tectonic environments of igneous rocks. A simple method is described here to replace the eye-fitted field boundaries between the groups with the probability based classifier surfaces. It involves projection of the assignment rules obtained from linear discriminant analysis onto the binary or ternary discrimination diagram of raw variables. The method is illustrated by obtaining classifier surface between I- and S-type granites in Na₂O-K₂O space and between Anorthosite/Rapakivi (AR) and Ring Complex (RC) granites in Na₂O-MgO space. The need for explicitly ensuring adequacy of samples, selection of suitable variables, evaluation of rate of misclassification and a better understanding of the unique influence of tectonic environments on magmatic process is emphasized for optimizing the performance of the classifier surface.

Keywords

Igneous Rocks, Geochemical Discrimination, Tectonic Classification, I- and S-type Granitoids, Rapakivi and Ring-Complex Granites.

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Salil Agrawal ¹, Pradeep Bora ¹, Mahendra Singh ¹

Affiliations
1 Department of Geology, University of Rajasthan and Jaipur - 302 004, IN

Abstract

No Abstract.

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