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

R. K. Srivastava ¹, A. Maheshwari ², Rajani Upadhyaya ¹

Affiliations
1 Department of Geology, University of Rajasthan, Station Road, Udaipur 313 001, IN
2 Maharaja's College Campus, University of Rajasthan, Jaipur 302001, IN

Abstract

The felsic volcanics of Gurapratap Singh and Diri are mostly welded tuffs and belong to andesite-dacite-rhyolite association; they are a part of the Malani volcanic suite of Precambrian age. The volcanics are cut by rhyolite porphyry (comagmatic with volcanics) and a younger dolerite dyke. A few flows of ultrapotassic rhyolites are also found which do not conform to the main trend and presumably mark a different phase of magmatic activity.

The chemical data on 91 samples of different rock types and Harker diagrams reveal a systematic variation in all major elements with the progress of differentiation, suggesting that the entire sequence except the ultrapotassic rhyolite· is cogenetic and that the changes have been controlled by some sort of fractional crystallisation mechanism. The principal component analysis of the data reveals that in the earlier stages, the fractionating phases were soda-lime plagioclase and a mafic phase and in the later stages K-feldspar became a dominant phase. This is supported by phenocryst assemblages in these rocks. The suite also has very high inter-elemental correlation indicating that the rocks are cogenetic and a single process was responsible for their evolution. The mixing line calculation also confirms the operativeness of a single process.

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R. K. Srivastava ¹, A. Maheshwari ², Rajani Upadhyaya ¹

Affiliations
1 Department of Geology, University of Rajasthan, Station Road, Udaipur 313001, IN
2 Maharaja's College Campus, University of Rajasthan, Jaipur, IN

Abstract

Felsic volcanics of the Gurapratap Singh and Diri, belong to the Malani volcanic suite of Precambrian age. Major element chemistry of these volcanics shows systematic variations with progress of differentiation. The distribution and behaviour of trace elements in these rocks is in accord with their major element data and the distribution patterns are normal with no anomalous concentrations of the elements studied. With the progress of fractionation there is a smooth decrease in Sr and increase in Rb which along with Ba are the main trace contents of these rocks. Unlike Sr and Rb which have a linear distribution pattern, Ba shows a curvilinear trend, i.e., an initial increase from andesite to dacite and rhyodacite followed by a decrease amongst rhyolites. The behaviour of ratios like Ni/Co, Fe/Zn, Mg/Li and K/Rb agrees with a model of fractional crystallisation. Furthermore, the trace element abundances of the volcanics point to their crustal source. The work also shows that in felsic magmas Ba and Sr are sensitive indicators for tracing possible differentiation and that rhyolitic rocks depleted in Ba are enriched in Rb and that in last stages of differentiation Ba/Rb ratio shows rapid decrease.

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A. Maheshwari ¹, M. Coltorti ², A. N. Sial ³, G. Mariano ³

Affiliations
1 Department of Geology, University of Rajasthan, Jaipur-302 004, IN
2 Istituto di Mineralogia, Universita di Ferrara, IT
3 Department of Geology, UFPE, Recife, 50.732-970, BR

Abstract

The Malani volcanics of Gurapratap Singh and Din belong to a basalt-andesite -dacite-rhyolite association of Precambrian age. The preponderance of rhyolites over basic and intermediate rocks is a significant feature of the area. The rhyolities are characterised by chemical features, which indicate their A-type affinity. The study rules out their cogeneticity and suggests the crustal involvement in the genesis of Malani rhyolites.

Keywords

Petrology, Oxygen Isotopes, Malani Volcanics, Rajasthan.

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