Charnockitic magmatism in southern India |
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| Authors: | H M Rajesh M Santosh |
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| Institution: | (1) Department of Geographical Sciences and Planning, University of Queensland, 4072 St Lucia, Queensland, Australia;(2) Department of Natural Environmental Science, Faculty of Science, Kochi University, 780-8520 Kochi, Japan |
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| Abstract: | Large charnockite massifs cover a substantial portion of the southern Indian granulite terrain. The older (late Archaean to
early Proterozoic) charnockites occur in the northern part and the younger (late Proterozoic) charnockites occur in the southern
part of this high-grade terrain. Among these, the older Biligirirangan hill, Shevroy hill and Nilgiri hill massifs are intermediate
charnockites, with Pallavaram massif consisting dominantly of felsic charnockites. The charnockite massifs from northern Kerala
and Cardamom hill show spatial association of intermediate and felsic charnockites, with the youngest Nagercoil massif consisting
of felsic charnockites. Their igneous parentage is evident from a combination of features including field relations, mineralogy,
petrography, thermobarometry, as well as distinct chemical features. The southern Indian charnockite massifs show similarity
with high-Ba-Sr granitoids, with the tonalitic intermediate charnockites showing similarity with high-Ba-Sr granitoids with
low K2O/Na2O ratios, and the felsic charnockites showing similarity with high-Ba-Sr granitoids with high K2O/Na2O ratios. A two-stage model is suggested for the formation of these charnockites. During the first stage there was a period
of basalt underplating, with the ponding of alkaline mafic magmas. Partial melting of this mafic lower crust formed the charnockitic
magmas. Here emplacement of basalt with low water content would lead to dehydration melting of the lower crust forming intermediate
charnockites. Conversely, emplacement of hydrous basalt would result in melting at higher {ie565-01} favoring production of
more siliceous felsic charnockites. This model is correlated with two crustal thickening phases in southern India, one related
to the accretion of the older crustal blocks on to the Archaean craton to the north and the other probably related to the
collision between crustal fragments of East and West Gondwana in a supercontinent framework. |
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| Keywords: | Charnockitic magmatism southern India intermediate charnockite felsic charnockite petrogenetic model |
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