The petrogenetic characterization of intermediate and silicic charnockites in high-grade terrains: a case study from southern India |
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Authors: | H M Rajesh |
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Institution: | (1) Department of Geology, University of Johannesburg, Auckland Park Kingsway Campus, P.O. Box 524, Auckland Park, 2006 Johannesburg, South Africa |
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Abstract: | Large charnockite massifs occur in some of the Precambrian high-grade terrains like the southern Indian granulite terrain.
The Cardamom Hill charnockite massif from the Madurai Block, southern India, consists of an intermediate type and silicic
type, with the intermediate type showing similarities to high-Ba−Sr granitoids with low K2O/Na2O ratios and the silicic type showing similarities to high-Ba–Sr granitoids with high K2O/Na2O ratios. Within the constraints imposed by near basaltic composition of the most mafic samples and their relatively high
concentrations of both compatible and incompatible elements, comparison with recent experimental studies on various source
compositions, and trace- and rare-earth-element modeling, the distinctive features of the intermediate charnockites can be
best explained in terms of assimilation–fractional crystallization (AFC) models involving interaction between a mantle-derived
basaltic magma and lower crustal materials. Silicic charnockites on the other hand are high temperature melts of moderately
hydrous basaltic magmas. A two-stage model which involves an initial partial melting of hydrous basaltic magma and later fractionation
explains the geochemical features of the silicic charnockites, with the fractionation stage most probably an open system AFC.
It is suggested that for massifs showing spatial association of intermediate and silicic charnockites, a model taking into
account their compositional difference in terms of the effect of variations in the conditions (e.g., temperature, water fugacity)
that prevailed, can account for plausible petrogenetic scenarios. |
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