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Journal of Petrology | Volume 44 | Number 12 | Pages 2139-2171 | 2003
© Oxford University Press 2003; all rights reserved
Early Proterozoic Melt Generation Processes beneath the Intra-cratonic Cuddapah Basin, Southern India
1 DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF CAMBRIDGE, CAMBRIDGE CB2 3EQ, UK
2 DEPARTMENT OF EARTH SCIENCES, INDIAN INSTITUTE OF TECHNOLOGY, MUMBAI, POWAI, 400 076, INDIA
3 DEPARTMENT OF EARTH SCIENCES, OPEN UNIVERSITY, MILTON KEYNES MK7 6AA, UK
4 DEPARTMENT OF GEOLOGY, McMASTER UNIVERSITY, HAMILTON, ONT. L8S 4M1, CANADA
* Corresponding author. Present address: Planetary Geosciences Institute, 306 GS Building, University of Tennessee, Knoxville, TN 37996, USA. Telephone: +1-865-974-6024. Fax: +1-865-974-2368. E-mail: anandm{at}utk.edu
Early Proterozoic tholeiitic lavas and sills were emplaced during the initial phase of extension of the intra-cratonic Cuddapah Basin, southern India. 40Ar–39Ar laser-fusion determinations on phlogopite mica, from the Tadpatri Fm mafic–ultramafic sill complex, constrain the age of the initial phase of extension and volcanism in the basin at 1·9 Ga. Despite their Early Proterozoic age, the igneous rocks are unmetamorphosed, undeformed and remarkably fresh. They exhibit a wide range in MgO contents (4–28 wt %) and have undergone varying degrees of accumulation or crystal fractionation. Variable La/Nb ratios (1·2–3·7) and 
Nd values (1 to -10) suggest that some, but not all, of the mafic rocks have been affected by crustal contamination. This appears to have taken place in magma chambers at 
9 kbar, i.e. the base of the continental crust. Forward modelling of major and trace elements (Fe and Nd) and inverse modelling of rare earth elements suggest that the primary Cuddapah melts were generated by 10–15% partial melting of a lherzolite mantle source. This corresponds to a mantle potential temperature of 1500°C. The thickness of the mechanical boundary layer predicted by the geochemical modelling is 70 km with a minimum initial lithospheric thickness of 120 km. This corresponds to a stretching factor of 1·6–1·8. Richter's (1988) secular cooling model for the Earth predicts that, at 1·9 Ga, the ambient mantle had a potential temperature of 1500°C (i.e. 200°C hotter than Phanerozoic mantle). If the cooling model is correct then Proterozoic lithospheric stretching and mantle melting beneath the intra-cratonic Cuddapah Basin could have been caused by passive rather than active rifting.
KEY WORDS: Proterozoic; intra-cratonic; Cuddapah Basin; rare earth element inversion; lithosphere
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