Abstract

Diapiric, comparatively massive trondhjemites were emplaced in the western part of the Dharwar craton of southern India about 3000 Ma ago. This age coincides with the age of (i) closure of Rb–Sr systems that now form the youngest isochrons in the predominantly gneissic terrane, and (ii) metasomatic enrichment of the gneisses in U. Younger events, principally about 2500 Ma ago, are recorded by sparse granites and by deformation of supracrustal sequences without major metamorphism. Thus, the trondhjemites appear to have formed during the last extensive thermochemical event in the craton. The trondhjemites are not associated with any mafic rocks and show very little fractionation within and among the various bodies.Presumably they formed as a single product of partial melting. Likely source materials for the magmas are rocks of basaltic composition (probably amphibolites) in the lower crust or along the crust–mantle boundary. Very low Zr and high Cr contents in the trondhjemites may indicate a slightly ultramafic (possibly basaltic komatiite) source. A lack of fractionation of Zr and Y and low light/heavy rare-earth element ratios in the trondhjemites may indicate an absence of equilibration of the magmas with major amounts of garnet. Lack of significant garnet equilibration could have resulted from production of hydrous magmas, either by partial melting of amphibolite or by introduction of water from an external source during the melting process.

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