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Abstract

Extensive swam of thick (10 to 80 m) en echelon Proterozoic dolerite dykes have intruded the Archaean gneissic/granitic rocks of the Eastern Dharwar Craton of Peninsular India. Dykes in the southern half of the craton, including a cluster of 12, strike mostly E-W, whereas those exposed in the northern half including two clusters of 25 and 6 dykes, strike NW and ENE, as well as E-W and N-S. All the dykes sampled from the southern group plot along a continuous tholeiitic fractionation trend and are characterised by low concentrations of FeO_(T), TiO₂, and P₂O₅ (an LFTP group) averaging 11.5,0.85, and 0.15 wt% respectively. About two-thirds of the 25 dykes sampled from the northern group are also LFTP dykes, but the remaining third (a HFTP group) contain much higher FeO_(T), TiO₂, and P₂O₅ (averaging 152.2, and 0.45 wt/%). The HFTP is clearly enriched in high field strength and heavy rare earth elements compared to the LFTP group, suggesting independent magma sources.

The LFTP dyke population is chemically rlnlike any plume or island are related magmatism and differs from most continental-rift related, within-plate tholeiites. It is, however, similar to the Mesozoic rift related olivine normative dolerite dyke population of Eastern North American province and its low- Ti (LTQ) counterpart. The chemical composition of the LlTP population is consistent with a MORBlike source that has experienced considerable crustal contamination.

The HFTP dyke population is chemically typical of continental flood basalt (CFB) magmatism, such as the Deccan basalt, and is interpreted as magma derived from a plume source that has assimilated some lithospheric rock.

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