GEOCHEMISTRY, PETROGENESIS AND TECTONIC SETTING OF PROTEROZOIC MAFIC DYKE SWARMS, EASTERN DHARWAR CRATON, INDIA

Extensive swarms of thick (10 to 80 m) en echelon Proterozoic dolerite dykes have intruded the Archaean gneissic/granitic rocks of the Easte rn Dharwar Craton of Peninsular India. Dykes in the southern half of t he 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 fro m 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 h igher FeO(T), TiO2, and P2O5 (averaging 15, 2.2, and 0.45 wt/%). The H FTP is clearly enriched in high field strength and heavy rare earth el ements compared to the LFTP group, suggesting independent magma source s. The LFTP dyke population is chemically unlike any plume or island a re related magmatism acid 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 Am erican province and its low-Ti (LTQ) counterpart. The chemical composi tion of the LFTP population is consistent with a MORB-like source that has experienced considerable crustal contamination. The HFTP dyke pop ulation is chemically typical of continental flood basalt (CFB) magmat ism, such as the Deccan basalt, and is interpreted as magma derived fr om a plume source that has assimilated some lithospheric rock.