GEOCHEMISTRY OF EARLY PROTEROZOIC MAFIC AND ULTRAMAFIC ROCKS OF JHAROL GROUP, RAJASTHAN, NORTHWESTERN INDIA

The deep water Jharol sediments of Aravalli Supergroup host sporadic b odies of mafic-ultramafic rocks. Field evidence suggests that at least some of these rocks are extrusive in nature. Low to medium grade meta morphism and intense deformation has obliterated the original mineralo gy and texture of these rocks. The ultramafic rocks are relatively mor e altered (serpentinized) with highly disturbed geochemical compositio n which cannot be used for petrogenetic interpretations. On the other hand, the mafic rocks are relatively less altered with some relict pla gioclase feldpars. Geochemically, these mafic volcanics are Mg- and Fe - rich tholeiites. These tholeiites have nearly flat to slightly enric hed REE patterns (about 30 to 40x % chondrite La and 20 to 25x % Chond rite Yb), with (La/Yb)N ratio ranging between 1.29 and 1.54 and (Gd/Yb )N ratio ranging between 0.97 and 1.07. All samples show slight to str ong positive Eu anomaly. Immobile incompatible-element geochemical pat terns and other binary plots suggest that these tholeiites have enrich ed chemistry and that probably they have been derived from enriched so urce(s) and resemble closely with within plate and initial rift tholei ites. Trace elements, including REE based petrogenetic modelling indic ate that (a) the source had about 3x% chondoritic REE abundances with nearly flat to slightly depleted pattern and (b) that these rocks were generated by about 8 to 12% partial melting of the modelled source fo llowed by some olivine and clinopyroxene fractionation. Field as well as trace element characteristics indicate rift related (lithospheric e xtension) generation of these basalts with some influence of asthenosp here (deep mantle) on the lithospheric source(s) as indicated by posit ive Nb and Sr anomalies.