Geochemistry and petrogenesis of radioactive palaeoproterozoic granitoids of kinwat crystalline inlier, Nanded and yeotmal districts, Maharashtra

摘要

Kinwat crystalline inlier exposes Palaeoproterozoic granitoids belonging to the northern extensions of younger phase of Peninsular gneissic complex (PGC) within Deccan Trap country in Eastern Dharwar Craton (EDC) and bounded in south by a major NW-SE trending lineament (Kaddam fault). Geochemically, the Kinwat granitoids are similar to high-K, calc-alkaline to shoshonite magnesian granitoids and subdivided into two major groups, i.e. felsic group (pink and grey granites) and intermediate to felsic group (hybrid granitoids). The felsic group (~67-74% SiO_2) shares many features with Neoarchaean to Palaeoproterozoic high potassic granites of PGC such as higher LILE and LREE content and marked depletion in Eu, P and HFSE, especially Nb, Ti, relative to LILE and LREE. The hybrid granitoids (～58 - 67% SiO_2) have comparatively higher Ca, Mg and Na contents and slightly lower REE content than the granitoids of felsic group. Both, felsic and hybrid granitoids are metaluminous to weakly peraluminous and belong to highly fractionated I-type suite as evidenced by negative correlation of SiO_2 with MgO, FeO ~t, CaO, Na_2O, A1_2O_3, whereas K _2O, Rb and Ba show sympathetic relationship with SiO_2. Moderate to strong fractionated REE patterns (Ce/Yb_N: ～54-387) and strong negative Eu anomalies (Eu/Eu*: 0.13 - 0.41) are quite apparent in these granitoids. The geochemical characteristics together with mineralogical features such as presence of biotite±homblende as the dominant ferromagnesian mineral phases point towards intracrustal magma source, i.e. derivation of magma by partial melting of probably tonalitic igneous protolith at moderate crustal levels for felsic granites, whereas hybrid granitoids appear to be products of juvenile mantle-crust interaction, in an active continental margin setting. Anomalous radioelemental concentration (upto 0.033% U _3O_8 and 0.30% ThO_2; n=61) has been recorded in Kinwat granitoids, especially close to the contact zones with mafic intrusives and shear zones. Distribution patterns of heat producing elements (U, Th and K) in these granitoids have indicated substantially high heat production values (9.55-97.78 μWm~(-3)), which might have played important role in uranium remobilisation and concentration by supporting hydrothermal gradient along the dilatant structures. Considering highly labile nature of uranium in this reactivated terrain, the Kinwat granitoids form a fertile provenance to mineralisation under favourable conditions, especially in Neoproterozoic cover sediments in adjoining areas.