Geochemical evolution of the massif-type anorthosite complex at Bolangir in the Eastern Ghats Belt of India

The Bolangir mass-type anorthosite in the Eastern Ghats Belt of India is composed of homogeneous anorthosite and subordinate volumes of schlieric leuconorite in its marginal parts. The massif is separated from granulite grade country gneisses by coarse-grained garnetiferous granitoids. Extremely iron-rich ferrodiorites (FeO* 39-30 wt %, SiO2 36-43 wt %, delta(18)O = 7.7-8.8 parts per thousand occur at the immediate contact with the massif, which they include in cross-cutting veins and sheets. They show remarkably high concentrations of high yield strength elements (HFSE) and rare earth elements (REE) (Zr 5900-4200 ppm, Y 240-30 ppm, Nb 290-230 ppm, La 480-440 ppm, Ce 1070-1065 ppm, Yb 22-14 ppm, Th 195-65 ppm). The ferrodiorites are interpreted to be late-stage residual melts that evolved through extensive anorthosite crystallization from high-Al basic magmas. Following their extraction from the anorthosite-leuconorite solids at an advanced stage of crystallization the ferrodioritic melts became enriched in HFSE and REE through selective assimilation of zircon, monazite and apatite from crustal sources (felsic melts). Locally, ferromonzodioritic varieties developed by mixing between the Fe-rich melts and the bordering felsic melts. The border zone peraluminous granitoids (K2O/Na2O = 2.4-3.5; Ba 1500-3000 ppm; mg-number 20-10, weakly fractionated REE patterns with enrichment at the LREE end; delta(18)O = 8.4-9.2 parts per thousand) are interpreted to be crustally derived melts coeval with the anorthosite, and presumably formed by advective heating of crustal zones overlying the fractionating high-al basaltic magma pools. Following emplacement of the anorthosite complex at mid-crustal levels, the terrane was folded and sheared (D-2), and the igneous assemblages were affected by granulite facies recrystallization [750-800 degrees C, 7-8 kbar, a(H2O) <0.25] during cooling from the igneous to the regional thermal regime.