A HFSE-ENRICHED AND REE-ENRICHED FERRODIORITE SUITE FROM THE BOLANGIRANORTHOSITE COMPLEX, EASTERN GHATS BELT, INDIA

The massif-type anorthosite complex at Bolangir in the northern part o f the Eastern Chats belt occurs in a milieu of predominantly supracrus tal granulite-grade rocks. The massif is separated from the host gneis ses by coarse-grained garnetiferous granitoid gneisses which are inter preted as coeval crustally-derived melts. Melanocratic ferrodiorite ro cks occur at the immediate contact with the anorthosite massif which t hey intrude in cross-cutting dikes and sheets. The emplacement age of the anorthosite diapir and the associated igneous suites is deemed to be pre-D-2. Recrystallization of the igneous assemblages of the ferrod iorite suite (750-800 degrees C, 7-8 kbar, a(H2O) < 0.25, logf(O2) < - 18 bar) during a period of near-isobaric cooling from the igneous crys tallization stage to the regional P-T regime led to extensive developm ent of coronitic garnet at the interface of plagioclase phenocrysts wi th the mafic matrix assemblage (opx + fay + cpx + ilm +/- amph, bio). Abundant accessory phases are zircon, apatite and thorite. The mafic p hases have extremely ferrous compositions (X-Fe gar: 0.93-0.87, fay: 0 .90-0.87, opx: 0.80-0.60, cpx: 0.70-0.47, amph: 0.81-0.71) reflecting the low Mg-number (16-8) of the rocks. Compared to worldwide occurrenc es of similar rocks, the Bolangir ferrodiorites (SiO2: 36-58 wt.%, FeO : 39-10 wt.%) are characterized by exceptionally high concentrations of HFSE and REE (TiO2: 4.8-1.0 wt.%, P2O5: 1.7-0.5 wt.%, Zr: 5900-1300 ppm, Y: 240-80 ppm, La: 540-100 ppm, Ce: 1100-200 ppm, Yb: 22-10 ppm, Th: 195-65 ppm). Well defined linear variation trends for major and t race elements reflect progressive plagioclase accumulation towards the felsic members of the suite. The ferrodiorites are interpreted to rep resent residual liquids of anorthosite crystallization which after seg regation and extraction from the ascending diapir became enriched in H FSE and REE through selective assimilation of accessory phases (zircon , monazite, apatite) from crustal felsic melts. Ferromonzodioritic roc k presumably formed through hybridization between the ferrodiorite and overlying felsic melts.