Journal of Petrology, Volume 39, Issue 6: June 1 1998.

Geochemical Evolution of the Massif-type Anorthosite Complex at Bolangir in the Eastern Ghats Belt of India

A. BHATTACHARYA¹, M. RAITH², S. HOERNES² AND D. BANERJEE³

¹DEPARTMENT OF GEOLOGY & GEOPHYSICS, INDIAN INSTITUTE OF TECHNOLOGY KHARAGPUR,KHARAGPUR 721 302, INDIA ²MINERALOGISCH-PETROLOGISCHES INSTITUT, UNIVERSITÄT BONN, POPPELSDORFER SCHLOß, 53115 BONN, GERMANY ³DEPARTMENT OF GEOLOGY, UNIVERSITY OF WESTERN ONTARIO, LONDON, ONTARIO N6A 5B7, CANADA

The Bolangir massif-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]¹⁸O = 7·7-8·0%°) occur at the immediate contact with the massif, which they intrude in cross-cutting veins and sheets. They show remarkably high concentrations of high field 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 atthe LREE end; [delta]¹⁸O = 8·4-9·2%°) are interpreted to becrustally 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 ofthe anorthosite complex at mid-crustal levels, the terrane wasfolded and sheared (D₂), and the igneous assemblages were affected by granulite facies recrystallization [750-800°C, 7-8 kbar, a(H₂O) <0·25] during cooling from the igneous to the regional thermal regime.

Keywords: Eastern Ghats Belt of India;Bolangir; massif-type anorthosite; geochemistry; petrogenesis

Pages 1169-1195

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