IRON-RICH KORNERUPINE IN SHEARED PEGMATITE FROM THE WANNI-COMPLEX, ATHOMAGAMA, SRI-LANKA

Kornerupine occurs sparingly in a sheared pegmatitic vein about 15-20 cm thick and 1-2 m in extent in cordierite gneiss of the Wanni Complex at Homagama, 17 km SE of Colombo, Sri Lanka. The pegmatite consists o f dominant quartz, microcline, and cordierite, minor plagioclase, and traces of kornerupine, tourmaline, dumortierite, muscovite, biotite, c hlorite, garnet, andalusite, siderite-magnesite, monazite, zircon, apa tite, gahnite, magnetite, ilmenite, rutile, ilmenorutile(?), and pyrit e; sillimanite occurs elsewhere in the pegmatite. Kornerupine contains 14.4-15.0 wt% Fe as FeO and 0.14-0.18 wt% BeO, among the highest valu es reported for this mineral. The kornerupine-bearing portion of the p egmatite contains 44 ppm Be, 30 ppm B, and 12 ppm Cs, that is, the tra ce-element enrichment that one might expect in a granitic pegmatite. T wo periods of crystallization are inferred, which correspond to the fo llowing stable assemblages: (1) cordierite +/- garnet + early andalusi te (+/- sillimanite) + K-feldspar + oligoclase + quartz, possibly at T greater-than-or-equal-to 540-degrees-C, P less-than-or-equal-to 3 kba r (assuming X(H2O) almost-equal-to 0.3), from a pegmatitic melt, and ( 2) siderite-magnesite + late andalusite + quartz + cordierite at T nea r 500-degrees-C and X(CO2) > 0.8 (for P almost-equal-to 3 kbar) during metamorphic recrystallization. Distinct kornerupine + cordierite (X(F e) = 0.31-0.33) + quartz and tourmaline + andalusite + K-feldspar + co rdierite (X(Fe) = 0.37) + quartz domains developed during (1). At the greater-than-or-equal-to 540-degrees-C temperatures inferred for cryst allization of the Homagama pegmatite, kornerupine was probably not sta ble with plagioclase and quartz, and thus crystallized from the pegmat itic melt with cordierite in isolation from plagioclase.