WIDESPREAD OCCURRENCES OF HöGBOMITE (2N2S) IN UHT METAPELITES FROM THE BETROKA BELT, SOUTHERN MADAGASCAR: IMPLICATIONS ON MELT/FLUID ACTIVITY DURING REGIONAL METAMORPHISM

Högbomite occurs within ultrahigh temperature metamorphism metapelites over an area of 60 km x 12 km at northern part of Betroka Belt, southern Madagascar. Metamorphic P-T estimates of the studied rocks yielded 880-1060°C and 4.5-6.6 kb for peak mineral assemblage. The studied högbomite mineral belongs to the 2N2S polysome type and exhibit a wide range of chemical composition. The högbomite was formed in polyphase domains hosted by discrete magnetite, ilmenite, and/or hematite in close association with spinel, corundum, and/or rutile. Thin films of Pl±Bt±Qtz surrounding garnet porphyroblasts and Fe-Ti oxide grain boundaries and biotite veinlets in the cracks of garnet porphyroblasts represent melt/fluid infiltration during the högbomite formation. The equilibrated association implies that the högbomite crystallization occurred at T≥800°C and P≥4 kb during various retrogressive reactions under relatively high oxygen fugacity. The textural features and compositional relationship of högbomite and spinel suggest that högbomite crystallized mainly at expense of spinel by introduction of hydrous anatectic melts formed during granulite facies condition or introduction of hydrous fluids along shear zones over a wide area. We propose högbomite as an indicator mineral for rock-melt/fluid interaction during retrogressive stage of regional metamorphism.