Fluorine content of biotite in granulite-grade metapelitic assemblages and its implications for the Eastern Ghats granulites

Abstract

Biotite inclusions showing high fluorine content (up to 3.3 wt.%) within porphyroblastic aluminous orthopyroxene (>10 wt.% Al 2 O 3 ) and cordierite suggest high temperature of melting for the prograde assemblages. Textural data indicate that biotite dehydration melting produces peak assemblages orthopyroxene + spinel + quartz + melt and orthopyroxene + sapphirine + quartz ± cordierite + melt in different microdomains depending on the bulk composition. A comparative study of the biotite composition with those of experimental data and present one indicates a minimum temperature of 950°C for the terminal stability of biotite-bearing assemblages. This also explains the appearance of observed peak assemblage sapphirine + orthopyroxene + quartz + cordierite + spinel directly from F-rich biotite as a consequence of topological changes in the KFMASH grids. Enrichment of fluorine in retrograde biotite can be explained by interaction of solid phases with the in situ melt fraction during subsequent retrogressive stage of the granulites. This is the first report of its kind from the northern part of the Eastern Ghats Belt, India.