Sapphirine, Kornerupine, and Sillimanite + Orthopyroxene in the Charnockitic Region of South India

Sapphirine, kornerupine, and the associations sillimanite-orthopyroxene and sillimanite-gedrite are reported from 23 localities (or areas of closely spaced localities) in Fermor's (1936) charnockitic region and in transitional zones to Fermor's non-charnockitic region in south India (Andhra Pradesh, Tamil Nadu, and southern Karnataka). The rocks containing these minerals and mineral associations are believed to have a pelitic sedimentary precursor. They are in general closely associated with khondalite (garnet-sillimanite gneiss), quartzite, and calc-silicate rocks, and less commonly with metabasites of presumed metavolcanic origin. However, the sapphirine occurrences in the Sittampundi anorthosite complex, where sapphirine and sillimanite-gedrite are found in reaction zones between anorthosite and chromite layers, are an exception. Assemblages of sapphirine with garnet and sillimanite are not rare, but sapphirine in direct contact with quartz, which has not previously been reported from south India, was found at Paderu, Vishakhapatnam District, A. P. Kornerupine occurs with sillimanite at three localities but with orthopyroxene only at Ganguvarpatti, a new locality for kornerupine. Temperatures and pressures calculated from pyroxenes, garnets, and feldspars in associated rocks suggest that sapphirine, kornerupine, and sillimanite-orthopyroxene crystallized at 800-850°C and 6-10 kbar in the charnockitic region and sapphirine at 780°C in a transition zone between the granulite and amphibolite-facies near Sargur. Water partial pressure during the metamorphism was significantly less than total pressure. Regional differences in pressure are reflected in the distribution of mineral assemblages: sillimanite-orthopyroxene is found in Madras and Andhra Pradesh, but is conspicuously absent in the Palni Hills, where spinel-cordierite is an important assemblage. Sapphirine and sillimanite-orthopyroxene (and sillimanite-gedrite) are restricted to rocks relatively rich in Al₂O₃ with high Mg/Fe ratios relative to khondalite. Kornerupine appears to be confined to rocks containing boron. These special rock compositions (except Sittampundi) are believed to reflect unusual compositions of the precursor pelitic sediments rather than changes in chemistry during metamorphism.