Spinel + Quartz association from the Kerala khondalites, southern India: evidence for ultrahigh-temperature metamorphism

摘要

Granulite facies metapelites (khondalites) from Chittikara are characterized by the assemblage Grt + Sil + Bt + Crd + Spl + Kfs + Pl + Qtz ± graphite. The rocks show a series of mineral reactions such as: (1) Bt + Sil + Qtz = Grt + Crd ± Kfs, (2) Grt + Sil + Qtz = Crd, (3) Grt + Crd + Sil = Spl + Qtz , (4) Spl + Qtz = Crd, and (5) Grt + Sil = Spl + Crd. Spinel and quartz occur in direct grain contact and their stability considerations in appropriate reaction grids indicate > 950℃ at 8 kbar. This, together with the ca. 1000℃ temperatures derived from mesoperthites suggests ultrahigh temperature metamorphism in the khondalite belt. Electron microprobe data on the various minerals have been used to compute pressure-temperature estimates from mineral phase equilibria thermobaromtery. Temperature estimates from spinel-cordierite thermometry are around 900℃, while garnet-cordierite thermometer yields an upper estimate of 818℃. Estimates based on garnet-sillimanite-plagioclase-quartz barometer indicate pressures up to 8 kbar. Evaluation of the microstructures and reactions in the khondalites using available reaction grids for pelitic rocks provide evidence for peak metamorphism at high or ultrahigh temperatures followed by steep decompression from around 8 kbar down to 4 kbar. The P-T path attests to a rapid exhumation history probably associated with the extensional collapse following collision of continental blocks during the assembly of Gondwana in the late Pan-African.