Multistage gedrite in gedrite-hypersthene-bearing high-grade granulites from Daltonganj, Chhotanagpur granite-gneissic complex, Jharkhand, as evident from TEM and textural relations

The studied rock consists of mineral phases garnet-cordierite-gedrite-hypersthene-biotite-quartz and lies nearly 14km southwest of Daltonganj in the western part of Chhotanagpur granite-gneissic complex. Textural relations of mineral phases, mineral chemistry and transmission electron microscopy (TEM) analyses of the rock samples suggest the early (gedrite1), middle (gedrite2) and late stages (gedrite3) formation of gedrite. Hypersthene appears through the metamorphic reaction gedrite + quartz = orthopyroxene + garnet + cordierite +H2O due to the breakdown of gedrite3 at the thermal peak of 869 degrees C/7.79kbar. The bundles, prismatic and fibrous forms of three gedrites have been observed in the TEM images. TEM images and selected area electron diffraction patterns show the distribution of metallic element position at the different lattice site. The P-T estimates of the rock from garnet-orthopyroxene, garnet-cordierite and garnet-biotite exchange geothermometers and garnet-sillimanite-cordierite-quartz geobarometers vary from 775 degrees to 869 degrees C, 642 degrees to 703 degrees C, 480 degrees to 617 degrees C and 6.76 to 7.79 kbar, respectively.