EVIDENCE FOR A NORMAL METAMORPHIC P-T GRADIENT AND TOP-NORTH DISPLACEMENT IN THE CENTRAL BHUTAN HIMALAYA: SHEAR SENSE AND P-T ESTIMATES FROM THE CHEKHA FM AND THE GREATER HIMALAYAN SEQUENCE
Five samples from near the base of the Chekha Fm are characterized by the mineral assemblage Grt-Bt-St. Garnet crystals from these samples show growth zoning; however, embayed and irregular grain boundaries and reversals in Fe/Fe+Mg suggest diffusional modification and variable amounts of garnet dissolution due to net transfer reactions. One schist sample from the GHS is characterized by the mineral assemblage Grt-Ky-Bt and was partially melted. Garnet from this sample show diffusional zoning profiles and evidence for significant dissolution most likely due to retrograde net-transfer reactions.
Preliminary Grt-Bt-Plag thermobarometry from the Chekha Fm yields temperatures from 525 °C to 625 °C and pressures from 6 to 8 kbar. The sample from the GHS yields P-T conditions of ~8 kbar and ~675 °C. Temperature estimates in several samples are inconsistent with the interpreted peak mineral assemblage and likely reflect disequilibrium due to garnet dissolution and Fe-Mg diffusional re-equilibration.
Several samples from the Chekha Fm show top-north shear bands with normal displacement. The sample from the GHS shows top-northwest and top-southeast conjugate shear bands, consistent with a component of pure shear. Shear bands in samples from both the GHS and the Chekha Fm deform all metamorphic minerals, indicating that top-north shear postdates peak metamorphism. Shear sense across the GHS and into the Chekha Fm is consistent with a model of extrusion such that lower structural levels are dominated by top-south shear whereas the upper structural levels are dominated by top-north shear. Pressure estimates may reflect a steep gradient as opposed to a discrete break between the GHS and the Chekha Fm.