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

Upper amphibolite facies metasedimentary rocks at the base of the Chekha Fm overlie the partially melted Greater Himalaya Sequence (GHS) and likely correlate with the Everest Series in the Mt. Everest massif. From the base of the Chekha Fm, metamorphic grade decreases to slates and unmetamorphosed rocks of the Tethyan sedimentary series (TSS) with increasing elevation. The Chekha Fm is separated from both the GHS and the overlying TSS by normal-sense shear zones related to the south Tibetan Detachment zone (STDZ).

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.