GEOCHEMISTRY OF ECLOGITE AND GNEISS FROM THE TSO MORARI UHP TERRANE

Ultrahigh pressure rocks occur in two locations in the Himalaya, separating Indian and Asian rocks: the Kaghan Valley of northern Pakistan, and near Tso (Lake) Morari in northwestern India. UHP metamorphism in Tso Morari rocks are confirmed by preserved coesite. UHP rocks crop out within dominantly felsic Indian supracrustal rocks. The sample set is a 17 sample, 15 m traverse across a contact in the UHP unit between an eclogite boudin and host quartzofeldspathic gneiss. Rocks were sampled very densely at the contact to order to fully capture any chemical exchange between the units. This approach allows a full investigation of typically heterogeneous, metasomatic reactions, and also serves to identify and characterize the retrograde overprint common to high pressure terranes. The whole-rock chemistry of rocks in the contact zone supports a significant role for metasomatism at the reactive interface between eclogite and gneiss. The concentrations of fluid mobile trace elements such as potassium, cesium, barium, rubidium, and H₂O are enriched; and the Ba/Rb and Cs/Rb ratios are systematically lower in the rocks approaching the eclogite-gneiss contact. Major element chemistry indicates that the samples in the contact region are heterogeneous and have intermediate compositions to the eclogite and gneiss, suggesting metasomatic alteration at the reactive, lithologic interface. Preliminary oxygen isotope data was collected from quartz, white mic and garnet in eclogite and gneiss from the Tso Morari traverse. Select samples were crushed and clean mineral separates were picked. White mica δ¹⁸O is 10.5‰ in the gneiss, 8‰ in the eclogite, and an intermediate 9.5‰ in the contact region. The whole rock geochemistry and oxygen isotope data from the traverse support metasomatic recrystallization at the contact between eclogite and host gneiss.