DELINEATING THE METAMORPHIC AND DEFORMATIONAL HISTORY OF THE QOMOLANGMA FORMATION, MOUNT EVEREST, NEPAL

This study is based on new samples collected from the Qomolangma Formation, an Ordovician limestone which comprises the upper 125 meters of Mount Everest. Phase equilibria geothermometry, electron backscattered diffraction analysis and correlation of microstructures to specific dislocation creep regimes, have been used to infer the metamorphic and deformational history of this formation. Results show that the Qomolangma Formation was a zone of distributed ductile shear that was partially overprinted by brittle deformation associated with exhumation and cooling of the Greater Himalayan Slab. Our data show a gradation in strain and metamorphism across the Qomolangma Formation, increasing toward the exposed base of the unit near the South Summit of Everest. Samples collected from the structural top of the formation have a penetrative mylonitic foliation and yield temperature estimates of ~250 ⁰C. In contrast, samples from the base are distinguished by a significant increase in recrystallized grain size of calcite and widespread dynamic recrystallization of quartz; here, metamorphic temperature has been calculated at ~400 ⁰C. Coexisting aggregates of muscovite, chlorite and biotite are found within samples from near the base of the Qomolangma Formation, and occur in association with tourmaline. The occurrence of these phases within a carbonate rock is interpreted to be a result of widespread metasomatism associated with the emplacement of leucogranite sills into pelitic rocks of the North Col Formation, structurally subjacent to the Qomolangma Formation. We also infer that metasomatic processes related to these phases played an important role in thermally altering the Qomolangma Formation, resulting in the transitional nature of metamorphic grade across this formation. Numerous studies posit that the Qomolangma Formation is composed of 'unmetamorphosed' Tethyan limestone, however, our findings present an altogether new understanding of this formation.