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 carbonate that comprises the upper 125 meters of Mount Everest. Ti-in-biotite geothermometry, electron backscattered diffraction analysis, correlation of microstructures to specific dislocation creep regimes, and geochemical data have been used to infer the metamorphic and deformational history of this formation. Results show that the Qomolangma Formation comprised a zone of distributed ductile shear which was partially overprinted by brittle deformation associated with exhumation and cooling of the Greater Himalayan slab. Samples preserve a stronger shear fabric toward the top of Mount Everest, raising questions about the location and nature of the Qomolangma detachment, a part of the South Tibetan Detachment System believed to be at the base of the Qomolangma Formation. Furthermore, our data show a gradation in 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 with significant grain size reduction of calcite (~12 µm) 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 (~51µm) and widespread dynamic recrystallization of quartz; here, metamorphic temperature has been calculated at ~500-600 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 dravitic tourmaline. The occurrence of these phases is interpreted to be a result of widespread metasomatism associated with the emplacement of leucogranite sills and dikes into pelitic rocks of the North Col Formation, structurally subjacent to the summit limestone. We also infer that metasomatism has 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.