Deformation and metasomatism of the Qomolangma Formation: a geochemical and microstructural analysis of the summit limestone, Mount Everest, Nepal

Abstract

This study is based on a suite of new samples that represent a structural transect across the Qomolangma Formation, an Ordovician limestone that comprises the uppermost ~125 meters of Mount Everest. Past studies posit Everest's summit limestone to be mildly sheared and unmetamorphosed; however, this study shows that the Qomolangma Formation has endured more strain and metamorphism than previously thought. Ti-in-biotite and Ti-in-quartz geothermometry, electron backscattered diffraction analysis, and geochemical analysis of tourmaline has been used to infer the metamorphic and deformational history of this formation. Results show that the Qomolangma Formation has experienced significant ductile shear throughout, though samples preserve a stronger shear fabric toward the top of the formation. Furthermore, our data show a gradation in the degree of metamorphism across the Qomolangma Formation, increasing toward the base of the unit. Samples collected from the structural top of the formation (Summit samples) have a penetrative foliation with significant grain size reduction of calcite (~8 microns) and yield temperature estimates of > or = 250°C. In contrast, samples from the base (South Summit samples) are distinguished by an increase in recrystallized grain size of calcite (~66 microns) and yield temperatures estimates of 500-600°C. Another important difference between Summit and South Summit samples is aggregates of the coexisting minerals muscovite, chlorite, tourmaline, rutile and biotite found throughout South Summit samples. Geochemical analysis of tourmaline suggests these minerals crystallized from metasomatic fluids (hydrothermal fluids) released from leucogranite bodies emplaced into rocks structurally subjacent to the summit limestone. As a result, metasomatic fluids caused localized metamorphism at the base of the Qomolangma Formation, producing the significant increase in metamorphic grade observed in South Summit samples. It is interpreted that the fabric preserved in Summit samples was ingrained during initial thrust faulting in the Eocene to Oligocene, whereas the fabric and metamorphic grade observed in South Summit samples is the result of detachment faulting and leucogranite emplacement in the Early Miocene. Lastly, microstructural observations suggest that metasomatic fluids may have promoted faulting on the Qomolangma detachment, a splay of the South Tibetan Detachment believed to be at the base of the Qomolangma Formation.