Raman geobarometry of quartz inclusions in eclogitic garnet constrains the metamorphic evolution and exhumation of the Sumdo oceanic slab in Tibet

Abstract Due to the high density at mantle depths, subducted oceanic crust rarely exhumes to the surface of Earth, making it challenging to determine the evolution and dynamic behavior of oceanic crust. The Sumdo eclogite-bearing metamorphic belt, located between the north and south Lhasa terranes in Tibet, is a natural laboratory for studying the metamorphic evolution of oceanic eclogites. We combined phase equilibrium modeling with detailed petrological investigations for the newly discovered Xilang eclogite from this belt and obtained near-peak metamorphic conditions of 539 °C at 1.84 GPa and a steep exhumation P–T path. These values are much lower than published for the other areas of this belt. Quartz inclusions sealed in garnet from high-pressure rocks are known to preserve residual pressures during exhumation, which can be used to reexamine the peak metamorphic conditions of eclogites in the Sumdo metamorphic belt. We analyzed quartz from three different areas in the belt, including Xilang, Jilang, and Xindaduo, with laser Raman micro spectroscopy. Xilang eclogite preserved the highest residual pressure of 0.53 ± 0.07 GPa in the quartz stability field. A systematic comparison of the residual pressures of different eclogites revealed that not only the peak metamorphic conditions but also the exhumation rate of the host rocks controlled the residual pressure. Hence, the Xilang eclogite may record lower peak P–T conditions, because it exhumed differently than the eclogites from the other areas in the Sumdo metamorphic belt. Combined with numerical modeling, the residual pressures obtained by Raman spectroscopy may be a new way to compare the metamorphic conditions of eclogites and to obtain geochronological information in addition to isotopic investigations.