PSEUDOTACHYLYTE RECORDING COSEISMIC FRACTURING DURING UHP SLAB EXHUMATION, WESTERN TIANSHAN

Dark pseudotachylyte veins are hosted by pelitic schists from the Western Tianshan Ultra-High Pressure Metamorphic (UHPM) Belt, which are millimeter-to-centimeter in width and crosscut the foliation and garnet porphyroclasts of the host schist. The matrix of pseudotachylyte consists of very fine-grained (<0.1 μm) fibrous intergrowth of muscovite + chlorite ± biotite that encloses lenticular feldspar and quartz augen, irregular carbonates, and minor garnet fragments. The pseudotachylyte veins are bordered by cataclasite selvages that consist of a greater abundance of angular garnet and omphacite clasts. The pseudotachylyte is trachyte-rhyolite in composition, having higher alkali and lower mafic concentrations than the cataclasite. Its rare-earth element (REE) compositions show enrichment in light REEs, pronounced negative Eu anomaly, and flat heavy REE patterns. The bulk-rock chemistry indicates that the pseudotachylyte developed in response to not only mechanical pulverization but also melting that led to chemical differentiation.

In the schist, garnet, omphacite and glaucophane are shattered but largely retain their crystallographic continuity; phengite flakes display dense micro-kinks. The garnet zoning pattern is similar to previously documented in this belt, and records a UHP peak condition (550 °C, >3 GPa) that is followed by near-isothermal decompression. The phengite in the host schist has silica contents as high as 3.51 (per 11 O) in contrast to the cataclastite veins (3.31-3.36). The plagioclase augens in the pseudotachylyte have higher Ca and K contents than the retrograde plagioclases in the cataclasite or schist and correspond to temperatures ~650 °C according to the immiscibility of ternary feldspars. The phase relation analyses as well as the textures reveal a transient thermal pulse of 650-700 °C at 1.5 GPa near the solidus. This temperature, >100 °C higher than the regional retrograde background, is likely a quenching temperature as the incipient frictional melting commonly entails large thermal oversteps. The pseudotachylyte locates at an inferred structural boundary between a UHP/HP and an HP slices, so might reflect a local seismic faulting event between thrust slices during slab exhumation to lower-crustal depth (~45 km).