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A New HP/LT Metamorphic Terrane in the Northern Altyn Tagh, Western China

Field observations and petrological data suggest a new high-pressure/low temperature (HP/LT) metamorphic terrane in the northern Altyn Tagh. Newly recognized eclogites, blueschists, and metapelites constitute part of a coherent HP/LT metamorphic terrane that occurs as a tectonic slab in an ophiolitic mélange. Geobarometry and geothemometry show that peak metamorphic conditions of the eclogites were T = 430-540°C and P = 20-23 kbar. Petrographic data, P-T estimates, and amphibole compositions indicate distinctly different metamorphic histories for two outcrops about 1.2 km apart. For blueschists from outcrop I, the retrograde P-T path is a relatively straight line directed towards the greenschist facies, suggesting simultaneous cooling and decompression. In contrast, interlayered blueschist and eclogite in outcrop II underwent isothermal decompression from eclogite/blueschist facies to epidote-amphibole facies conditions during exhumation. This implies that rocks from these two outcrops experienced different tectono-thermal histories. Preliminary data regarding geological setting, lithologies, and P-T estimates reveal significant similarities between the HP/LT metamorphic terranes in the north Altyn Tagh and the north Qilian Mountains, and suggest that they may be parts of the same early Paleozoic HP/LT belt, subsequently displaced by the Altyn Tagh fault.