~(39)Ar-~(40)Ar geochronology of high-pressure/low-temperature blueschist and eclogite in the North Altyn Tagh and their tectonic implications

Blueschists and eclogites are newly recognized in the Hongliuquan area,North Altyn Tagh. These blueschists and eclogites,together with metapelites,calcareous schists and quartz schists,form part of a coherent high-pressure (HP)/low-temperature (LT) metamorphic terrane,which occurs as a tectonic slab in faulted contact with ophiolitic melanges. P-T estimates show that the peak pressure-temperature metamorphic conditions for the eclogites were 2.0-2.3 GPa and 430-540℃. Phengite from an eclogite sample and paragonite from a blueschist sample were selected for Ar-Ar dating. Phengite from eclogites yields an Ar-Ar plateau age of (512±3)Ma and an isochron age of (513±5)Ma,and paragonite from blueschists gives an Ar-Ar plateau age of (491±3)Ma and an isochron age of (497±10)Ma. These new age data suggest that the North Altyn Tagh HP/LT metamorphic belt probably formed earlier than the North Qilian HP/LT metamorphic belt and imply the existence of diachronism of subduction of the Early Paleozoic oceanic crust beneath the North Altyn Tagh-North Qilian.