Lawsonite-bearing eclogites in the north Qilian and north Altyn Tagh: Evidence for cold subduction of oceanic crust.

Lawsonite-bearing eclogites are recognized respectively in the north Qilian (NQL) and north Altyn Tagh (NAT), based on petrography and mineral chemistry. Lawsonite coexists with omphacite and phengite as inclusions in garnet, indicating eclogite-facies metamorphism. Peak metamorphic conditions for equilibrium lawsonite-omphacite-phengite-garnet peak assemblages are estimated to be 2.1 - 2.4 GPa and 420-500°C, which is in the stability field of lawsonite eclogite-facies. A low geotherm of 6-8 °C/km is referred, which is in consistent with a cold subduction. The occurrences of lawsonite-bearing eclogites in both NQL and NAT provide further evidence that the NAT HP/LT metamorphic belt is possibly the northwestward extension of the NQL HP/LT metamorphic belt offset by the Altyn Tagh left-lateral slip fault. The NQL and NAT lawsonite-bearing eclogites are formed by subduction of oceanic crust before collision of the Alashan and Qilian blocks in early Paleozoic. [ABSTRACT FROM AUTHOR]