A cold Early Palaeozoic subduction zone in the North Qilian Mountains, NW China: Petrological and U-Pb geochronological constraints

The north Qilian high-pressure (HP)/low-temperature (LT) metamorphic belt is composed mainly of blueschists, eclogites and greenschist facies rocks. It formed within an Early Palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved HP/LT metamorphic belts in China. Here we report new lawsonite-bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the North Qilian Mountains. Five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet, indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks. Peak pressure conditions estimated from lawsonite omphacite-phengite-garnet assemblages were 2.1-2.4 GPa at temperatures of 420-510 degrees C, in or near the stability field of lawsonite eclogite, and implying formation under an apparent geothermal gradient of 6-8 degrees C km(-1), consistent with metamorphism in a cold subduction zone. SHRIMP U-Pb dating of zircon from two lawsonite-bearing eclogitic metabasites yields ages of 489 +/- 7 Ma and 477 +/- 16 Ma, respectively. CL images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism. An age of 502 +/- 16 Ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph-bearing eclogite, which is in agreement with the formation age of Early Ordovician for some ophiolite sequences in the North Qilian Mountains, and may be associated with a period of oceanic crust formation. The petrological and chronological data demonstrate the existence of a cold Early Palaeozoic subduction zone in the North Qilian Mountains.