Proterozoic blueschist belt in western China: Best documented Precambrian blueschists in the world

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Abstract
A coherent blueschist terrane near Aksu, northwestern China, is unconformably overlain by unmetamorphosed sedimentary rocks of Sinian age (ca. 600-800 Ma). The pre-Sinian metamorphic rocks, termed the Aksu Group, have maintained stratal continuity and were derived from shale, sandstone, basaltic volcanic rocks, and minor pelagic sediments. They have undergone multistage deformation and transitional blueschist-greenschist facies metamorphism, and are represented by strongly foliated chlorite-stilpnomelane-graphite schist, stilpnomelane-phengite psammitic schist, greenschist, blueschist, and minor quartzite, metachert, and metamorphosed ironstone. Metamorphic minerals present in blueschists include crossitic amphibole, epidote, chlorite, albite, quartz, and actinolite. Mineral parageneses and composition of sodic amphibole suggest blueschist facies recrystallization at about 4 to 6 kbar and about 300-400 °C. Post-tectonic alkali diabase dikes cut the Aksu Group. The basal conglomerate of the overlying Sinian to Eocambrian sedimentary succession contains clasts of both the metamorphic and dike rocks, clearly demonstrating that conditions required for blueschist facies metamorphism were attained at least 800 m.y. ago.

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