Geochemical characteristics and tectonic implications of HP-UHP eclogites and blueschists in Southwestern Tianshan, China*

Four rock assemblages in correspondence with tow different tectionic settings have been recognized in the NEE-SWW extending HP-UHP metarmorphic belt in southwestern Tianshan, northwest China, Eclogite assemblage ECI is geochemically akin to alkaline within-plate oceanic island basalt (OIB). EC2 shows affinity to enriched mid-oceanic ridge basalt (EMORB). Rare earth element (REE) and other immobile trace element characteristics of blueschist assemblage BSI resemble those of normal mid-oceanic ridge basalt (NMORB). These three assemblages are likely formed on a seamount setting, and the prevalent presence of carbonate minerals and omphacite quartzite stripes/gobbets suggests ancient pelagic sediments including marls are probably developed upon the basaltic seamount. Whereas the geochemical characteristics of BS2 assemblage are of volcanic arc basalt-type. The seamount with the pelagic sediments on it is brought into the subduction zone, and volcanic arc basalts formed on the active continental marg...