Geochemical characteristics and tectonic implications of HP-UHP eclogites and blueschists in southwestern Tianshan, Chin

Four rock assemblages in correspondence with two different tectonic settings have been recognized in the NEE-SWW extending HP-UHP metamorphic belt in southwestern Tianshan, northwest China. Eclogite assemblage EC1 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 BS1 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 margin and trench sediments are eroded and enwrapped in the subducting mass, they are altogether subjected to high to ultrahigh pressure metamorphism and subsequent exhumation towards surface. The HP-UHP metamorphic belt is thus interpreted as a subduction-accretionary complex formed by tectonic juxtaposition and imbrication of seamount, seafloor, trench and volcanic arc sequences during oceanic crust subduction.

作 者： AI Yongliang ZHANG Lifei LI Xuping QU Junfeng   作者單位： AI Yongliang,ZHANG Lifei,QU Junfeng(MOE Key Laboratory of Orogenic Belt and Crust Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871,China)

LI Xuping(School of Geosciences and Resources, China University of Geosciences, Beijing 100083, China) 

刊 名： 自然科學進展（英文版）  SCI 英文刊名： PROGRESS IN NATURAL SCIENCE  年，卷(期)： 2006 16(6)  分類號： N1  關鍵詞： southwestern Tianshan   HP-UHP metamorphic belt   subduction-accretion complex   trace element geochemistry