Geochemistry, Sm-Nd and U-Pb isotope study of gneisses (schists) enclosing eclogites in the North Qaidam Monutains--deeply subducted Precambrian metamorphic basement?
柴北缘夹榴辉岩的片麻岩（片岩）地球化学、Sm-Nd和U-Pb同位素研究——深俯冲的前寒武纪变质基底

Orthogneisses and paragneisses ( schists) enclosing eclogites crop out along the Yuqia-Xitieshan-Shaliuhe of the North Qaidam Mountains ( NQD) . Our geochemical analyses and Sm-Nd isotope data indicate that the Orthogneisses are similar in geochemical composition and Nd model ages( 1. 88 ~ 2. 18Ga) to the paragneisses. In combination of geological relationship that the paragniesses occur as xenolith within the orthogneiss, the Orthogneisses were probably derived through in situ melting of the paragneisses ( schists). Zircon U-Pb data suggest that the magma of the orthogneiss was formed at 952Ma ago . Moreover, the gneisses ( schists) enclosing eclogites also show similar characteristics in Sm-Nd isotope composition and Nd model TDM (1. 87 ~ 2. 26Ga) to those of the high-grade metamorphic rocks enclosing no eclogites ( non-eclogite bearing basement gneisses) in NQD and metamorphic basement rocks of the Qilian block. This implies that they have close affinity, and probably are derived from same metamorphic basement with a crust formation age of late Paleoproterozoic. However, in contrast to the high-grade metamorphic rocks enclosing no eclogites (non-eclogite bearing basement gneisses) in NQD and basement gneisses in the Qilian orogenic belt, the gneisses enclosing eclogites ( eclogite bearing gneisses) were obviously overprinted by early Paleozoic metamorphism as indicated by zircon U-Pb data of a lower intercept age of 478 44Ma which is in agreement with the age of metamorphic zircon from eclogite within error range. In addition, newly reported coesites in paragneisses enclosing eclogites in Dulan area also suggest that the gneiss and enclosed eclogite as a whole underwent early Paleozoic HP-UHP metamorphism. Therefore, although the gneisses enclosing eclogites have similar early evolution histories to adjacent Precambrian high-grade metamorphic basement rocks, they experienced deep subduction together with the enclosed eclogite during early Paleozoic orogeny.