METAMORPHISM AND EXHUMATION OF ULTRAHIGH PRESSURE ECLOGITES AND GNEISSES, NORTH QAIDAM, CHINA

Early Paleozoic orogenesis in the Qilian Shan, China, produced several high and ultrahigh pressure metamorphic belts. In the North Qaidam belt (Luliang Shan, 38°N, 95°E), eclogite blocks occur in leucocratic to melanocratic gneisses. Leucocratic gneisses contain K feldspar, quartz, plagioclase, garnet, muscovite, biotite, hornblende, and epidote. Coesite inclusions in zircons from the gneisses in the region (Yang et al., 2001) indicate ultrahigh-pressure metamorphism. Mafic portions of the gneisses are characterized by epidote-amphibolite facies assemblages (hornblende, plagioclase, rutile, epidote, quartz ±biotite, chlorite). Eclogite blocks with 443-494 Ma zircon ages display three stages of mineral growth. Stage I minerals occur as inclusions in garnets, and include blue amphibole, omphacite, epidote, rutile, white mica, quartz, and zircon. Stage II minerals include garnet, omphacite, rutile, and quartz and represent the main eclogite-facies assemblage. Garnets are zoned from Ca- and Fe-rich cores to more magnesian rims. The stage III assemblage is locally symplectic, retrograde replacement of stage II, including amphibole zoned from taramite to hornblende, white mica, plagioclase (An7), ilmenite, quartz, sphene, epidote, ±chlorite and calcite. Stage II cpx and garnet commonly display dense, random networks of microfractures filled by stage III minerals, suggesting that retrograde metamorphism was in part controlled by brittle to semibrittle deformation associated with fluid ingress. High-F sphenes (1.5 wt%) imply elevated F₂ and reduced H₂O fugacities. Stage I minerals correspond to the epidote-blueschist facies (<600°C, 1.5-2.3 GPa). Stage II conditions were 600-700 °C, >2 GPa. Final reequilibration in the epidote-amphibolite facies (0.5-1.0 GPa, ~600 °C) is implied by assemblages in eclogites and gneisses. Results indicate a clockwise P-T path, with nearly isothermal decompression. Zircon occurs only in stage I, suggesting that 443-494 Ma ages immediately predate peak eclogite-facies conditions. Our data are consistent with (1) deep burial of tectonically eroded lower Qaidam crust during Ordovician, south-directed subduction of the Qilian terrane, and (2) rapid exhumation to mid-crustal epidote-amphibolite facies, perhaps by diapiric flow.