THE MINERALOGY, PETROLOGY, METAMORPHIC PTDT TRAJECTORY AND EXHUMATIONMECHANISM OF BLUESCHISTS, SOUTH TIANSHAN, NORTHWESTERN CHINA

The south Tianshan blueschist belt extends for about 200 km along the south Tianshan suture from Changawuzhi to the upper Kekesu river. The high-pressure-low-temperature (high-P/low-T) metamorphic rocks contain sodic amphibole, garnet, phengite, albite, quartz, epidote, chlorite and stilpnomelane. These blueschists occur as small blocks, lenses, la minae and bedded slabs or sheets in greenschist facies country rocks. A few dismembered ophiolitic nappe slices and marble lenses are interl ayered in the high-P/T metamorphic belt. Metabasite blueschists are ma inly E-type MORE, less commonly N-type. The protoliths of the meta-sed imentary rocks are graywackes, which contain volcaniclastic material o f E-type MORBs, and probably some continental basement materials, deri ved from the Tarim plate. These protoliths may represent subduction co mplexes or meta-melanges, which formed within an accretionary wedge on the southern side of the Yili-Central Tianshan plate (YCTP) at the en d of the early Paleozoic. The mineral assemblages include sodic amphib ole (mainly crossite) + garnet + phengite + epidote + chlorite + albit e, estimated to have crystallized at 450 degrees C and 9-10 kbar. Thes e conditions correspond to the epidote-blueschist facies and are the p eak metamorphic conditions. The prograde metamorphism is represented b y lawsonite-blueschist facies and the retrograde is greenschist facies . The metamorphic PTDt trajectory of the blueschists runs clockwise an d reveals that the metamorphism revolved from laumontite facies (?), l awsonite-blueschist facies (?), epidote-blueschist facies (408 Ma?) to greenschist facies (345 Ma), during the creation and exhumation of th e blueschists in the south Tianshan suture. We have recognized six sta ges of deformation in the blueschists. The earlier three stages were c oncomitant with metamorphism, the later three postdated the metamorphi sm. On the basis of the deformation characteristics of the blueschists and coherent rocks, the regional geology and the metamorphic PTDt pat h we discuss the exhumation mechanism of the blueschists. The post-col lisional ductile thrust within the suture probably exhumed the bluesch ists and coherent rocks, allowing them to escape retrograde overprinti ng in the greenschist facies. The influence of erosion on the exhumati on of blueschists could not be considered directly and quantitatively because of the absence of Devonian strata and the few exposures of Ear ly Carboniferous elastic rocks, all without detritus of blueschists.