THE MINERALOGY, PETROLOGY, METAMORPHIC PTDT TRAJECTORY AND EXHUMATION MECHANISM 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, laminae and bedded slabs or sheets in greenschist facies country rocks. A few dismembered ophiolitic nappe slices and marble lenses are interlayered in the high-P/T metamorphic belt. Metabasite blueschists are mainly E-type MORE, less commonly N-type. The protoliths of the meta-sedimentary rocks are graywackes, which contain volcaniclastic material of E-type MORBs, and probably some continental basement materials, derived from the Tarim plate. These protoliths may represent subduction complexes or meta-melanges, which formed within an accretionary wedge on the southern side of the Yili-Central Tianshan plate (YCTP) at the end of the early Paleozoic. The mineral assemblages include sodic amphibole (mainly crossite) + garnet + phengite + epidote + chlorite + albite, estimated to have crystallized at 450 degrees C and 9-10 kbar. These conditions correspond to the epidote-blueschist facies and are the peak metamorphic conditions. The prograde metamorphism is represented by lawsonite-blueschist facies and the retrograde is greenschist facies. The metamorphic PTDt trajectory of the blueschists runs clockwise and reveals that the metamorphism revolved from laumontite facies (?), lawsonite-blueschist facies (?), epidote-blueschist facies (408 Ma?) to greenschist facies (345 Ma), during the creation and exhumation of the blueschists in the south Tianshan suture. We have recognized six stages of deformation in the blueschists. The earlier three stages were concomitant with metamorphism, the later three postdated the metamorphism. On the basis of the deformation characteristics of the blueschists and coherent rocks, the regional geology and the metamorphic PTDt path we discuss the exhumation mechanism of the blueschists. The post-collisional ductile thrust within the suture probably exhumed the blueschists and coherent rocks, allowing them to escape retrograde overprinting in the greenschist facies. The influence of erosion on the exhumation of blueschists could not be considered directly and quantitatively because of the absence of Devonian strata and the few exposures of Early Carboniferous elastic rocks, all without detritus of blueschists.