Metamorphism and Exhumation of Very High-Pressure Eclogites, North Qaidam, China

The North Qaidam metamorphic belt (38° N, 95° E) is located within the Early Paleozoic Qilian Shan orogeny. Here mafic eclogite occurs as discrete blocks in leucocratic gneisses of epidote-amphibolite grade. The gneisses contain the assemblage: K-feldspar, quartz, plagioclase, garnet, muscovite, biotite, hornblende and epidote. The eclogite blocks are not of uniform grade; they vary in extent of retrogression and fluid alteration with no apparent correlation to structural position. The most retrograded blocks do not contain eclogite facies minerals and are amphibolite to epidote-amphibolite grade. They have the assemblage: garnet, Ca-amphibole, plagioclase, sphene, chlorite +/-white mica, epidote. In blocks which display only partial replacement of the eclogite facies assemblage, three phases of mineral growth can be identified. The first defines the prograde path and is found as inclusions in garnet (quartz, amphibole, plagioclase, rutile, epidote, sphene, ilmenite +/-pyroxene, apatite, zircon, white mica and K-feldspar). Based on textural evidence, the main eclogite facies assemblage is garnet, omphacite, rutile, quartz, +/-phengite. The garnets contain abundant inclusions, but omphacite is inclusion free. The final stage is marked by the retrograde introduction of fluids while still in the eclogite facies leading to the growth of high-pressure amphiboles, taramite and katophorite zoned to pargasite, containing inclusions of omphacite. Upon further decompression, fine-scale symplectites of albite and augite followed by albite and hornblende grew in grain boundries. The garnets exhibit numerous micro-fractures implying brittle deformation occurred during exhumation aiding fluid flow. The prograde path corresponds to the epidote-blueschist facies (<625° C, 2.4-1.5 GPa). Peak eclogite conditions are just below the coesite-quartz phase boundry (700° C, 2.4GPa). Final reequilibration conditions correspond to the epidote-amphibolite facies (550-450° C, 1.0-0.8 Gpa), which is the same as in the surrounding gneiss. The P-T path defines an open clockwise loop with nearly isothermal decompression. The variability observed in the eclogite blocks can be explained by discrete, non-uniform fluid influx during exhumation and probably does not represent different P-T histories. Our data are consistent with deep burial of continental materials during south-dipping subduction of the North China plate beneath the Qilian terrane and rapid exhumation from 80 km to mid-crustal levels, perhaps by diapiric flow. Based on regional correlation this event occurred during the development of the Ordovician-Silurian Qilian magmatic arc, and is apparently unrelated to continent-continent collision.