Metamorphic evolution and geochronology of the tectonic melange of the Dongbatu and Mogutai blocks, middle Dunhuang orogenic belt, northwestern China

Mafic granulite and amphibolite in the Dongbatu and Mogutai blocks, middle Dunhuang orogenic belt, northwest China, southernmost Central Asian orogenic belt, occur as lenses within the matrix of metapelite and marble, exhibiting typical block-in-matrix fabrics of tectonic melange. Three stages of metamorphic mineral assemblages were identified in these lenses. Clockwise metamorphic pressure-temperature (P-T) paths were obtained through geothermobarometry and thermodynamic pseudosection modeling, passing from 656 degrees C and 10.9 kbar through 830 degrees C and 16.5 kbar to 657 degrees C and 4.9 kbar for the mafic granulite, and from 564-645 degrees C and 3.2-9.6 kbar through 634727 degrees C and 6.1-14.2 kbar to 615-664 degrees C and 3.2-4.2 kbar for the amphibolites, respectively. Metamorphic peak P-T conditions in the metapelitic country rocks were estimated to be 635-675 degrees C and 6.0-6.9 kbar. The metamorphic peak of the mafic granulite approaches the high P-T facies series, indicative of a subduction zone. Secondary-ion mass spectrometry and laser ablationinductively coupled plasma-mass spectrometry U-Pb dating of metamorphic zircons suggests that the metamorphic event occurred between ca. 420 and 372 Ma. These data further certify that the subduction of the continental margin and subsequent uplift of the Dunhuang orogenic belt represent a long-lived tectono-metamorphic event in the Paleozoic.