DIVERGENT METAMORPHISM OF THE DUNHUANG COMPLEX, NORTHWEST CHINA: IMPLICATION FOR THE TECTONIC EVOLUTION IN A SUBDUCTION CHANNEL

The Dunhuang orogenic belt (DOB) lies to the east of the Tarim craton, to the south of the Beishan orogen and to the north of the Altyn Tagh fault, and mainly comprise low- to high-grade metamorphic complex termed as the Dunhuang complex. It has long been considered as meta-recrystallized basement formed in the Early Precambrian. Nevertheless, our recent studies including the discovery of Devonian eclogite, show that this area is actually a Paleozoic orogenic belt and typifies a fossil subduction zone. Garnet-bearing granulites and amphibolites commonly occur as lenses or boudinages enclosed within metasediments, exhibiting typical block-in-matrix fabrics of tectonic mélange. Here, we report the new finding of HP mafic granulite in the northwestern DOB and discrepant metamorphic evolution of different components (or slices). Three stages of metamorphic mineral assemblages were identified in these metabasites. For two HP mafic granulites, they recorded P-T variations passing from 660-670 °C / 7.4-9.7 kbar through 820-840 °C / 13-15 kbar to 690-700 °C / 5.2-5.7 kbar. The derived metamorphic P-T paths show similar tight clockwise and the metamorphic peaks of the granulites approach the high P-T facies series, indicative of the subduction metamorphism. High resolution secondary-ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircons reveals the two granulite blocks experienced HP metamorphism in fact asynchronously at ca. 460 Ma and ca. 415 Ma, respectively. The garnet bearing amphibolite recorded a clockwise P-T path with peak condition of 620-640 °C / 9-10 kbar. Compared to the HP metamorphism, amphibolite facies metamorphism happened at ca. 415 Ma, however, at a much shallower burial depth. New data suggest that the DOB shares a similar metamorphic history of the Beishan orogen. This study recognizes a mixing of rocks from significantly different depths to create a tectonic mélange in a subduction channel. Rock slices were possibly subducted and equilibrated at various depths asynchronously, and in consequence, witnessed heterogeneities in the timing of metamorphic equilibration and P-T evolution, pointing to independent tectonic evolutions of different slices inside a subduction channel.