PALEOZOIC SUBDUCTION OF THE NORTHWESTERN DUNHUANG OROGENIC BELT, SOUTHERNMOST CENTRAL ASIAN OROGENIC BELT: METAMORPHISM, GEOCHRONOLOGY AND TECTONIC IMPLICATION

The Dunhuang area, northwest China, is traditionally considered as an ancient stable terrane formed in the Precambrian and its tectonic attribution is in great debate. Our recent studies, including the first discovery of Devonian eclogite, show that this area was in fact a Paleozoic orogenic belt and the metamorphic event occurred between the Silurian to Devonian. The Dunhuang Orogenic Belt (DOB) was dismembered by Cenozoic faults to discrete tectonic blocks formed at different metamorphic grades. We firstly report the finding of HP mafic granulite in the Dahongshan block, northwestern DOB, southernmost Central Asian orogenic belt (CAOB). HP granulites and (garnet-bearing) amphibolites commonly occur as lenses or boudinages enclosed within metapelite, exhibiting typical block-in-matrix fabrics of tectonic mélange. Three stages of metamorphic mineral assemblages were identified in these metabasites. Clockwise metamorphic pressure-temperature (P-T) paths were obtained through geothermobarometry and thermodynamic pseudosection modeling, passing from 660-670 °C / 7.4-9.7 kbar through 820-840 °C / 14.3-15.5 kbar to 690-700 °C / 5.2-5.7 kbar for the HP mafic granulite. The derived metamorphic P-T paths show similar tight clockwise loops including nearly isothermal decompression processes, typical of orogenic metamorphism. Secondary-ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircons indicates the HP metamorphism occurred at ca. 414-418 Ma. The protolith of one mafic granulite was revealed to be formed at ca. 710 Ma, and preliminary geochemical study shows it has mid-ocean ridge basalt (MORB) affinity, probably representing a metamorphosed fragment of the Paleozoic oceanic crust. These data further suggest that the DOB underwent subduction and subsequent exhumation caused by the closure of the Paleozoic Hongliuxia ocean (the southern segment of Paleo-Asian Ocean) during the Silurian to Devonian.