LATE TRIASSIC VOLCANICS IN GREAT XING’AN RANGE, EASTERN SEGMENT OF THE CENTRAL ASIAN OROGENIC BELT AND THEIR IMPLICATIONS

The Great Xing’an Range, situated between the Siberian and Sino-Korean (North China) Cratons, is regarded as the eastern segment of the Central Asian Orogenic Belt. This range is the hot spot of studying tectonic evolution, magmatic activity and metallogenic event of northeast Asia, which has experienced and documented the geological processes in different periods and different tectonic domains. It has been previously suggested that this part of CAOB is a segment that has experienced two stages of evolution under different tectonic systems. In the Paleozoic, tectonic development was controlled by the evolution of the Paleo-Asian Ocean between the Siberian and North China Cratons. However, in the Triassic- Early Jurassic, there was a switch, with the development now controlled by Paleo-Pacific subduction in the east and effects of Mogol-Okhotsk Ocean closure in the west of NE China. In the past years, researchers from different areas have carried out plenty of data and competing interpretations of the geological evolution of this range. However, in the transition period, only a few igneous rocks were reported. Fortunately, we found a series of complex volcanics in Moguqi area. In addition, these volcanics locate among the northeast Hegenshan-Heihe tectonic belt, which is known as an important tectonic belt in NE China. So the petrology, geochemistry, petrogenesis, forming age and tectonic setting of these volcanics have significant meaning to reveal the tectonic features of the eastern segment of CAOB and the characteristics of transition of two different tectonic systems.

Based on the petrology features and TAS diagram, we classify these volcanics into three groups: basaltic-trachyandesites, trachytes and trachydacites. Their REE patterns decline slightly to right and they are enrichment in light REE and depleted in HREE and HFSE. Through our integrated study, we think these complex volcanic rocks formed in the whole extensional setting after the collision orogenesis. The decrease of the pressure in lithosphere mantle caused the upwelling of the mantle, which melted and mixed the residual oceanic slab causing the eruption of these volcanics. So we eventually draw the conclusion that the whole tectonic setting in the transition period of two different tectonic systems in eastern CAOB is extensional.