Late Palaeozoic Extensional Event And Its Evolution History In The Middle Part Of Xing’an Mongolian Orogenic Belt

The Central Asian Orogenic Belt(CAOB) is a giant accretionary orogen, lying between the Siberian and Tarim-North China cratons. The eastern CAOB extending across Inner Mongolia and Northeast China is named the Xing’an-Mongolian Orogenic Belt(XMOB). The debate on tectonic evolution of the XMOB is concentrated on the closure time of the Palaeo-Asian Ocean(PAO). Some authors considered the PAO had closed before the Carboniferous, while some proposed it didn’t close until the late Permian. Therefore, research on the Carboniferous-Permian igneous rocks and sedimentary rocks will place better constrains on the tectonic evolution of the XMOB during the late Palaeozoic.Previous published data revealed that the PAO was divided into two separate oceanic basins by Xilinhot Block. During the late Silurian, southern oceanic basin closed along Solonker-Xar Moron suture zone and gave rise to the Southern Orogenic Belt. During late Devonian, the northern oceanic basin closed along Erenhot-Hegenshan suture zone and gave rise to the North Orogenic Belt. Late Devonian continental molasse basin developed in middle part of XMOB. The continued collision lead to regional uplift that a hiatus formed during early Carboniferous and a regional angular unconformity developed between the Upper Carboniferous and its underlying geological body.Based on comprehensive petrology, chronology and geochemical study of Late Carboniferous to early Permian igneous rocks and lithofacies palaeogeography of the sedimentary strata, we found out that the late Palaeozoic strata distribute in bands and indicate a tendency of water depth changing from shallow to deep and again to shallow. These strata are dominated by terrigenous detrital materials, showing characteristics of near provenance and fast deposit. Igneous rocks also distribute in bands. Bimodal volcanic rocks developed along Xi Ujimqin Qi, Sonidyouqi and Mandula, companied by S-type granites emplacement, suggesting that the tectonic system had changed from compressional to extensional during late Carboniferous and gave rise to an extensional tectonic belt.The development of extensional tectonic belt reached its peak during the early Permian. The emergence of argillaceous rocks and Bouma sequence from Shoushangou Formation indicates the max of water depth. Mafic-ultramafic rocks developed in some intensely extensional area, such as Daqing pasture-Diyanmiao in West Ujimqin, Weijing, Bulitai, Mandula and Solonker. Relatively complete ophiolite suites were distinguished in West Ujimqin and Solonker. Early Permian Dashizhai Formation from Xi Ujimqin Qi is typical bimodal volcanic rocks, while Permian volcanic rocks from Weijing are mainly composed of mafic rocks; however, Permian volcanic rocks in the study area have relatively high εHf(t) value and younger modal age, suggesting these rocks is the product of partial melting of juvenile crust.During the middle Permian, the extentiaonal tectonic belt started to shrink, followed by the decreasing of water depth. The final closure of the extensional tectonic belt occurred during the late Permian, when a tectonic mélange belt developed along Xi Ujimqin Qi, Bulitai and Weijing and carbonaceous slate and fossilised plant appeared in Linxi Formation, indicating a continental environment. The closure of the extensional belt was also documented by the syn-collisional or post-collisional granites along the north margin of the NCC.Therefore, we concluded that the early Palaeozoic and late Palaeozoic tectonic evolution of XMOB belong to two different cycles, not just related to the evolution of one ocean.