Carboniferous-Early Permian Magmatic Evolution Of The Southern Bogda Range(Xinjiang,Northwest China):Implications For The Late Paleozoic Accretionary Tectonics Of The SW Central Asian Orogenic Belt

The Bogda Range is located in the eastern segment of the Chinese Tianshan orogenic belt,separates the Junggar Basin and Altai Orogen to the north from the Tianshan Orogen and Turpan-Hami Basin to the south.It is a key area for understanding the Paleozoic geodynamic evolution of the CAOB.Several different viewa about the Late Paleozoic tectonic environment of Bogda range,including island arc,intracontinental rift,and mantle plumbe related large ignous province,etc.In particular,the geochemical characteristics and tectonic attributes of the Carboniferous to Permian volcano rocks at the northerm and southern margin of Bogda are still in hot debate.In order to furthr understading the formation mechanism of the late Paleozoic Bogda volcanics,three sections of the northern and southern margin of Bogda has been studied,in terms of field investigation,petrology,geochemistry and isotopic analysis in detail.Following results are obtained:(1)The Carboniferous and Permian volcanic and sedimentary sequences,and present new zircon U-Pb ages and whole-rock geochemical data of volcanic rocks of the Daheyan section,southern Bogda Range.One Carboniferous rhyolite is dated at 298±8 Ma(U-Pb zircon),and a Permian basalt yielded many Proterozoic zircon xenocrysts,while its maximum age of?297 Ma is based on the detrital zircon ages of the sandstone that stratigraphically underlies it.These volcanic rocks belong to calc-alkaline series.(2)The geochronological data from the Mulei section,northern Bogda range,display the minimum Zircon U-Pb age of diabase is 306 Ma,and 329 Ma for the trachy basalt,and the emergence of Paleoproterozoic zircon xenoliths.Sandstones in lower strata showing the depositional age limit of 299 ± 5 Ma.(3)Most of Proterozoic xenolith zircons in the Carboniferous and Early Permian magmatic rocks show generally positive ?Hf(t)values,suggesting possible existence of Proterozoic juvenile crustal rocks in the basement of the Bogda arc.Based on the above result,we summarize substantial amout of published geochemical and isotopic data from Bogda volcanic and sedimentary rocks,and carried them on the comprehensive interpretation.According to the available data,the magma activity in the region lasted between 350 Ma and 280 Ma.To to the regional tectonic magmatic evolution,we drew the following conclusions:(1)the Carboniferous to Early Permian magmatic rocks of the Bogda Range generally show consistent continent-based arc-type features.Increasing mantle input through time suggests intra-arc extension within the mature active margin.The localized occurrence of Early Permian alkaline pillow basalts and deep water sediments close to the major shear zone advocate a transtensional crust thinning due to the transition from Carboniferous convergence into Permian transcurrent tectonics;(2)occurrence of a substantial number of Proterozoic zircon xenocrysts in Carboniferous magmatie rocks,and detrital zircon age patterns from clastic sediments suggest a continental or transitional nature for the basement of the Bogda Arc;(3)subduction terminated in the Bogda area prior to the accumulation of Middle Permian to Jurassic terrestrial sediments,without any evidence for collision.As for further instruction,we also collect and summarize the geochemical,geochronological data of Carboniferous-Permian volcanic and sedimentary rocks from the Junggar basin,East Junggar and North Tianshan,have found that there are both similarities and differences with their geochemical characteristics and tectonic attributes.Paleoarchean to Neoproterozoic(600-3200Ma)zircon xenocrysts have discovered in the sedimentary rocks of these regions,suggesting that they may be somehow related to the Archean basement of the ancient continental crust.Therefore,whether an ancient basement of the Bogda orogenic belt exists is still worth giving more research and exploration.