Sedimentary Evolution Of The Western Hubei Trough And Its Tectonic Implications-with Discussions On The Source Of The PTB Bentonite In South China

The Late Permian-Early Triassic Western Hubei Trough located at the northern margin of South China Craton was controlled by the Jianshi-Enshi fault.Its sedimentary succession is dominated by high TOC chert and mudstone with volcanic ash layers(bentonite)occasionally interbedded.Since the trough was formed at special geological period when Pangea supercontinent formed and PTB(Permian-Triassic boundary)mass extinction happened,its strata may contain important information for a lot of highly debated geological issues.This research presents sedimentological,petrological,mineralogical,whole-rock geochemical and(mudstone and bentonite)zircon U-Pb age,trace element and Hf isotopic data from the central and western part of the Western Hubei Trough.The sedimentary succession of the trough was measured and analyzed in detail and the source of the clastic sediment and the bentonite was located.Based on these analyses,this paper mainly discusses the tectono-sedimentological evolution of the Western Hubei Trough,the timing of the collision of the North and South China cratons and the source of the PTB bentonites in South China.The trough succession is composed of Upper Permian Dalong Formation in the lower part and Lower Triassic Daye Formation(lower part)in the upper part.The Dalong Formation is mainly composed of high TOC chert and mudstone which were deposited at deep-water intra-platform basin below the storm wave base.The lower part of the Daye Formation is mainly composed of low TOC mudstone and limestone which were deposited at shallow-water intra-platform basin above the storm wave base.In the western Hubei area,facies differentiation occurred above the Upper Permian Wujiaping Formation carbonate rocks since late Wuchiapingian(~ 255 Ma)when carbonate platform margin facies belt,such as platform margin reefs,formed at the western margin of the study area,while the main part of the study area was covered by deep-water intra-platform basin high TOC chert and mudstone.This period represents the opening and expanding stage of the Western Hubei Trough.During latest Changhsingian(latest Permian)to the Early Triassic Induan,the expansion of the Western Hubei Trough ended and the study area was mainly covered by shallow-water intra-platform basin mudstone and limestone,which period can be called rapid filling and vanishing stage.Since the latest Induan(~250 Ma),the western Hubei area was covered by shallow-water carbonates,indicating the trough has been filled up and became part of the Yangtze carbonate platform.Whole-rock geochemical features of mudstones in the Dalong and Daye formations indicate they were mainly sourced from active continental margins.Particularly,the LaTh-Sc signatures of these mudstones resemble those from average upper crusts of North China Craton and Qinling tectonic belt instead of South China Craton.LA-ICP-MS UPb dating of detrital zircons from Western Hubei Trough yielded one major age population of 250-330 Ma,four minor age populations of 400-650 Ma,1030-1300 Ma,1600-2000 Ma and 2300-2800 Ma.When compared with tectono-magmatic events from South China Craton,North China Craton and Qinling tectonic belt,the detrital zircon UPb age distribution,trace element and Hf isotopic features indicate that these zircons were mainly sourced from Qinling tectonic belt,the South and North China cratons contributed as minor sources.These provenance analyses indicate that the North and South China cratons collided along the southern margin of the East Qinling Belt no later than late Wuchiapingian(~ 255 Ma).The collision of the North and South China cratons controlled the movement of the Jianshi-Enshi fault and thus controlled the tectono-sedimentological evolution of the Western Hubei Trough.At late Wuchiapingian,the collision occurred along the southern margin of the East Qinling Belt and induced the reactivation of the JianshiEnshi fault and the formation of the Western Hubei Trough.During Early Triassic Induan,along with the ongoing compression between these two continents,the trough was closed quickly possibly because of basement uplifting,rapidly shoaling of the water-column and filling up.Three bentonite layers were found at measured Shuanghe section in the study area.Zircon LA-ICP-MS U-Pb dating yielded 252.2 ± 3.6 Ma,251.6 ± 4.9 Ma and 250.4 ± 2.4 Ma,respectively,for these three volcanic ash layers.Trace element and Hf isotopic features indicate that PTB bentonites in South China were mainly sourced from coeval intermediate to felsic volcanisms at Simao-Indochina active continental margin,the coeval Qinling active continental margin is another possible source.