| The Dabie orogenic belt, one of the important parts of the Central Orogenic Belt lying in the eastern China, has outcropped massive the HP-UHP metamorphic rocks, and has been the typical area for international studies of continental deep subduction and UHP metamorphic rocks. A series of achievements have been gained in continental deep subduction, continental collision, the HP-UHP metamorphic rocks exhumation, basin-mountain coupling and so on in the past decades. However, studies of the Dabie orogenic belt of the Paleozoic tectonic evolution history are still not thorough known, as well as the understandings of the Paleozoic paleotectonic and paleogeography, and many controversies need to be solved. The Carboniferous outcropped in Beihuaiyang area is the only suite of unmetamorphosed or slightly metamorphosed sedimentary strata in the northern margin of the Dabie Mountains, which not only has recorded the information of the Paleozoic basins tectonic evolution history, and the characteristics of sedimentary environments and paleogeography, but also has recorded the provenance information and tectonic evolution processes during Paleozoic. Based on detailed field geological survey and many analytical methods, including petrography, petrology, heavy mineral, geochemistry, and detrital zircon geochronology etc., this study synthetically analyzes the sedimentary environments and sedimentary facies and provenance, and establish the Paleozoic tectonic evolution history of the Beihuaiyang tectonic zone. On this basis, it could provide a restriction for the Paleozoic tectonic evolution of the Dabie orogenic belt.The Carboniferous in the research area is about 2400meters thick, which can be divided into Upper and Lower Carboniferous. The Upper Carboniferous includes Huayuanqiang formation, Yangshan formation and Daorenchong formation, while the Lower Carboniferous contains Huyoufang formation, Yangxiaozhuang formation and Shuangshitou formation. Huayuanqiang formation with the thickness over 337 m, comprises of clastic rocks, and tis bottom never outcropped, which exposes mainly in Duanji Liulinwan of Henan province. It also has Late Devonian’s characteristics. Yangshan formation consists of coarse clastic rocks, intercalated with several sets of coal seam, which mainly exposes in Yangshan coal mine and Duanji Shimenkou, and unconformable contact with Huayuanqiang formation, and its thickness is over 367m. Daorenchong formation is composed of clastic rocks and a few carbonate rocks, with the thickness of 428 meters, which mainly appears in Duanji Erdaohe and Daorencong, and contains many kinds of marine fossils. The Upper Carboniferous Huyoufang formation is composed of fine grained clastic rocks, which has the widest distribution range and can be divided into two sides of east and west. The western side distributes in Wangqiao-Guoyao area. The eastern area mainly distributes in Leidianzi-Wujianshan area. This formation thick is more than 770 meters, which contains fossil plants and unconformable contacts with the overlying formations. Yangxiaozhuang formation consists of clastic rocks and a few coal seams, mainly distributed in Shangcheng Wangao and Suxianshi. Shuangshitou formation is made up of clastic rocks and carbonate rocks, with its upper part marblization commonly, which mainly appears in Shangcheng Wangao and unconformable contact with Yangxiaozhuang formation. Its thickness is about 300 meters.According to rock assemblages and sedimentary characteristics of the Carboniferous rocks,16 types of lithofacies can be identified in the studied area. They are matrix-supported massive conglomerate facies, matrix-supported layered conglomerate facies, clast-supported massive conglomerate facies, clast-supported massive layered conglomerate facies, layered conglomerate facies, colluvial breccia facies, tabular cross-bedding sandstone facies, horizontally bedded sandstone facies, grade-bedding sandstone facies, ripple-bedded fine-sand facies, low angle cross-bedded sandstone facies, flute-filling sandstone facies, flame structure mudstone- siltstone facies, layered mudstone- siltstone facies, massive sandstone facies, globular pelitic siltstone facies, and coal seams. All kinds of facies marks are synthesized and the study area can be divided into 11 sedimentary facies and 6 sedimentary systems, including alluvial fan facies, fan delta facies, braided river delta facies, lacustrine facies, lake delta facies, deepwater turbidite fan facies, gravelly braided fluvial facies, sandy braided fluvial facies, barrier littoral facies, barrier-free clastic littoral facies, and mixed platform facies. They respectively belong to alluvial fan sedimentary system, delta sedimentary system, lacustrine sedimentary system, fluvial sedimentary system, clastic littoral sedimentary system, sedimentary system, and onshore-shallowsea sedimentary system. And the sand bar sediment in Yangshan formation is found firstly. Grain-size analysis and gravel fabric analysis show that each formation of Carboniferous differs in their characteristics of sediment environments. According to sedimentary facies assemblages, 3 sedimentary models are put forward, including barrierless littoral- onshore shallowsea restricted platform sedimentary model, alluvial fan- fan delta-tidal flat lagoon onshore sedimentary model, fluvial- delta-lacustrine- deepwater turbidite sedimentary model. During the early and later stages of the Carboniferous, some changes took place in tectonic paleogeography, which caused two different stages of sedimentary evolution. In the early Carboniferous, the sedimentary environments of Huayuanqiang formation belong to the shallow sea and littoral-neritic sea. Later, it was affected by early Hercynian tectonic movement and uplifted as a whole, and gradually transformed from littoral-neritic sea to marine-continental transitional facies in Yangshan formation. And it also developed molasse formation and coal-bearing deposits. Then in the late early Carboniferous, large-scale transgression happened in the study area leading the Daorenchong formation deposited in littoral-neritic sea and contains plenty of marine fossils which similar to Qinling-Qilian area. During late Carboniferous, due to the extensional tectonic setting, the study area suffered large-scale regional rift, which caused the Huyoufang formation formed in various different sedimentary environments, including faulted basin sedimentary and deep water turbidite fan. In the middle- late of the late Carboniferous, the study area belonged to sea-continental transitional zone, causing Yangxiaozhuang formation contain coal-bearing deposit, which similar to Huabei area. In the late period, large-scale transgression happened, which caused Shuangshitou formation transformed from clastic littoral-neritic sea into restricted carbonate platform. Hence, during Carboniferous, the study area had experienced the sedimentary evolution processes from marine to marine-continental transitional to marine to continental to marine-continental transitional back to marine cycle. The evolution of sedimentary environments and paleontologic characteristics indicate that Beihuaiyang area has been united with North and South China in the Carboniferous, and these sea areas communicated in the same biota range and without any separated ocean.The features of detrital component and heavy mineral components and geochemical characteristics show that the types of material source mainly include sedimentary rocks, low-grade metamorphic rocks, and acidic magmatic rocks. They were mainly from stable craton margin, recycled orogenic belt, metamorphic basement and some magmatic arc. There are some fossils found in Yangshan formation carbonate gravels, like coral:Heloilites cf. Anhuiensis, late Ordovician conodont:Panderodus gracilis Branson et Mehl. Silurian-Devonian ostracods:Kirkbyella sp. The gravels in Huyoufang formation conglomerate are mainly composed of sedimentary rocks and low-grade metamorphic rocks, which shows that provenance may include early Paleozoic sedimentary strata of the Yangtze Block and metamorphic crystalline basement and intermediate-acid intrusion. Trace elements geochemical characteristics show that most of the source rocks were island-arc tectonic setting, which was similar to the early Paleozoic island arc of Qinling orogen. The detrital zircons analysis (392 detrital zircons) about six sandstone samples of the Carboniferous show that the age characteristics of detrital zircons mainly include three sections, with the peak of 400-485Ma belonging to the early Paleozoic (about 60%), the second is 900～1000Ma middle to late Proterozoic (about 30%) and a few at 1800-2300Ma Paleoproterozoic (less than 10%). This reveals that the source area mainly come from the middle Paleozoic island arc of North Qinling, and some Meso-Neoproterozoic basement of the Yangtze Block, also a few from Paleoproterozoic metamorphic basement. Synthesizing all kinds of provenance analysis results, the Carboniferous provenance in study area came mainly from the early Paleozoic sedimentary cover of the Yangtze Block, the middle Paleozoic island arc in North Qinling, the Meso-Neoproterozoic basement of the Yangtze Block, and a few Paleoproterozoic metamorphic basement. Besides, the age characteristics of detrital zircons show that the source area had significant changed in the late Carboniferous, the provenance in Shuangshitou only comprised of the Meso-Neoproterozoic basement of the Yangtze Block, and lacked of the middle Paleozoic island arc in North Qinling provenance. All these show that the Carboniferous early provenance in Beihuaiyang area mainly came from the early Paleozoic geological body, secondly came from the Meso-Neoproterozoic basement. Until the last phase it came only from the Meso-Neoproterozoic basement of the Yangtze Block. What’s more, Palaeocurrent analysis shows that the sediments mainly came from the southwest direction.The trace elements and REE elements of detrital zircons in the study area show that the average of U/Yb is 1.61, which indicates that the source rocks were all from continental crust, the oceanic crust were excluded, and imply that the ocean basin may necessarily been existed during early Paleozoic in Dabie Orogen, which differs from the Qinling orogen. |
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