| The Central Asian Orogenic Belt(CAOB),as one of the largest accretionary orogens in the world,is bounded by the East European block and the Siberian craton to the north and by the Tarim and the North China cratons to the south.Recent investigations indicate that this vast orogen was built up through punctuated interplay of a series of asynchronous geological entities such as accretionary complexes,continental/island arcs,sea-mountains and micro-continents,resulting from Neoproterozoic to Mesozoic accretion and collision during the development of Paleo-Asian Ocean(PAO).However,many issues remain unresolved,especially regarding the timing and location of the final closure of the PAO,which strongly restrict the studies of the tectonic evolution of CAOB.Located in the eastern segment of the CAOB,the Erenhot area,Inner Mongolia,belongs to two tectonic units,namely the Uliastai Continental Margin and the Erenhot-Hegenshan Ophiolitic Belt.This study carries out petrological,geochemical and detrital zircon U-Pb and Hf isotopic analyses on Paleozoic strata,as well as Late Paleozoic granites,from the Erenhot area to better constrain the maximum depositional ages,source terranes and tectonic settings that can help determine the opening and closure of the Hegenshan Ocean,the relationship between the Hegenshan Ocean and the PAO and the tectonic evolution of the eastern segment of CAOB.Paleozoic(meta)sedimentary rocks,including feldspathic arenites,lithic arenites,siltstones,phyllite and mica-schists,have been reported in the Erenhot area.However,thier depositional ages and tectonic settings are far from fully understood,for they were poorly exposed,widely intruded by Paleozoic granites and cut by complex fault system,causing low continuity and making difficulties comparing among different regions.In this study,Paleozoic strata were systematically sampled,whose detrital zircons generally show typically oscillatory compositional zoning and possess high Th/U ratios(>0.4),indicating an igneous origin.Combined with previous investigations,the youngest detrital zircons are used to clarify their maximum depositional ages,leading to suggestions that the Wubinaobao formation should be deposited after Late Ordovician,rather than Early-Middle Ordovician as previously assumed,and the uncertain sedimentary rocks in the Bengbatu area should be formed no earlier than Middle Permian,but not Middle Devonian or Early Carboniferous.From the Chagan Obo Temple area in northern Erenhot,detrital zircons from Late Ordovicianto Middle Devonian strata yield one major age population at ca.511-490 Ma and three subordinate age populations at ca.982-891 Ma,834-790 Ma and 574 Ma,showing a large variation in ?Hf(t)values from-20.8 to +16.9.By contrast,detrital zircons from Late Carboniferous to Early Permian strata are dominated by Paleozoic(ca.500-300 Ma with peaks at 410 Ma and 336 Ma)grains with mostly positive ?Hf(t)values(+1.3 to +14.9);rare Precambrian zircons were detected.From the Bengbatu area in southern Erenhot,detrital zircons from Middle Permian strata are dominated by ca.517-261 Ma grains with minor Precambrian(2687-544 Ma)ones.Precambrian zircons show dominantly negative ?Hf(t)values,while 500-350 Ma and 300-261 Ma grains have a large variation in ?Hf(t)values from-14.0 to +15.3,and 350-300 Ma ones exhibit mostly positive ?Hf(t)values(+0.1 to +16.0).Based on the above detrital zircon U-Pb-Hf isotopic data,the following speculations can be reached.Detrital materials for Late Ordovician-Middle Devonian strata in the Chagan Obo Temple area were derived dominantly from Neoproterozoic-Early Devonian(particularly Cambrian)magmatic rocks.Similar detrital zircon age patterns and magmatic events have been found in the Mongolian arcs.Likewise,detritus for Late Carboniferous-Early Permian strata were probably sourced from Cambrian-Early Carboniferous magmatic rocks that are well-documented in the Northern Accretionary Orogen and the Mongolian arcs.The Paleozoic strata were probably formed in an active continental margin,based on their predominant younger(post-Neoproterozoic)detrital zircon compositions and juvenile Hf isotopes.The contrasting detrital zircon age patterns and Hf isotopes of Early and Late Paleozoic strata,indicating a marked shift of provenance terranes from multiple sources to a single source and Hf isotope compositions from mixed to positive values,were probably related to a sudden change in the tectonic setting,possibly as the result of the opening and expansion/subduction of the Hegenshan Ocean in the Carboniferous.Detrital materials from Middle Permian strata in the Bengbatu area were sourced primarily from Cambrian-Middle Permian magmatic rocks,which can be found in the Mongolian arcs,the Erenhot-Hegenshan ophiolitic rocks and the Northern Accretionary Orogen.These strata were probably deposited in an active continental margin setting,similar to coeval strata in theChagan Obo Temple area.Combined with previous investigations,the Hegenshan Ocean most likely opened as a back-arc ocean during ca.350-330 Ma,as evidenced by the considerable juvenile materials generated in source terranes during that time.Moreover,the Hegenshan Ocean probably opened at ca.350-330 Ma and closed at ca.300 Ma,as manifested by two marked shifts between mixed and highly positive zircon ?Hf(t)values.More involvement of ancient crustal materials in post-Carboniferous(after ca.300 Ma)strata possibly reflects a transition in tectonic environment from syn-orogenic to post-orogenic extensional setting.Late Carboniferous to Early Permian high-K calk-alkaline granites are found in the Erenhot region in this study.Detailed petrological and geochemical,as well as zircon Hf isotopic,data suggest that these granites were generated from the partial melting of juvenile crustal rocks with minor involvement of mantle components as heat engine,and may suffered from fractional crystallization and crustal sediments input to some extent during magmatic evolution.Combined with tectonic distinguishing diagrams,I suggest that the formation of these granites witnessed a tectonic transition from continental collision to post-collisional extension,genetically related to the closure of the Hegenshan Ocean and the subsequent crustal collapse.On the basis of data in this study and results from previous investigations,following conclusions can be drawn:(a)the Hegenshan Ocean did not open in Late Ordovician-Middle Devonian,with the Erenhot region controlled by the northward subduction of the PAO;(b)the Hegenshan Ocean possibly opened as a back-arc basin due to slab rollback of the PAO in the Carboniferous;(c)the Hegenshan Ocean closed during Late Carboniferous to Early Permian times,resulting in the collision between the Uliastai continental margin and the Northern Accretionary Orogen along the Erenhot-Hegenshan suture;and(d)emplacement of the Erenhot-Hegenshan ophiolitic rocks possibly occurred in the Middle Permian,associated with a tectonic transition from syn-orogenic to post-orogenic extension in the Erenhot region,while subduction of the PAO was still ongoing. |
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