Mesozoic Magmatism And Metallogenesis In The Bengbu Uplift,Southeastern North China Craton

Because the magmatic rock is not only the probe of the earth's deep materials but also the carrier of ore-forming materials,the study of its formation age,type,geochemical characteristics,magma sources,and magma properties can provide many important information for regional rock-and ore-forming mechanism,geodynamics background,and tectonic evolution.In this paper,magmatic rocks collected from Jingshan,Laoshan,Mayishan,Donglushan,Xilushan,Caoshan,Zhuishan,and Huaiguang plutons,and Jiangshan and Dagongshan deposits in the Bengbu uplift are selected as the research objects.This paper has carried out detailed whole-rock geochemistry and whole-rock Sr-Nd-Pb isotope,zircon U-Pb geochronology,zircon Lu-Hf isotope and mineral chemistry of these magmatic rocks,aiming to(1)Determine their types,formation ages,sources,petrogenesis,partial melting conditions,magmatic processes,and tectonic background;(2)Explore the relationship between different magmatism and gold mineralization in the Cretaceous,and identify ore-causative rocks;(3)Determine the role of ore-causative rocks for gold mineralization;(4)Conduct a comparative study with the Jiaobei magmatic rocks,and explore the relationship between the Mesozoic tectonic evolution and rock-and ore-forming mechanism in the eastern North China Craton.The Late Jurassic leucogranites are characterized by high Si O₂,alkali,and Al₂O₃contents.In addition to two samples exhibiting the rare-earth element tetrad effect,all rocks have high differentiation index values,extremely low iron–magnesium–titanium oxide contents,and low O isotopic compositions,low Zr/Hf and Nb/Taratios,suggesting a highly fractionated granite.Furthermore,the samples contain Neoproterozoic and Triassic zircons,have low Ti-in-zircon temperatures,low radiogenic Pb isotopic compositions,and highly enriched Sr–Nd–Hf isotopic compositions.Taken together,the geochronological and geochemical results suggest that the BLG was formed by high-degree fractional crystallization after hydrous partial melting of the North Dabie granitic gneisses,which have been subducted beneath the Bengbu area by Triassic continental subduction.The early Early Cretaceous magmatic rocks in the Bengbu area were formed at115–130 Ma and are mainly composed of adakitic granites and a few mafic dikes.These rocks have a close spatiotemporal relationship with the gold orebodies.The adakitic granites samples are characterized by pronounced enrichment in light rare earth elements(REE),depletion in heavy REE,an absence of Eu anomalies,and they have relatively low Mg#values and variable Sr–Nd–Pb–Hf isotopic compositions.These features indicate that the parental magmas of the adakites were derived from partial melting of thickened lower crust of the South China Plate and/or the North China Craton.In addition,based on mineral chemical composition of apatite and zircon,the adakitic magmas were highly oxidized and hydrated,which would favor transpotation for ore-forming materials.The mafic dikes have low Si O₂ and high Mg O contents;high Ba/Rb ratios(>18),low Sr-Nd values;and a wide range of radiogenic Pb isotopic ratios,indicating an enriched mantle source metasomatized by the upper crust of the subducted South China Block.Their mineral chemical composition of apatite and amphibole suggest that the mafic magmas were more highly oxidized and hydrated,which are favorable conditions for activation of ore-forming source and transportation.The late Early Cretaceous magmatic rocks in the Bengbu area were formed at 109–114 Ma and are mainly composed of calc-alkaline granites.There is no close temporal and spatial relationship between them and gold ore bodies in the field.Compared with granites of the first two stages,the granites have low Sr,high Y and Yb content,small negative Nb–Ta anomalies,high Nb/La,low and homogeneous Nd–Pb–Hf isotopic compositions,indicating they were derived from the middle to lower crust of the North China Craton.By comparing the Mesozoic magmatism and mineralization of the Bengbu Uplift and the Jiaobei Uplift,based on the regional tectonic evolution,this paper suggests that the Mesozoic magmatism and mineralization in the Bengbu area are importantly related to the subduction of the South China Block and the Izanagi Plate.During the Middle Triassic,the South China Block subducted northward under the eastern North China Craton.The continental subduction caused Neoproterozoic pyritic sedimentary rocks with Au and S anomalies in the South China upper crust to be transported to the deep part of the North China Craton.In the Middle Jurassic,the Izanagi plate subducted westward to the North China Craton,and the fluids released by the subducted slab hydrated the overlying lithospheric mantle,reducing its stability.During the Late Jurassic,the subduction direction of the Izanagi Plate slowly changed to the NWN direction,leading to the emplacement of the Late Jurassic intrusions.Subsequently,the subduction of the Izanagi Plate accelerated,and the subduction direction continued to deflect to the north.As the subduction angle steepened,the subducting Izanagi Plate rolled back during the middle Early Cretaceous,causing the large-scale tectonic extension along the Tanlu fault zone.The strong extensional movement induced partial melting of the lithospheric mantle and the lower crust,generating gold-related mafic and granitic melts,respectively.In addition,the pre-existing NE-NNE striking faults adjacent to and inside of the Tanlu fault zone reactivated as normal faults in this stage.These gentler dipping faults provided structural sites for precipitation of the ore-forming minerals.At the same time or slightly later,lithospheric thinning was completed after 115 Ma.Regional thermal anomalies triggered partial melting of the thinned lower crust,generating calc-alkaline granitic magma.