Geochemical Feature And Petrogenesis Of Shangcheng Pluton And Xinxian Pluton,Dabie Orogenic Belt,China

Many Mesozoic intermediate-acid igneous rocks occur in the Dabie orogenic belt that is located between the North China and Yangtze blocks.In this study,we chose the Shangcheng pluton and the Xinxian pluton to study their geological and geochemical features by the investigations of field work,elemental and isotopic measurements,zircon U-Pb dating.The Shangcheng quartz monzonite and monzonitic granite crystallized in 138~133 Ma and can be divided into three units with minor mafic veins formed in ~132 Ma.The Shangcheng pluton belongs to high-K calc-alkaline series and its chondried-normalized REE patterns portray strong enrichment in LREEs,depletion in HREEs and dispensable negative Eu anomaly.The Shangcheng pluton is characterized by high Sr and low Y content,enrichment of LILEs(e.g.K,Ba and Sr),depletion of HFSEs(e.g.P and Ti),especially Nb-Ta anomalies,and belongs to C-type adakite.These indicate that its magma source was derived from the garnet stable zone and generated from the partial melting of thickened lower crust with contamination by upper crust.The comparison of the Shangcheng pluton and the two stages rocks from the Shapinggou deposit shows that Shangcheng pluton has similar geologic and geochemical features with Stage I intrusion in Shapinggou deposit,and they likely have consistent magma origin and evolution process and are products from same magma system.The Xinxian monzonitic granite crystallized in 133~129 Ma and can be divided into three units based on grain sizes.It contains Paleoproterozoic inherited zircons.The monzonitic granite is characterized by Si-and alkali-rich,depletion of LILEs(e.g.Ba and Sr)and HFSEs(e.g.P and Ti),indistinctive Nb-Ta anomalies,and belongs to slight peraluminous high-K calc-alkaline series.The chondried-normalized REE patterns portray strong enrichment in LREEs,depletion in MREEs,weak enrichment in HREEs and obvious negative Eu anomaly.These features indicate that the granites were derived from an amphibole stable zone in an extensional environment of the thickened crust,rather than a garnet stable zone in the thickened lower crust.This implies that the magma originated mainly from ancient crustal rocks,such as the Kongling and/or Douling Groups.A comparison of the Xinxian monzonitic granite and the ore-bearing rocks from the Qian' echong and Baoanzhai deposits shows that they have consistent geochemical features,inferring closely genetic relationship or products of different stages of the same magma.