Title: ZIRCON U-Pb AGE AND GEOCHEMISTRY OF MENGYANG INTRUSION AND ITS TECTONIC IMPLICATIONS IN THE LIANGHE,WESTERN YUNNAN
Abstract: Field investigation reveals that Mengyang intrusion consists of biotite monzonitic granite and granodiorite with abundant dioritic enclaves,which shows gradual transition relation with its host rock.Zircon LA-ICP-MS dating indicates that the biotite monzonitic granites,granodiorite and diorite enclaves were formed at 127.7 Ma±0.7 Ma,115.2 Ma±1.1 Ma and 122.6 Ma±0.8 Ma,respectively,which provides evidence for mixed magmatic origin.Mengyang intrusion was formed in the Early Cretaceous.Geochemical data show that the diorite enclaves are metaluminous and calc-alkaline serirs,characterized by low SiO2,high MgO,high K2O and Mg#,with Sm/Nd ratio of 0.18~0.20,LREE/HREE ratio of 2.91~4.64,weak to moderate negative Eu anomalies,enrichment in U,Th,Rb and Ba,depletion in Tb,Nb,Zr and Hf.The Mg#of the diorite enclaves approach to the mantle protomagma,indicating that dioritic magma originated from partial melting of mantle.The host rock of biotite monzonitic granites is rich in Si,K,poor in P,belong to peraluminous and high-K calc-alkaline to k black series,with Sm/Nd ratio of 0.17 ~0.25,enrichment in U,Th,Rb and Ba,depletion in Tb,Nb,Zr and Hf,similar to calc-alkaline rocks in island arc or active continental margin.It was considered that the host rock and enclave may be derived form the same magmatic processes.The REE pattern diagram and normalized diagram for trace elements of the biotite monzonitic granite and granodiorite with dioritic enclaves are similar.They are rich in U,Th,Rb and Ba,depletion in Tb,Nb,Zr and Hf,similar to intrusion rocks in island arc or active continental margin.These characteristics indicate that the host rock and enclave may be derived form the same magmatic processes.According to geochemistry and the trace element discrimination diagrams for tectonic interpretation,the Early Cretaceous granitoids are considered as the post-collision type formed in an island arc tectonic setting,and the diorite enclaves as th partial melting of mantle-wedge peridotite.The study shows that the biotite monzonitic granite was derived from partial melting of the Mid-Proterozoic metamorphosed basement(Gaoligong Group),and granodiorite from the mixing of mantle-derived magma and partial melting crust.The early Cretaceous intrusive rocks in the Tengchong block were closely related to the closure of the Bangonghu-Nujiang-Longling-Ruili ocean basin,subduction of ocean crust and collision orogenesis of the Tengchong block with Zuogong-Baoshan block.
Publication Year: 2013
Publication Date: 2013-01-01
Language: en
Type: article
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