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Geochronology and geochemistry of the Early Palaeozoic granitoids in Baoshan block,western Yunnan and their implications

DONG MeiLing1,DONG GuoChen1,2**,MO XuanXue1,2,ZHU DiCheng1,2,NIE Fei1,XIE XuFeng1,WANG Xia1 and HU ZhaoChu3 1.China University of Geosciences,Beijing 100083,China 2.State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Beijing 100083,China 3.State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan 430074,China  
The Pinghe granitoid pluton,located in the Baoshan block of the Tethyan belt,western Yunnan,consists mainly of granite and monzogranite with subordinate granodiorite.Zircon U-Pb ages of 480 ~ 486Ma for four samples from this pluton indicate that this pluton was emplaced during the Early Ordovician.The Pinghe granitoids are high-K calc-alkaline and peraluminous,with K2O/Na2O 1 and A/CNK = 1.07 ~ 1.13.These rocks are enriched in large-ion lithophile elements(LILEs),light rare-earth elements(LREEs) [(La/Yb) N =4.33 ~7.05]and Pb,and depleted in high field-strength elements(HFSEs).They show similar chondritenormalized REE patterns,with negative Eu anomalies(δEu = 0.25 ~ 0.48).58 analyses from four samples have a wide range zircon εHf(t) values(-12.4 ~-3.0) and varying Hf-isotope crustal model ages(2.2 ~ 1.7Ga).Such geochemical signatures indicate that the Pinghe granitoids are S-type granite in character and were derived mainly from anatexis of ancient crustal materials(such as greywacke) with varying contributions from mantle-derived components.The Pinghe granitoids are geochemically comparable to the coeval silicic metavolcanic rocks reported in the central Lhasa Terrane.It is suggested that the Baoshan block may represent a part of an Early Paleozoic magmatic arc in the Gondwana continental margin facing the proto-Tethyan Ocean.
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