Geochronology and geochemistry of the Early Palaeozoic granitoids in Baoshan block, western Yunnan and their implications
滇西保山地块早古生代花岗岩类的年代学、地球化学及意义

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.