Petrogenesis of Lincang granites in Sanjiang area of western Yunnan Province: Constraints from geochemistry, zircon U-Pb geochronology and Hf isotope
滇西三江地区临沧花岗岩的岩石成因:地球化学、锆石U-Pb年代学及Hf同位素约束

The Lincang granites in southern Lancangjiang zone of Sanjiang area,located in the west of Yunnan Province,are mainly composed of biotitic monzogranites.The SiO2 and Al2O3 of the granites range 66.84% ～ 73.99% and 12.94% ～ 15.23%,averagely 69.72% and 14.44% respectively.The K2O/Na2O ratio is high,ranging 1.42 ～ 30.1,with average of 8.66.A/CNK varies from 1.06 to 8.59,averagely 2.61,and mostly > 1.1.So Lincang granites are high-K calc-alkaline and peraluminous to strongly peraluminous series.Lincang granites are also enriched in LILEs and Pb,but strongly depleted in HFSEs.The REE content is between 198.2 × 10-6 and 359.2 × 10-6(average 252.5 × 10-6),with(La/Yb) N ratios of 7.87 ～ 17.62(average 11.19) and δEu of 0.34 ～ 0.57(average 0.48).The intrusion is remarkably characterized by enriched LREE and depleted HREE.The chondrite-normalized REE patterns show strong negative Eu anomalies.The zircon U-Pb ages of the two samples are 219.19 ± 0.99Ma and 219.69 ± 0.67Ma,belonging to Late Triassic.The SiO2-P2O5,SiO2-Zr,K2O-Na2O relationship and ACF plot indicate that the Lincang granites are S type granites,which mainly derive from clay-poor psammite.Lincang granites can be dropped into the group of post-collisional granites in the Rb-Y + Nd discrimination diagram.In addition,Lincang granites have the features of low-Sr and high-Yb in the Sr-Yb discrimination diagram,which is in accordance with the granites in southeastern China.Lincang granites have homogeneous zircon Hf isotopic composition.Their zircons have negative εHf(t) values(-14 ～-11) and old Hf isotope crust model ages(1.95 ～ 2.15Ga),suggesting that Lincang granites were formed by partial melting of old crust.Therefore,we consider that Lincang granites were formed in post-collisional stage between the Burma-Thai-Malaysia and Simao block in the Late Triassic.