Geochemistry, Geochronology And Petrogenisis Of Lincang Granites In Southern Lancangjiang Zone Of Sanjiang Area

The Lincang granite in southern Lancangjiang zone of Sanjiang area, located in West of Yunnan Province, has the largest area of outcrop in Yunnan Province. The composite granite batholith is an important part in the paleo-Tethyan Tectonic Region. However, its petrogenesis and age are still in dispute. Based on Field investigation, the Lincang granite is systematically researched in this study, in which the subjects of U-Pb isotope chronology, petrology, elemental geochemistry and zircon Hf isotopic geochemistry are involved. Correlated with the known U-Pb dating, the purpose of this study endeavors to bring up the geological constraints for its petrogenesis, age and regional magmatic evolution under the tectonic setting.The Lincang granite is mostly biotitic monzogranite in terms of lithology. The 19 representative samples have shown that SiO₂ and Al₂O₃ of the granitoid range 66.07-73.99% and 12.94-15.23%, averagely 68.88% and 14.30% respectively,The K₂O/Na₂O ratio is high,ranging 1.41-2.21, with average of 2.06,A/CNK varies from 0.97 to 2.01,averagely 1.27,and mostly >1.1. So Lincang granites are high-K calc-alkaline and peraluminous to strongly peraluminous series. They are also enriched in LILE and Pb,but strongly depleted in HFSE. The REE content is between 173.58×10^-6 and 359.16×10^-6(average 240.13×10^-6), with (La/Yb)_N ratios of 6.69-15.72 (average 11.54) andδEu of 0.41-0.75(average 0.53). 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 age of the six samples is 227.00±0.80 Ma,219.19±0.99 Ma,219.69±0.67 Ma,203.38±0.70 Ma,211.9±1.8Ma and 214±2Ma, belonging to Late Tertiary. The SiO₂-Zr relationship and ACF plot indicated that the Lincang granite was S type granite. It mainly derives from clay-poor psammite. Lincang granite can be classified into the group of post-collisional granites in the Rb-Y+Nd discrimination diagram. In addition, Lincang granites are plotted into the low-Sr and high-Yb granites in the Sr-Yb discrimination diagram, in accordance with the granites in Southeast China, and should be formed during the transferring setting from compression to extension. Lincang granites have homogeneous zircon Hf isotopic composition. Their zircons have negativeεHf(t) Values(-14^-10) and old Hf-isotope Crust model ages (1.90².15Ga), suggesting that Lincang granite was formed by partial melting of old crust. Therefore we consider that Lincang granite was formed in post-collisional stage between the Burma-Thai-Malaysia and Simao block in the late Triassic.