The Shale granites from the southern part of the Lancangjiang tectonic belt,western Yunnan: Zircon U-Pb age,geochemistry and geological implications

The Shale granites from Shale,Nanjian,western Yunnan consist of biotite monzogranite,granodiorite and a small amount of orthophyre. The LA-ICP-MS zircon U-Pb dating gives an age of 246.4±2.6 Ma for biotite monzogranite,and 245.7±3.6 Ma for granodiorite,indicating that the Shale granites were formed during the Early Triassic. The Shale granites are characterized by SiO₂ contents ranging from 71.44% to 76.39%,Al₂O₃ contents ranging from 12.72% to 16.15%,and K₂O/Na₂O ratios ranging from 1 to 1.30,suggesting the high-K calc-alkaline peraluminous and strongly peraluminous granites. The samples of the Shale granites mostly projected into the fields of volcanic arc granites,within plate granites and collision-type granites indicated by the geochemical signatures and trace element discriminantion diagram for tectonic interpretation,implying that the granites were formed in the subduction-collision magmatic arc transitional environments. The Sr-Yb diagram for the granite classification has disclosed that most of the Shale granite samples are assigned to the low-Sr and high-Yb granites, and a few of them belong to the low-Sr and low-Yb granites,both of which are derived from the sandy metamorphic mudstones. It can be seen from the zircon U-Pb age determinations and regional tectonic setting that the Shale granites were originated during the initial stages of the collision and mountain building processes of the Myanmar-Thailand-Malaysia continent and Simao landmass. The initial collision of the Changning-Menglian ocean may be traced back to the Early Triassic.