The identification of early Indosinian tectonic movement in Tengchong block, western Yunnan： Evidence of zircon U-Pb dating and Lu-Hf isotope for Nabang diorite

Abstract：The Tengchong block is an important part of the southeastern Tibetan Plateau, corresponding to the Lhasa block. The discovery of the Songduo eclogite belt in recent years proves the existence of the subduction and collision event in Lhasa block, and the problem has cropped up whether the same event existed or not in Tengchong block. The Nabang diorite in Tengchong block is a metaluminous, K-rich and calc-alkalic magmatic rock body. Zircon LA-ICP-MS U-Pb dating shows the diorite was emplaced at 245.0±2.9 Ma. The zircon εHf(t) values range from +7.8 to +14.9, with corresponding single-stage model ages ranging from 298 to 590 Ma. All samples fall in the region between meteoritic Hf evolution (CHUR) and depleted mantle (DM) lines in the plot of εHf(t) values versus U-Pb ages and in the subduction-related fluid field in the Th/Yb versus Ba/La discriminant diagram. These data, in combination with high Mg# values, low Yb/Hf ratios (＜1.2) and negative anomalies of Th and U, indicate that the materials were derived from mantle-derived magma mixed with a little crust-derived magma. The strong enrichment of large-ion lithophile elements relative to the primordial mantle，the negative anomalies of Th, U, Nd, Ta, and Ti in the primitive mantle-normalized trace element spider diagram, and the La/Yb versus Sc/Ni and La/Yb versus Th/Yb tectonic discriminant diagrams suggest that the diorite was formed in a tectonic setting related to subduction and collision. The emplacement age and tectonic setting of the diorite in Tengchong block is identical with the data of eclogite in the Lhasa block, and regional angular unconformity with the lack of P2 to T1, suggests that the subduction-collision in Tengchong block did happen during early Indosinian, like the event that happened between Lhasa and northern Australia blocks from P2 to T1.