Petrogenesis and zircon U-Pb ages of the Thien Ke granitic pluton in the Tam Dao region: Implications for early Paleozoic tectonic evolution in NE Vietnam

摘要

Granites of the Thien Ke pluton that are exposed in the Tam Dao region are of sub-alkaline affinity, belong to the high-K calc-alkaline series, and display typical features of S-type granite. The rocks have high AS1 values of 1.14-1.41 and are characterized by high initial ~(87)Sr/~(86)Sr ratios (0.7264-0.7270) and low ε_Nd (456 Ma) values (-6.5 to -8.4). All of these characteristics, combined with ～ 1.8 Ga T_(DM) model ages and low Mg# of 13-22, suggest an origin in the partial melting of crustal source rocks. LA-ICP-MS zircon U-Pb analytical results from two samples revealed emplacement ages of the granite at 456 ± 28 and 449 ± 12 Ma. These ages are entirely different from those reported in earlier publications and indicate the existence of a Caledonian tectonothermal event in the study area. The Nd-Sr isotopic data, in conjunction with inherited zircon ages (721-1876 Ma), imply Paleoproterozoic to Neoproterozoic crustal material as the major components of the underlying basement. The new geochronological data provide evidence for early Paleozoic magmatism in the Tam Dao region, which has not been previously identified.