Geochronology, geochemistry and Sr-Nd-Hf isotopic compositions of Late Cretaceous-Eocene granites in southern Myanmar: Petrogenetic, tectonic and metallogenic implications

摘要

The geochronology, petrogenesis and tectonic setting of granites associated with tin-tungsten mineralization in southern Myanmar remain unclear. This work presents whole-rock geochemical, Sr-Nd isotopic, and zircon U-Pb and Hf isotopic data for felsic intrusions in the Sibusima terrane. Zircon U-Pb dating indicates formation in the Late Cretaceous-Early Eocene (similar to 84-48 Ma) and suggests that these intrusions likely represent a southward extension of the coeval magmatic belt in the Tengchong terrane. Primarily classified as high-K talc-alkaline, the granites show strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb, Ta, P, and Ti, and negative Ba and Sr anomalies on primitive mantle-normalized diagrams. P2O5, CaO, Al2O3, MgO, TiO2 contents, and Nb/Ta ratios decrease with increasing SiO2 contents, possibly consistent with evolution trends in fractionated l-type granitic magmas. In addition, these intrusions display a wide range of negative epsilon(Nd)(t) (-14.6--.5.5) and zircon epsilon(Hf)(t) values (-22.7-5.7), suggesting a dominant Sibusimacrustal source with a minor mantle contribution. Notably, the Eocene and a few Paleocene granites show more contribution of mantle material based on higher epsilon(Nd)(t) (-8.5--5.5) and zircon epsilon(Hf)(t) (-11.0--3.2) values. Large variations in zircon Hf isotopic compositions within an individual Eocene granitic intrusion (e.g.,-15.2-5.7) and corresponding biotite-rich enclave (-17.8-0.2) likely indicate magma mixing. Considering the spatial-temporal distribution of magmatism in the West Burma and Sibusima terranes, the studied Late Cretaceous-Eocene felsic magmas likely formed in a continental arc setting during normal subduction (similar to 100-60 Ma) and subsequent roll-back of the Neo-Tethyan oceanic lithosphere (similar to 60-50 Ma). Additionally, most of granites have the elevated Sn with increasing Rb/Sr ratios, and decreasing TiO2 contents and Nb/Ta ratios, suggesting Sn enrichment is primarily controlled by magmatic fractionation. Meanwhile, Sn and Li depletions in some granites deviate from the magmatic evolution trend, likely as a result of fluid exsolution. The exsolved Sn-rich fluids could have made a genetic contribution to hydrothermal Sn mineralization in southern Myanmar.