Geochemistry and U-Pb geochronology of the Wagone and Hermyingyi A-type granites, southern Myanmar: Implications for tectonic setting, magma evolution and Sn-W mineralization

Tin-tungsten-associated granites occur widely in the Dawei region of the Southeast Asian tin belt. In this contribution, we present a detailed study of zircon U-Pb geochronology and whole-rock and zircon elemental geochemistry for the Wagone and Hermyingyi granites in southern Myanmar, with the aim of determining their petrogenesis, mineralization associations, and tectonic setting. These granitic rocks, which belong to the high-K calc-alkaline series A(2)-type granites, are characterized by high concentrations of SiO2 (75.2-82.5%), Rb (264-927 ppm), Th (18.4-58.3 ppm), U (10.4-18.3 ppm), and Y (33.5-212 ppm), but low contents of TFeO (0.57-3.45%), CaO (0.23-0.86%), MgO (0.02-0.19%), MnO (0.01-0.38%), TiO2 (0.01-0.10%), P2O5 (0.003-0.012%), Ba (3-162 ppm), and Sr (4.3-34.4 ppm). Hydrothermal zircons from the mineralized Wagone pluton show an anomalous lack of zoning and vermicular, spongy internal structures, and they contain high concentrations of Ti, Y, U, and light REEs but low Th/U ratios (mostly < 0.1). These features suggest slow crystallization from a U-enriched and Th-depleted residual magmatic fluid under relatively cool and reducing conditions. LA-ICPMS U-Pb dating of magmatic and hydrothermal zircons from the two granites yielded concordant ages of 61-60 Ma, indicating an Early Paleogene magmatic-mineralization event. Geochemical and geochronological data suggest that these A-type granites have a crustal origin and were produced by partial melting of a felsic clay-rich source in a back-arc extensional setting. The magmas may have ascended relatively slowly in the crust and experienced crystal fractionation and upper crustal contamination, contributing to intensive Sn-W mineralization in the Dawei area.