Petrogenesis of Late Devonian volcanic rocks in the Jinghong area, southwestern Sanjiang region and its geological implications

In this study, SHRIMP zircon U-Pb age, Hf-O isotopes, and whole-rock geochemical data are reported for the newly discovered Late Devonian dacitic volcanic rocks from the Jinghong area, southwestern Sanjiang region. New dating results reveal that the volcanic rocks were formed at 362.3 +/- 3.4Ma. These rocks have moderate SiO2 (62.87% similar to 66. 29%), MgO (2.15% similar to 2.49%), and Mg-#(44 similar to 47) values. They are characterized by high Na2O contents (4.77% similar to 5.51%) with Na2O/K2O ratios of 2.1 similar to 3.3, and low Y (12.5 x 10(-6) similar to 15.3 x 10(-6)) and Yb (1.38 x 10(-6) similar to 1.70 x 10(-6)) contents. These geochemical results are similar to those of high-SiO2 adakites, indicating that they were derived by partial melting of the subducted oceanic slab. The low zircon epsilon(Hf) (t) values (+0.87 similar to +3.27) and relatively high zircon delta O-18 (6.31 parts per thousand similar to 7.64 parts per thousand) values, and whole-rock Th/Yb (4.86 similar to 7.78) and (La/Sm)(N) (3.62 similar to 4.56) suggest that subducted oceanic sediments were assimilated into or sediment-derived melt incorporated into the magma. In combination with regional Paleozoic magmatism and stratigraphy data, it can be concluded that the southern Lancangjiang belt underwent the Late Devonian oceanic slab subduction, and the process is likely to represent the subduction of the back-arc oceanic basin revealed by Dazhonghe and Dapingzhang volcanic rocks and volcanic-hosted massive sulfide (VHMS).