Petrogenesis and tectonic implications of Eocene-Oligocene potassic felsic porphyries in the Sanjiang Region, southeastern Tibetan Plateau

This study presents zircon U-Pb dating and Hf isotope, whole-rock geochemistry and Sr-Nd isotopes of the potassic felsic porphyries from western Yunnan in the Sanjiang Region. Zircon U-Pb dating reveals that quartzmonzonite porphyry, granite porphyry, and adakitic granite porphyry were emplaced during 36-33 Ma. Geochemical data show that quartz-monzonite and granite porphyries have enriched LREE ((La/Yb)(N) = 39-67) and depleted HFSEs (e.g. Nb, Ta, P and Ti), with weak Eu anomalies. Moreover, they display similar Sr-87/Sr-86 ratios of 0.7068-0.7070 and negative epsilon Nd(t) values of-6.7 to-5.4, with zircon epsilon Hf(t) values of -5.65 to 3.06. The adakitic granite porphyries have high Sr (1273-1377 ppm), low Y (9.2-10.7 ppm), and low Yb (0.9-1.0 ppm) contents and high Sr/Y (128-138) and (La/Yb)(N) (17.7-19.8) values, typical of adakite rocks. In addition, these adakitic granite porphyries possess negative whole-rock epsilon Nd(t) values -0.2 to -0.3 and positive zircon epsilon Hf (t) values 2.7-6.0, associated with low Mg#, MgO, Ni and Cr contents. These geochemical features suggest that three kinds of potassic felsic porphyries could be produced by partial melting of mafic rocks (e.g., amphibolite) in a thickened lower crust. Our data, combined with previous studies and regional geology, suggest that delamination of lithospheric mantle and asthenosphere upwelling beneath the Sanjiang Region, both contributed to Cenozoic magmatic activities.

Automated summary

This study presents the U-Pb dating, Hf isotope, geochemistry, and isotopic composition of potassic felsic porphyries from western Yunnan in the Sanjiang Region. The results indicate that these porphyries were emplaced during 36-33 Ma and could be produced by partial melting of mafic rocks in a thickened lower crust. The study also suggests that delamination of lithospheric mantle and asthenosphere upwelling played important roles in the Cenozoic magmatic activities in the region.