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

Abstract

• Eocene-Oligocene (36–33 Ma) potassic felsic rocks occur in the Sanjiang Region. • Eocene potassic felsic rocks produced by partial melting of mafic rocks in a thickened lower crust . • Eocene magmatic activities caused by post-orogenic delamination of lithospheric mantle and asthenosphere upwelling in the Eocene. 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 quartz-monzonite 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 87 Sr/ 86 Sr ratios of 0.7068–0.7070 and negative εNd(t) values of −6.7 to −5.4, with zircon εHf(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 εNd(t) values −0.2 to −0.3 and positive zircon εHf(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.