Chronological and Geochemical Study of the Cenozoic Potassic Felsic Igneous Rocks in Western Yunnan, SE Tibet: Implications for their Tectonic Mechanisms

Whether there is a relationship between widespread Cenozoic igneous rocks and the activated Ailao Shan–Red River shear zone(ASRR) in western Yunnan, SE Tibet, remains highly controversial. New geochronological, and geochemical data derived from Cenozoic igneous rocks both within and far from the ASRR offers answers. Although these coeval felsic rocks(36.9–33.5 Ma) are enriched in large-ion lithophile elements relative to high field strength elements, they also display some special geochemical features, such as high Y and Yb values of syenitic porphyries, which contrast with the high Sr and low Y contents of monzoporphyries and granitic porphyries. The syenitic porphyries exhibit geochemical characteristics similar to those of coeval mafic magmas, e.g., high K2O, Y, and Yb contents, indicating that the former were probably generated by fractional crystallization of the latter. However, the high Sr/Y(56.1–109) monzoporphyries and granitic porphyries, combined with their low MgO, and Cr(or Ni) contents, suggest that they were mainly derived from partial melting of a thickened mafic lower crust. Given that these Cenozoic magmatic events exhibiting planar distributions occurred outside of the activation of the ASRR(31–20 Ma), then they were not controlled by the latter. Instead, they were tied to convective removal of a thickened lower lithospheric mantle in western Yunnan during the Paleogene.