Domains and enrichment mechanism of the lithospheric mantle in western Yunnan: A comparative study on two types of Cenozoic ultrapotassic rocks

Geochemical comparison of two suites of Cenozoic potassic volcanic rocks in western Yunnan reveals the existence of two mantle domains beneath this region, which correspond to their respective tectonic affinity. The Erhai ultrapotassic rocks (42-24Ma) in western Yangtze Craton are characterized by LILE enrichment, HFSE depletion, low TiO2 content (<1%), high initial ⁸⁷Sr/⁸⁶Sr (0.7064–0.7094) and negative eNd (−3.84–−4.64). Geochemically similar to K-rich volcanism in subduction setting, they were probably originated from a refractory spinel harzburgitic source metasomatized by subduction-related fluids. In contrast, the Maguan potassic magmas (<16Ma) in the South China Block show an OIB-type trace elemental signature, high TiO2 content (>2%), low initial 87Sr/86Sr (0.7041–0.7060) and positive eNd (5.46–7.03). These features resemble the typical intraplate potassic rocks around the world. These rocks are thus interpreted as melting products of a fertile garnet lherzolitic source which has been infiltrated by small-volume, asthenosphere-derived silicate melts. The temporal and spatial distribution of these two types of K-rich rocks cannot be explained by any unified tectonic model. It is proposed that the Oligocene magmatism in the Erhai area may have resulted from convective thinning of the thickened lithosphere, whereas the post-Miocene volcanism in the Maguan area was related to the opening of South China Sea.