Domains and enrichment mechanism of the lithospheric mantle in western Yunnan; a comparative study on two types of Cenozoic ultrapotassic rocks.
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摘要
A 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 affinities. The Erhai ultrapotassic rocks (42-24 Ma) in western Yangtze Craton are characterized by LILE enrichment, HFSE depletion, low TiO (sub 2) content (<1%), high initial (super 87) Sr/ (super 86) Sr (0.7064-0.7094) and negative Nd (-3.84 to -4.64). Geochemically similar to K-rich volcanism in terms of subduction setting, they probably originated from a refractory spinel harzburgitic source metasomatized by subduction-related fluids. In contrast, the Maguan potassic magmas (<16 Ma) in the South China Block show an OIB-type trace element signature, high TiO (sub 2) content (>2%), low initial (super 87) Sr/ (super 86) Sr (0.7041-0.7060) and positive Nd (5.46-7.03). These features resemble the typical intraplate potassic rocks from around the world. These rocks are thus interpreted to be the 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 Oligocene magmatism in the Erhai area may have resulted from convective thinning of the thickened lithosphere, whereas post-Miocene volcanism in the Maguan area was related to the opening of the South China Sea.