Cenozoic Potassic Volcanic Rocks And Mantle Xenoliths In Maguan, Southeastern Yunnan: Geochemical Characteristics And Implications For Lithospheric Dynamics

Maguan (southeastern Yunnan) is situated to northwest of South China Sea and extends along the border between China and Vietnam. It is located geologically in the eastern side of the south section of the Ailaoshan-Red River fault belt and structurally within the South China block on the western margin of the Yangtze Plate, where Cenozoic lava is extensively distributed. This paper mainly deals with the elemental and isotope geochemistry of the Cenozoic potassic lava in this area, the petrology, mineral chemistry, elemental and isotope geochemistry as well as the equilibrium temperature of the mantle-derived peridotite xenoliths in the potassic lava. The data are used to explore the magmatic processes, the nature of the source regions and the formation mechanisms of the potassic lava, and to investigate the compositions and dynamic processes of the upper mantle in the study area.The potassic lava in the Maguan area is characterized by the low-medium value of Mg# (0.56-0.69), which is lower than that for the primary magma in equilibrium with mantle-derived peridotite (0.68-0.75). This suggests that the Maguan potassic lava was not primary magma, but may have undergone crystallization and differentiation to various degrees. The lack of correlation between Sr isotope ratio (0.7041-0.7060) and 1/Sr, theconsistently high Ce/Pb (6.43-25.4, Avg. 16.6) and Nb/U (31.2-62.1, Avg. 48.9) ratios as well as the low La/Nb (0.37-0.65, Avg. 0.45) and high εNd ( > 5.64 ) values argue against contamination of the potassic rocks by crustal components. Consequently, the elemental and isotope signatures inherited the characteristics of the magmatic source regions for the lava. The high contents of basaltic components like Al2O3, Fe2O3, CaO and Na2O imply a fertil mantle source, while the relatively depleted HREE composition indicates that the magma was derived from the partial melting of a garnet-bearing mantle. The high TiO2 content (>2%), similar to that for intraplate highly potassic lava, and the OIB type trace elemental signatures imply that its source could be fertile garnet-bearing lherzolite which has been metasomatized by small volume silicate melt derived from the asthenosphere.The Erhai ultrapotassic lava (42-24 Ma) that is located on the west margin of the Yangtze plate is enriched in LILE but depleted in HFSE, and characterized by low TiO2 contents ( <1% ), high initial 87Sr/86Sr ratios(0.7064-0.7094) and negative εNd values (-3.84~-4.64). Similar to typicalpotassic lava associated with plate subduction in the world, the Erhai ultrapotassic lava was also derived from depleted spinel phase harzburgite metasomatized by fluids originated from the subduction of the Paleotethys domain.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. It is noted that the magmatism in this area is roughly coeval with and compositionally similar to those in the circum region of South China Sea (e.g., Guangdong, Hainan Island and VietName) (16-0 Ma). There is amagmatic hiatus (32~17 Ma) during the opening of China Sea. It is likely that the magmatism in the Maguan area was related to the stress release subsequent to the cease of the opening of South China Sea, but the detailed dynamical mechanism needs further investigation.The peridotite xenoliths in the Maguan potassic lava include fertil lherzolite, cpx-poor lherzolite and harzburgite. Characterized by relatively simple compositional variation, these samples represent the residues of partial melting to various degrees of the upper mantle. Mantle metasomatism, if any, is limited to a very small number of samples. Cpx is depleted in LREE and shows typical MORB-type Sr-Nd isotopic signatures ( 87Sr/86Sr = 0.7064-0.7094, εNd = 9.5-12.3). These characteristics are rather similar to those for oceanic lithospheric mantle. The relatively refractory peridotites (Fo>90) show an equilibrium temperature of 928-959 ℃, which is lower than that for fertile peridotites (Fo<90, 955-1110 ℃ ). This implies that the lithospheric mantle has a stratified structure with refractory peridotites sitting above the fertile ones. Such a lithospheric architecture is most likely related to the decompressed melting of an upwelling asthenosphere. The high thermal state and the MORB-type isotope signatures of the peridotite xenoliths in the study area suggest that the Maguan peridotites may represent the newly accreted lithospheric mantle, which have been converted from the upwelled asthenosphere with a MORB-like signature. This is consistent with the extensional dynamic setting for the study area since the late Cenozoic.