The composition and age of subcontinental lithospheric mantle beneath Maguan, Yunnan Province: Constraints from Re-Os isotopes of mantle-derived peridotitic xenoliths
云南马关地区岩石圈地幔组成和年龄:地幔橄榄岩包体的Re-Os同位素限制

This study carried out petrological and geochemical research on the mantle-derived peridotitic xenoliths borne in Cenozoic akali-basalts in Maguan, Yunnan Province, southwestern China. The first suite of Re-Os isotopic data of the peridotitic xenoliths was reported. The peridotitic xenoliths can be classified as spinel lherzolite, which show fertile characteristics in major elements and are depleted in light rare earth elements (LREE) and some high field strength elements (HFSE) and large ion lithophile elements (LILE), such as Nb, Zr, Ti and Ba. The Sm-Nd isotopic results of the peridotitic xenoliths imply that the subcontinental lithospheric mantle (SCLM) beneath Maguan area represented by the xenoliths is depleted and heterogeneous in Nd isotopes. Results of five whole-rock Re-Os isotopic analysis show that the Os concentrations of the peridotitic xenoliths are relatively high(3.29×10-9~3.78×10-9), comparable to those of peridotite massif from orogenic belts, and the Re concentrations change in a relatively wide variation, ranging from 0.24×10-9 to 0.54×10-9. The 187Os/188Os ratios of samples vary between 0.12295 and 0.12530. Neither 187Re/188Os ratios nor Al2O3 contents show correlation with187Os/188Os ratios, indicating that the Re-Os isotopic system is not solely controlled by melt extraction process. The Re-depletion age of the peridotitic xenoliths range from 254Ma to 604Ma, suggesting that the formation time of SCLM beneath Maguan area is at least no later than Neoproterozoic. The SCLM beneath Maguan area possessed a relatively complex structure and was not directly formed by the cooling of upwelling asthenospheric mantle induced by decompressional melting. Alternatively, ancient melt extraction process of primitive mantle gave birth to the original SCLM, which underwent modification and metasomatism by the melt/fluid derived from later geological processes, leading to parts of originally depleted peridotites transformed into relatively fertile peridotites, and leaving the other parts of SCLM intact with depleted characteristics. The melt/fluid modification process are probably attributed to the activities of Late-Permian Emeishan mantle plume and the Cenozoic potassic volcanisms occurred in Maguan area, which showed little effect on the Os isotopic systems due to their young eruption age.