中文摘要：

滇东南马关地区新生代钾质玄武岩携带的幔源包体为研究该地区上地幔性质提供了物质基础。分析表明，全岩及单矿物中的玄武质组分与MgO含量之间均有很好的负相关性，与世界尖晶石二辉橄榄岩的变化趋势一致，表明橄榄岩包体代表了成分均—的上地幔经不同程度部分熔融后的残余；绝大多数包体亏损LREE及除U、Th以外的其他不相容元素，Sr-Nd同位素组成单一（87^Sr／86^Sr=0．7022～0．7029，εNd=9．5～12．3），显示了MORB型地幔的成分特点。相对亏损橄榄岩（Fo〉90）的平衡温度（928～959℃）低于饱满型橄榄岩（Fo〈90，956～1110℃）。这些特征表明，马关橄榄岩包体很可能代表了MORB型软流圈地幔底辟上涌、减压熔融后新增生的岩石圈地幔。

英文摘要：

Maguan （southeastern Yunnan） is situated to the northwestern part 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, within the South China block on the west margin of the Yangtze Plate, where Cenozoic lava is extensively distributed. This paper mainly deals with the petrology, mineral chemistry, elemental and isotope geochemistry as well as the equilibrium temperature of the peridotite xenoliths in Cenozoic potassic lava in this area. The data are used to investigate the compositions of and the dynamic processes in the upper mantle in the study area. The Maguan peridotite xenoliths in the potassic lava are characterized by simple composition, and represent the remnants resulted from partial melting to various degrees of the upper mantle. Here mantle metasomatism is not remarkable and is limited to a very small number of samples. The Cpx is depleted in LREE and shows typical MORB-type Sr-Nd isotopic signatures （87^Sr/86^Sr = 0. 702 2 - 0. 702 9, εNd = 9. 5 - 12. 3）, and these characteristics are rather similar to those for oceanic lithospheric mantle. The relatively depleted peridotite （Fo 〉 90） shows an equilibrium temperature of 928 - 959 ℃, which is obviously lower than that for saturated peridotite （Fo 〈 90, 955 - 1 110 ℃ ）, implying that the lithospheric mantle has a stratified structure, which is characterized by saturated composition of the lower part and enriched composition of the upper part and is related to the decompressed melting of an upwelling asthenosphere. The features 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.