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

本文对马关地区新生代碱性玄武岩中的地幔包体进行了系统的岩石学和地球化学研究,并首次进行了包体的Re-Os同位素测试。马关地区的橄榄岩包体主量成分上表现为饱满肥沃的特征;具有不同程度的轻稀土亏损特征,亏损Nb、Ti和Zr等高场强元素(HFSE)以及Ba等大离子亲石元素(LILE);橄榄岩包体的Nd同位素特征表明橄榄岩包体代表的是不均一的亏损地幔。5个橄榄岩全岩样品的Re-Os同位素分析结果表明,样品的Os含量总体较高(3.29×10^-9~3.78×10^-9),接近于造山带橄榄岩体的Os含量,Re含量变化范围较大(0.24×10^-9~0.54×10^-9),与Re的迁移能力较强有关。样品的¹⁸⁷Os/¹⁸⁸Os值在0.12295~0.12530之间变化,与¹⁸⁷Re/¹⁸⁸Os值和Al₂O₃含量之间都不存在较好的相关性,说明Re-Os体系不单纯由熔体抽取过程所控制。橄榄岩包体的Re亏损年龄t_RD为254~604Ma,说明马关地区岩石圈地幔形成的时代应该在新元古代之前。马关地区岩石圈地幔并非是由软流圈上涌新增生的地幔,而是经历了如下演化历史:在新元古代之前,由原始地幔的部分熔融和熔体抽取作用形成了岩石圈地幔,之后经历了熔/流体交代和改造而发生了再富集作用,导致部分地幔橄榄岩逐渐从亏损难熔的特征向饱满肥沃转变,而未遭受熔/流体的改造的橄榄岩仍然保持了难熔亏损的特征。这种熔/流体交代和改造作用很可能与晚二叠纪峨眉山地幔柱的活动有关,而新生代以来印度-亚洲大陆碰撞导致地幔物质向东南方向的侧向流动,诱发软流圈上涌和马关地区的钾质岩浆的活动,也对马关地区岩石圈地幔的改造具有重要的影响,但由于喷发时间较新对Os同位素组成的影响还未显现出来。

The composition and age of subcontinental lithospheric mantle beneath Maguan, Yunnan Province: Constraints from Re-Os isotopes of mantle-derived peridotitic xenoliths

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.