橄榄岩—熔体相互作用过程中Re-Os体系的变化趋势:对华北新生代地幔橄榄岩Re-Os年龄含义的启示  

Variation of Re-Os isotopic system during peridotite-melt interaction:Implication for the meaning of Re-Os isotopic age of Cenozoic mantle peridotites from the North China Craton

作　　者：张宏福[1] 

机构地区：[1]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029

出　　处：《岩石学报》2008年第11期2457-2467,共11页Acta Petrologica Sinica

基　　金：国家自然科学基金委大陆动力学重点项目(40534022);<克拉通破坏>计划项目(90714008);群体项目(40721062);中国科学院知识创新工程重要方向项目(KZCX2-YW-103)资助成果

摘　　要：橄榄岩的 Re-Os 同位素体系作为探讨岩石圈地幔形成年龄的重要手段已受到人们越来越多的重视。然而,Re-Os体系是否会受到橄榄岩的低温蚀变如蛇纹石化和地幔交代作用或橄榄岩—熔体相互作用的影响并不十分清楚。本文在总结国际上典型造山带橄榄岩剖面研究和地幔橄榄岩捕虏体研究的基础上,发现橄榄岩的 Re-Os 同位素体系基本不受蛇纹石化等低温蚀变作用的影响;但橄榄岩—熔体相互作用会造成橄榄岩的 Re-Os 体系的变化即 Re 或 Os 的带入。当熔体/岩石比例较小时(≤1),橄榄岩—熔体相互作用会造成橄榄岩的 Re 含量显著增加,而放射成因的 Os 同位素即^(187)Os 增量有限。但当熔体/岩石比例很大时(≥1),会造成^(187)Os 同位素显著增加。同时,橄榄岩的 Re 含量与中等程度不相容主、微量元素如 Al、Ti、Yb 的正相关关系可以通过橄榄岩的两阶段演化模式来解释,即早期的部分熔融残留和后期的橄榄岩—熔体相互作用共同影响的结果。橄榄岩的 Re 含量与 Cr 含量的正相关关系进一步说明了这一问题。上述结果对华北东部岩石圈地幔的演化过程有重要的启示作用。因为华北东部古生代时期的确存在太古宙岩石圈地幔,然而新生代时期地、橄榄岩捕虏体普遍具有元古宙而非太古宙的事实证明橄榄岩—熔体的相互作用的确广泛存在,从而造成岩石圈地幔橄榄岩变年轻。因此,新生代地幔捕虏体的 Re-Os 同位素年龄并不是岩石圈地幔的形成年龄,而是混合作用的结果。汉诺坝地幔橄榄岩的 Re 含量和 Sr、Nd 同位素组成符合橄榄岩—多元熔体相互作用的混合趋势,其橄榄岩的全岩 Al_2O_3含量和橄榄石 Fo 的反相关性能够通过橄榄岩—熔体相互作用来解释。More and more attention has been paid to peridotite Re-Os isotopic system as this system behaves as an important method to probe the formation age of the lithospheric mantle. However, if the Re-Os system is affected by the low-temperature alteration such as serpentinization of the peridotite and mantle metasomatism or peridotite-melt interaction is still not very clear. This paper, based on the reviewing intergration of the studies on the typical orogenic peridotite massif profile and mantle peridotite xenoliths in the world, found that peridotite Re-Os system is generally unaffected by the low-temperature alteration. However, peridotite-melt interaction could change the peridotite Re-Os system, i.e. the introduction of the Re and/or Os. Periditite-melt interaction at the low melt/rock ratios （≤1） could result in an obvious increase in peridotite Re abundance and a limited increase in radiogenic Os, i.e. lSTOs. The peridotite-melt interaction at the high melt/rock （ ≥ 1 ） could result in a marked increase in 187 Os isotope. At the same time, positive correlations between the Re abundance of the peridotite and moderately incompatible major and trace elements such as AI, Ti, Yb have been interpreted by two stage model of evolution, namely as an resuh of early stage partial melting and later peridotite-melt interaction. The positive correlation between the Re abundance and the Cr observed in the peridotite further argue for such a suggestion. Above result has an important implication for the evolution of the lithospheric mantle beneath the eastern North China Craton. Archean lithospheric mantle indeed occurred in the Paleozoic in the eastern North China Craton. However, mantle peridotite xenoliths entrained in the Cenozoic basalts ubiquitously have Proterozoic rather than Archean Re-Os ages. This fact suggests that peridotite-melt interaction indeed exist extensively in the lithospheric mantle beneath the eastern North China Craton, leading to the lithospheric mantle becoming younger. Therefore, Re-Os is

关 键 词：Re—Os体系 橄榄岩/熔体相互作用 Re或放射成因Os带入 Os同位素年龄 华北东部 

分 类 号：P588.125[天文地球—岩石学] P597.3[天文地球—地质学]