Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications
Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications
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西藏那曲蛇绿岩碳质球粒状地幔来源的骨同位素证据及其意义
DOI：
10.1007/s11434-012-5384-8
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发表时间：
2013
影响因子：
--
通讯作者：
DING Lin
中科院分区：
文献类型：
--
作者：
LIU DeLiang;ZHANG XiaoRan;FAN ShuaiQuan;DING Lin
那曲蛇绿岩中的早结晶铬尖晶石（铬尖晶石）具有高Os、低Re的特点，在蛇纹石化、风化变质过程中不易蚀变。原始岩浆的化学成分保存在铬尖晶石中，这使得它适合于确定地幔源区的原始锇同位素组成。本文报道了西藏那曲蛇绿岩堆晶岩剖面中堆晶纯橄榄岩和辉长岩的铬-尖晶石-锇同位素和锆石U-Pb年龄。研究结果对岩石圈地幔的形成和演化具有重要意义。那曲蛇绿岩位于班公湖-怒江缝合带中段。它是新特提斯洋壳的残余，包含堆晶纯橄榄岩和辉长岩。辉长岩中锆石的206 Pb/238 U加权平均年龄为183.7±1 Ma。铬尖晶石的γ Os值为−0.2至−0.3，表明纯橄榄岩的地幔来源与碳质辉长岩相似。因此，西藏岩石圈主要是特提斯洋洋岩石圈的遗迹，特提斯洋岩石圈是由中生代正常软流圈地幔改造而成的。这是首次将尖晶石Os同位素联合收割机与精确锆石U-Pb年龄相结合来制约蛇绿岩地幔源区地球化学特征的研究。
Early-crystallizing chromian spinel (Cr-spinel) in the Nagqu ophiolite has high Os and low Re contents, and it is resistant to alteration during serpentinization, weathering and metamorphism. The chemical composition of primitive magma is preserved in Cr-spinel, which makes it suitable for determining the initial Os-isotope composition of the mantle source. This study presents Cr-spinel Os isotopes and zircon U-Pb ages for cumulate dunite and gabbro, respectively, in the same cumulate section of the ophiolite at Nagqu in Tibet. The results shed light on the formation and evolution of lithospheric mantle. The Nagqu ophiolite is located in the central part of the Bangong-Nujiang suture zone. It is a remnant of the Neotethyan oceanic crust, and contains cumulate dunite and gabbro. Zircon from the gabbro yielded a weighted mean206Pb/238U age of 183.7±1 Ma. Cr-spinel exhibitsγOsvalues of −0.2 to −0.3, suggesting that the mantle source for the dunite is similar to that of carbonaceous chondrites. Thus, the Tibetan lithosphere is primarily a relic of Tethyan oceanic lithosphere, which has formed by the transformation of the normal asthenospheric mantle in the Mesozoic. This is the first study to combine the spinel Os isotopes with accurate zircon U-Pb ages to constrain the geochemical characteristics of the mantle source for the ophiolite.
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作者：
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