Tibetan chromitites: Excavating the slab graveyard
Tibetan chromitites: Excavating the slab graveyard
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DOI：
10.1130/g36245.1
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发表时间：
2015-02
期刊：
影响因子：
5.8
通讯作者：
Nicole M. McGowan;W. Griffin;J. González-Jiménez;E. Belousova;J. Afonso;R. Shi;C. McCammon;N. Pear
中科院分区：
文献类型：
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作者：
Nicole M. McGowan;W. Griffin;J. González-Jiménez;E. Belousova;J. Afonso;R. Shi;C. McCammon;N. Pear
罗布萨地块(西藏东南部)贫化方辉橄榄岩中的豆荚状铬铁矿中含有钻石和高度还原的微量矿物组合。透辉石和柯石英从铬铁矿中出溶，反映了地幔过渡带上部(>；400公里)钙铁氧体结构的反转。然而，铬铁矿的痕量元素特征是蛇绿岩铬铁矿的典型特征，暗示着浅层的初级结晶。铬铁矿中OS-Ir金块的Re-Os模式年龄(TRD)为234±3 Ma，而原位Ru-Os-Ir硫化物的TRD年龄范围为290~630 Ma，峰值出现在325 Ma左右。铬铁矿中真面体锆石的U-Pb年龄为376±7 Ma，e Hf=9.7±4.6，d18O=4.8‰-8.2‰。硫化物和锆石年龄可以确定超俯冲带环境中类玻尼石熔体中铬铁矿形成的年龄，而Os-Ir金块的模式年龄可以确定泥盆纪俯冲后过渡带的局部还原。热机械模型显示了一种快速(≲10 m.y.)在印度板块的早第三纪和/或晚白垩世退回期间，浮力方辉橄榄岩从400公里深处上升。这一过程可能发生在其他碰撞带中；来自过渡带的地幔样品可能比目前所认识的更广泛。
Podiform chromitites enclosed in depleted harzburgites of the Luobusa massif (southeastern Tibet) contain diamond and a highly reduced trace-mineral association. Exsolution of diopside and coesite from chromite suggests inversion from the Ca-ferrite structure in the upper part of the mantle transition zone (>400 km). However, the trace-element signatures of the chromites are typical of ophiolitic chromitites, implying primary crystallization at shallow depths. Os-Ir nuggets in the chromitites have Re-Os model ages (T RD ) of 234 ± 3 Ma, while T RD ages of in situ Ru-Os-Ir sulfides range from 290 to 630 Ma, peaking at ca. 325 Ma. Euhedral zircons in the chromitites give U-Pb ages of 376 ± 7 Ma, e Hf = 9.7 ± 4.6, and d 18 O = 4.8‰–8.2‰. The sulfide and zircon ages may date formation of the chromitites from boninite-like melts in a supra-subduction-zone environment, while the model ages of Os-Ir nuggets may date local reduction in the transition zone following Devonian subduction. Thermo-mechanical modeling suggests a rapid (≲10 m.y.) rise of the buoyant harzburgites from >400 km depth during the early Tertiary and/or Late Cretaceous rollback of the Indian slab. This process may occur in other collision zones; mantle samples from the transition zone may be more widespread than currently recognized.