古老大陆岩石圈地幔再循环与蛇绿岩中铬铁矿床成因

不同地区、不同时代蛇绿岩中不同类型铬铁矿岩的Re-Os同位素研究表明,在铬铁矿石或围岩中均存在极度亏损的具有大陆岩石圈地幔属性的物质。新疆达拉布特古生代蛇绿岩带中萨尔托海富Al铬铁矿岩的Os同位素组成为0.1109～0.1256,对应的模式年龄为3.5～0.6Ga;西藏班公湖—怒江中生代蛇绿岩带中东巧富Cr铬铁矿石及围岩Os同位素组成介于0.1175～0.1261,对应的模式年龄为1.5～0.1Ga;雅鲁藏布江中生代蛇绿岩带中罗布莎富Cr铬铁矿岩的Os同位素变化范围为0.1038～0.1266,对应的模式年龄为3.37～0.28Ga,而该带中不含矿的泽当二辉橄榄岩的Os同位素组成为0.1256～0.1261,没有古老大陆岩石圈地幔属性的物质存在,与新特提斯洋地幔Os组成较为接近。推测在蛇绿岩形成过程中,古老大陆岩石圈地幔参与循环有利于形成铬铁矿床,明确提出"熔体与古老大陆岩石圈地幔反应成矿"的假说,指出蛇绿岩带中存在的古老微陆块可能是找矿的指示标志。

Recycling of Ancient Sub Continental Lithospheric Mantle Constraints on the Genesis of the Ophiolitic Podiform Chromitites

This study presents the comparison data of Os isotopic compositions of chromitites, chromitite bearing and free peridotites from Dongqiao and Luobusa ophiolites in Xizang(Tibet) and Sartohay chromitites in Xinjiang Autonomous Region with different ages. The results identify that chromitites and chromitite bearing peridotites show different Os isotope sources with chromitite free peridotites. The chromitite free peridotites (Zedang lherzolite) have a narrow range of n(187Os)/n(188Os) ratios from 0.1256±0.0008 to 0.1261±0.0008 consisting with Os isotopic compositions of Tibetan Tethys Ocean convecting upper mantle. But in contrast, the chromitites and the chromitite bearing peridotites yield a very wide range of n(187Os)/n(188Os) values as follows. The Os isotopic composition of the Palaeozoic Sartohay Al rich chromitites range from 0.1109 to 0.1256, corresponding Re depletion model ages (TRD) from 3.5 Ga to 0.6 Ga, the Cr rich chromitites from Mesozoic Tibetan ophiolites show the n(187Os)/n(188Os) ratios between 0.1175 and 0.1261 in the Dongqiao deposit with TRD = 1.5~0.1Ga, and n(187Os)/n(188Os) ratios of the Luobusa chromitites vary from 0.1038 to 0.1266, corresponding the TRD ranging mainly from Paleozoic to Archean. Their age distribution mirrors the tectonic history of the Tibetan Tethys Ocean and Paleo asian Ocean. These data suggest that part of these oceans is underlain by a fragment of ancient subcontinental lithospheric mantle (SCLM), left stranded in the oceanic lithosphere during the opening of these oceans. And in the supra subduction zone (SSZ) setting, the chromium, which is early partitioning into the bottom of mantle during the evolution of the Earth, removed again caused by high fO2 fluids or melts and enriched in the SSZ type ophiolite. Here we suggest that the recycling of the ancient SCLM plays a key role in the formation of the ophiolitic chromitites via the interaction of percolating melt with the ancient SCLM, and the ancient terrain surviving within the ophiolitic sutures likely becomes the diagnostic criterion.