元古代仁错蛇绿岩地幔橄榄岩的属性对拉萨岩石圈地幔演化的约束

The Lhasa Terrane is a microcontinent with at least Proterozoic basement. However, it remains unclear if Proterozoic (or even order) lithospheric mantle was ever existed beneath the Lhasa microcontinent. Moreover, the Shiquanhe–Yunzhug–Namu Tso ophiolite belt (SNOB), cropped along within the Lhara Terrane, comprises not only Mesozoic ophiolitic massifs, but also newly-identified Proterozoic RenTso ophiolite. However, it is still controversial if these ophiolites could represent oceanic relicts that developed within the Lhasa continental lithosphere during different geological periods. These might provide possible clue for the understanding and comparison of the difference and evolution between the Proterozoic and Mesozoic Lhasa lithospheric mantle, which could shed new lights on the origin and evolution of the Lhasa Terrane. Here, we intend to present detailed field geology, mineralogical, Re-Os isotope and PGE geochemical studies of mantle peridotites from the Proterozoic RenTso ophiolite, which are hoped to provide robust constraints on their petrogenesis, Re-Os isotope isochron and/or model ages, and tectonic provenance (subcontinental or suboceanic lithospheric mantle). Then, we hope to figure out if there is any difference in nature and evolution between the Proterozoic RenTso mantle peridotites and the Mesozoic Yunzhug peridotites (with our previous work strongly suggesting a subcontinental origin). Collectively, results from these mantle peridotites as well as their coexisted mafic rocks are integrated to provide constraints for the origin and evolution of the Lhasa lithospheric mantle and the SNOB.

拉萨地体是一个至少具有元古代基底的微型陆块，然而关于该陆块是否保留元古代甚至更老的岩石圈地幔记录尚不清楚；另外，出露于该陆块内部的狮泉河-永珠-纳木错蛇绿岩带不仅发育中生代的蛇绿岩，而且最近的研究还揭示出存在元古代的仁错蛇绿岩，那么，这些蛇绿岩是否代表了拉萨内部大陆岩石圈在不同时代裂解形成的古洋遗迹? 这为我们了解和对比元古代与中生代拉萨岩石圈地幔的差异和演化提供了可能, 并进一步对拉萨陆块的形成和演化提供新的约束。本申请以元古代仁错蛇绿岩的地幔橄榄岩为研究对象，拟通过野外地质、矿物学、Re-Os同位素与PGE等研究，查明其岩石成因、Re-Os同位素等时线或模式年龄、属性（大陆或大洋）等；对比相邻的中生代永珠蛇绿岩内具大陆属性的地幔橄榄岩（申请者已初步证实），揭示两者之间的异同和联系；综合共存的同时代基性岩的已有成果，进一步约束拉萨地块岩石圈地幔和狮泉河-永珠-纳木错蛇绿岩带的形成与演化。

出露于拉萨地体内部的狮泉河-永珠-纳木错带广泛发育中生代的蛇绿岩，而且最近的研究还揭示出在中部仁错地区可能存在元古代的蛇绿岩，由此引发了关于狮泉河-永珠-纳木错带是否存在多期蛇绿岩的激烈讨论。在项目的资助下，我们对仁错地区出露的镁铁-超镁铁质岩体进行了系统的野外考察、岩相学、矿物和全岩地球化学综合研究。研究表明，仁错新元古代辉长岩具有高放射性Os同位素组成，具有高Ti-Fe和MORB型地球化学成分特征，是软流圈幔部熔融产生的熔体经历干、低氧逸度结晶分异作用的产物。然而区域上与辉长岩密切共生的地幔橄榄岩来自中生代新增生的大洋岩石圈地幔，与侏罗纪MORB型镁铁质岩石构成了中生代蛇绿岩套。尚没有明确证据表明狮泉河-永珠-纳木错带存在多时期形成的蛇绿岩。侏罗纪永珠-仁错蛇绿岩可能整体上代表了威尔逊旋回初级阶段大陆裂解－洋盆形成的过程。