青藏高原拉萨地块钾质-超钾质火山岩源区特征、深部过程和岩石成因：Li同位素制约

Based on their lithogeochemical and isotope geochemical signatures, the potassic and ultrapotassic rocks in the Lhasa block, Tibetan plateau are characterized by the dual mantle- and crustal-affinities, which can be explained by low degrees of partial melting of a metasomatically enriched upper mantle. However, many key scientific issues, such as the mechanisms of their source enrichment, are still poorly understood and expounded. As a kind of "non-traditional stable isotopes", lithium isotope system has been used to study some important geological or geochemical problems. Recently, the research on Li isotope has become one of the most fast developing fields in the study of crust-mantle interaction. One of the most important features is that there is no fractionation during the magmatic process at high temperatures, so the lithium isotopic compositions of magmatism in the subducted belt can effectively restrict whether the magmatic source region has the subducted crustal components, especially the sediments. Consequently, the lithium isotope method is an effective tracing means to resolve the mechanisms of source enrichment of potassic and ultrapotassic rocks in the Lhasa block, Tibetan plateau. This research will be carried out on the potassic and ultrapotassic rocks in the Lhasa block and different geological endmembers in Tibetan plateau by the use of the lithium isotopic method combined with the traditional methods, such as major elements, trace elements, rare earth elements, Pb-Sr-Nd isotopes, etc, to investigate source characteristics of the potassic and ultrapotassic rocks in the Lhasa block and their deep-seated processes, to further discuss their petrogenesis. So it is very important in theory.

岩石地球化学和同位素地球化学研究表明，拉萨地块钾质-超钾质火山岩具有地幔和地壳双重特征，普遍认为是交代富集上地幔低度部分熔融的产物。然而，其源区富集机制这一研究程度最高的关键科学问题，至今尚未得到很好的理解和揭示。作为一种"非传统稳定同位素"，锂同位素在地质学、地球化学研究中有着广阔的应用前景，壳-幔相互作用过程的锂同位素地球化学研究已成为近年来国际上研究的热点之一。其重要特征之一是锂同位素在高温岩浆作用过程中不发生有意义的分馏，因此，俯冲带岩浆作用的锂同位素组成能够对源区是否存在俯冲地壳组分，尤其是沉积物提供有效的约束，对解决拉萨地块钾质-超钾质火山岩源区富集机制是一种有效的示踪手段。本项目选择拉萨地块钾质-超钾质火山岩和西藏不同地质端员开展锂同位素地球化学研究，将传统方法与新兴手段相结合，揭示钾质-超钾质火山岩的起源演化及其深部过程，探讨钾质-超钾质火山岩岩石成因，具有重要的理论意义。

作为一种“非传统稳定同位素”，锂同位素在地质学、地球化学研究中有着广阔的应用前景，壳–幔相互作用过程的锂同位素地球化学研究已成为近年来国际上研究的热点之一。其重要特征之一是锂同位素在高温岩浆作用过程中不发生有意义的分馏，因此，俯冲带岩浆作用的锂同位素组成能够对源区是否存在俯冲地壳组分，尤其是沉积物提供有效的约束，对解决拉萨地块富钾火山岩源区富集机制是一种有效的示踪手段。选择印度上下地壳、新生下地壳以及富钾火山岩开展锂同位素地球化学研究，将传统方法与新兴手段相结合，揭示了富钾火山岩的起源演化及其深部过程，探讨了其岩石成因。.完善了MC-ICPMS高精度测定Li同位素分析方法，7种常用地质标准物质的锂同位素组成为：δ7LiBHVO-2=+4.7‰±1.0‰，δ7LiJB-2=+4.9‰±1.0‰，δ7LiBCR-2=+4.4‰±0.8‰，δ7LiAGV-2=+6.1‰±0.4‰，δ7LiNKT-1=+9.8‰±0.2‰，δ7LiL-SVEC=-0.3‰±0.3‰，δ7LiIRMM-016=+0.0‰±0.5‰。.印度上地壳高的δ7Li（0.95.6 ‰）值是由于印度下地壳释放的高δ7Li流体造成的，而印度下地壳低的δ7Li（–4.4–0.1 ‰）值是由于残余印度下地壳部分熔融形成的。新生下地壳的锂同位素组成（+0.8+6.6 ‰）归因于次大陆岩石圈地幔的部分熔融，受到俯冲洋壳和沉积物的不同比例流体的交代。.富钾火山岩锆石Hf–O同位素分为两组：第一组具有高的δ18O（6.7～11.3‰）、低的εHf(t)（−17.0～−12.0）；第二组的δ18O为6.8～10.7‰，εHf(t)值为−11.8～−6.3。钾质火山岩、超钾质火山岩、富镁钾质岩的锂含量分别为12.7–64.9 ppm, 15.3–46.1 ppm和11.2–15.3 ppm，δ7Li分别为−4.9‰ to +3.2‰, −3.9‰ to +1.7‰和−1.2‰ to +3.5‰。.通过计算模拟，认为6–30%、6–10%和6–11%的印度下地壳分别参与了钾质火山岩、超钾质火山岩、富镁钾质岩的源区富集，据此提出了其岩石成因模式。