西藏中部班戈大型花岗岩基中镁铁质岩石和低Sr、低Y强过铝质花岗岩的成因

Petrogenesis and compositional differentiation of giant granitic batholiths within accretionary orogenic belts are central to understand the mechanism for the formation of continental crust, and thus receive much attention from international scientific community. To unravel the mechanism for the formation of continental crust in accretionary orogenic belts, this proposal sheds lights on the large Baingoin Batholith within the Lhasa-Qiangtang accretionary orogenic belt that is characterized by the presence of extensive accretionary complex, with special emphasis on the origin of mafic rocks (including mafic dykes and enclaves) and granitic rocks with distinct geochemical signatures (e.g., low Sr, low Y, and strongly peraluminous) within the batholith. On the basis of the route large-scale geological mapping and petrographical observations, as well as in-situ mineral compositional and whole-rock elemental and isotopic determinations, this proposal aims to (1) constrain whether the mafic rocks (especially the mafic dykes) represent the evolved mantle-derived basaltic magmas or the earlier cumulates crystallized from parental magmas, (2) recover the compositions of parental magma, (3) explore the origin of the granitic rocks with distinct geochemical signatures (e.g., low Sr, low Y, and strongly peraluminous), and (4) reconstruct the genetic links and compositional differentiation between the mafic rocks and other lithologies in the batholith. These results are used to discuss the mechanism responsible for the formation of continental crust within the Lhasa-Qiangtang accretionary orogenic belt. The expected result of this proposal is to provide a good example to explore the generation of giant granitic batholiths and the formation of continental crust within accretionary orogenic belts worldwide.

增生造山带大型花岗岩基的成因和成分分异过程对理解大陆地壳的形成机制具有重要意义，因此备受国际学术界关注。为揭示增生造山带大陆地壳的形成机制，本项目拟以发育大量增生杂岩、与微陆块或地体拼贴有关的拉萨－羌塘增生造山带内的班戈大型花岗岩基为对象，重点聚焦其中的镁铁质岩石和具独特地球化学特征（低Sr、低Y、强过铝质）的花岗质岩石。在野外开展大比例尺路线地质填图和岩相学观察基础上，通过系统的单矿物原位成分分析、元素和同位素地球化学测试，确定这些镁铁质岩石（尤其是镁铁质岩脉）究竟是演化的幔源岩浆还是母岩浆堆晶作用的产物，恢复母岩浆成分，限定低Sr、低Y强过铝质花岗质岩石的岩浆源区，探讨不同岩性之间的成因关联和成分分异过程，从岩浆成因角度讨论拉萨－羌塘碰撞带大陆地壳的形成机制，能够为探索增生造山带大型花岗岩基成因和大陆地壳形成机制提供借鉴。

以班戈岩基和邻近Gajin岩基为主要研究对象，查明了班戈岩基岩浆活动的时空分布特点和岩石组合，发现班戈岩基之下约 25 km和10 km深度发育中下地壳岩浆房，认为班戈岩基形成于班-怒洋俯冲晚期和拉萨-羌塘地体拼合后板片拆离背景下的母岩浆结晶分异和变沉积岩部分熔融，提出Gajin高硅花岗岩稀土元素的四分组效应可以由副矿物（独居石、褐帘石、磷灰石和锆石）的分离结晶产生，不需要岩浆热液参与，这意味着需要重新评估岩浆-热液相互作用在稀有金属成矿中的作用。本项目提出拉萨地体很可能位于澳大利亚北西部，其东段可能代表了中澳大利亚北部的外延，需要谨慎使用碰撞带上盘岩浆的富集组分来限定陆-陆碰撞的时间。在Tectonics、GRL、Lithos等刊物共发表SCI论文9篇，培养硕士研究生2名。