道县辉长岩-麻粒岩复合捕虏体研究对下地壳生长和变质的约束

The process of mantle-derived magma underplating and triggering the remelting of previous lower crust is a crucial way for the continental lower crustal growth and reworking. Understanding these processes and related geodynamics is critical for investigating the formation and evolution of the earth. However, it is still poorly known about the transition process from the magmatism to metamorphism in the continental lower crust. The Mesozoic basalts in Daoxian carried abundant lower crustal xenoliths, such as gabbro, granulite and gabbro+granulite composite xenoliths, which provides a rare opportunity to recognize the processes of growth and metamorphism of the lower crust. This project will conduct a comprehensive research of petrology and mineralogy on three types of xenoliths in the Daoxian basalts, which aims to investigate the element migration and isotopic fractionation and the dynamics of these variations during the granulite facies metamorphism. By integrated with zircon in situ U-Pb dating results and Hf-O isotopes, whole-rock geochemical and Sr-Nd-Pb-Hf isotopic compositions of the gabbro and granulite xenoliths, we can recognize the rock-forming process of the gabbro and granulite in the Daoxian lower crust, and illuminate the processes of mantle-derived magma underplating and previous lower crust remelting, which are important for understanding the continental lower crustal accretion and evolution.

幔源岩浆底侵并引起先存下地壳重熔是大陆下地壳生长和改造最重要的途径之一，了解这一过程及其发生机制对探讨地球的形成与演化等科学问题具有重要意义。但目前对于大陆下地壳经历岩浆作用到麻粒岩相变质作用的转变过程仍缺乏足够的了解，道县虎子岩中生代玄武岩携带的辉长岩、麻粒岩以及辉长岩+麻粒岩复合捕虏体为揭示下地壳变质过程提供了契机。本项目拟对这三类捕虏体开展系统的岩石学、矿物学和地球化学研究，查明麻粒岩相变质过程中的元素迁移与同位素变化特征及其行为机制；结合辉长岩和麻粒岩捕虏体的锆石U-Pb年龄和Hf-O同位素组成、全岩地球化学以及Sr-Nd-Pb-Hf同位素组成研究，恢复辉长岩和麻粒岩的成岩过程，提取幔源岩浆底侵及先存地壳重熔的信息，为认识大陆下地壳的生长、演化提供重要参考。

麻粒岩捕虏体由火成岩迅速携带至地表，反映下地壳的原位信息，是研究下地壳的直接样品，记录了岩浆底侵作用与先存下地壳再造之间的相互关系。道县辉长岩-麻粒岩复合捕虏体记录了岩浆-变质的结构变化，可能为下地壳的变质过程提供约束。针对这些现状，项目对道县中生代玄武岩中的各类捕虏体开展了矿物学、岩石学、地球化学、全岩Sr‒Nd‒Pb‒Mg同位素、锆石年代学及原位Hf–O同位素研究，获得以下认识：.（1）长英质和镁铁质麻粒岩捕虏体均记录了相似且连续的岩浆结晶（226–218 Ma）和变质作用年龄（218–193Ma），指示了两者的成因联系。复合捕虏体的矿物化学特征显示变质作用可能导致了Rb、Th和U的亏损，其他主、微量元素组成没有明显变化。地球化学和同位素证据显示镁铁质捕虏体（含复合捕虏体）来自幔源岩浆底侵，底侵导致了先存下地壳的重熔最终形成长英质捕虏体。镁铁质麻粒岩具有比平均地幔更轻的Mg同位素组成，指示其地幔源区经历了沉积物的再循环交代作用。.（2）捕虏体中含有年龄丰富的锆石，记录了华夏下地壳从晚太古代到早中生代多阶段的演化过程。这些锆石U‒Pb年龄和Hf‒O同位素数据与华夏地壳显生宙岩浆活动分布趋势一致，支持了华夏地壳中生代岩浆底侵作用导致先存地壳物质熔融并产生广泛花岗质岩浆活动的演化模型。.（3）温压估算结果显示复合捕虏体来自下地壳的上部约35–40 km，而镁铁质麻粒岩捕虏体可能来自下地壳底部45 km左右，暗示了早中生代岩浆底侵作用导致下地壳增生加厚。镁铁质捕虏体高的Mg#可能指示Fe-Ti氧化物的分离结晶，重的、富Fe-Ti的物质的“拆沉”可能是华夏现今下地壳减薄的原因之一。.本项目细化了华夏地块下地壳的增生和再造历史，约束了下地壳的Mg同位素组成，为认识华夏地块下地壳加厚与减薄提供不同角度，相关成果部分发表在Lithos期刊，部分正在撰写论文。