徐淮下地壳包体薄片下锆石和榍石原位U-Pb定年研究：对加厚下地壳演化的启示

A rare suite of eclogite- and HP granulite-facies lower crust xenoliths was reported from the Xu-Huai region in the southeastern margin of the North China Craton, which provides a precious opportunity to investigate the formation and evolution of a thickened lower continental crust. However, due to the small size, mafic-ultramafic chemical composition, and sophisticated thermal history of these lower crustal xenoliths, zircons are difficult to separate and precise ages are difficult to yield. The nature of the protolith and the P-T history for most of the xenoliths are still unclear as well, which hampered our understanding of the Xu-Huai eclogite- and HP granulite-facies lower crustal xenoliths. This project plan to utilize femtosecond laser ablation system (fs-LA) combined with sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) to study the U-Pb geochronology of zircon and titanite in petrographic thin sections of the Xu-Huai lower crustal xenoliths, with high spatial resolution (15-30 microns) and high precision, in order to yield more systematic and precise age data and establish the geochronological framework for the xenoliths. Together with analysis of zircon Hf, titanite Nd, and plagioclase Pb isotope in thin sections and with metamorphic petrology, our study will provide a comprehensive knowledge of origin, nature of the protolith, and P-T-t path of the Xu-Huai eclogite- and HP granulite-facies xenoliths. Then, this project will provide further constraints on the timing, genetic mechanism, and evolution history of the thickened lower continental crust in the southeastern margin of the North China Craton, which may deepen our understanding in the thickening and foundering of lower continental crust.

华北克拉通东南缘徐淮地区罕见的榴辉岩相和高压麻粒岩相变质下地壳包体为研究加厚大陆下地壳的形成演化提供了难得的窗口。但受限于包体过小的体积、基性-超基性的成分和复杂的演化历史，分选出的锆石数量少且颗粒小，难以获得包体的准确年龄；而且多数包体的原岩性质和温度-压力历史不清楚，大大制约了对徐淮榴辉岩相和高压麻粒岩相包体的认识。本研究拟利用飞秒激光单接收磁质谱对徐淮下地壳包体开展岩石薄片下锆石和榍石原位高分辨率（15-30微米）和高精度的U-Pb定年，以获得更全面、准确的年龄数据，建立徐淮包体的年代学框架。同时结合薄片原位锆石Hf、榍石Nd和长石Pb同位素分析，以及变质岩石学研究，系统认识徐淮榴辉岩相和高压麻粒岩相包体的来源、原岩性质和P-T-t轨迹，从而进一步制约华北克拉通东南缘加厚大陆下地壳的形成时间、成因机制和演化历史，深化人们对大陆下地壳加厚拆沉机制的认识。

大陆下地壳由于位于地下十几至几十公里的深度，通常无法直接接触；但该部分地壳在壳幔物质交换过程中又相对活跃，记录了丰富的演化历史，也是地球中极为重要的地质储库。本项目围绕华北克拉通东南缘加厚大陆下地壳形成与演化历史的总体研究目标，采用激光剥蚀多接收等离子体质谱薄片原位锆石、榍石和金红石定年技术，重点对华北克拉通东南缘徐淮下地壳包体开展精细年代学研究，建立准确年代学框架；在矿物同位素地球化学和变质岩石学研究基础上，确定包体的来源、原岩性质和岩浆-变质历史。三年来本项目围绕上述科学目标，开展了大量野外和室内分析测试，结合前人已有研究成果，进一步揭示了华北克拉通东南缘加厚大陆下地壳的形成时间、成因机制和演化历史。该加厚下地壳主要形成于古元古代造山事件中，叠加了三叠纪大陆碰撞造山事件，并在早白垩世区域构造-岩浆热事件中发生了破坏和减薄。这些下地壳包体在古元古代还经历了多种类型的交代作用，演化出现今复杂的铅同位素特征，对于认识大陆地壳铅同位素演化有重要意义。此外本项目还在华南陆核区域早期大陆地壳生长演化历史和副矿物激光等离子体质谱高精度定年方法等方面开展了研究。以上述研究为基础，受资助期间发表SCI论文2篇，其中2篇论文发表在影响因子>3.0的国际刊物上。1人主持国际会议分会场并作口头报告；1人获湖北省自然科学奖一等奖；1人增任国际地质学刊物Tectonics副编辑（Associate Editor）。总之，本项目不仅达到了预期科学目标，还在多方面开展了探索性研究，为今后深入研究奠定了良好基础。