独居石的地球化学特征对A型花岗岩岩石学成因的指示作用

The A-type granite that occurs in extensional tectonic environments has been a focus point of investigation due to its particular chemical characteristics and the close relationship to large-scale mineralization of tungsten, tin, uranium, niobium, tantalum and rare earth. The Late Mesozoic A-type granite in the Eastern Segment of the Yangtze Block is characterized with high εNd(t). However, it is still controversial on the origin of the granite with high εNd(t), which limits our understanding for the formation and evolution of the continental crust in this region. At present, because the study of Nd isotope in this region is only limited to the whole-rock scale, the change of Nd isotope composition in magmatic evolution cannot be effectively discriminated. This project intends to investigate the monazite of the Late Mesozoic A-type granite in the Eastern Segment of the Yangtze Block. Combined with EPMA and LA-ICP-MS to obtain the corresponding the major and trace element compositions and geochronology, LA-MC-ICP-MS will be used to determinate the Nd isotopic compositions of the monazite, which will reflect the spatial change of the Nd isotopic compositions from magmatic system components in the process of generation, emplacement and evolution of A-type granitic magmas. And then the data will provide evidence for the theoretical basis to constraint sources of the granitic melt with high εNd(t) and solve the petrogenesis of the A-type granite.

A型花岗岩以其独特的地球化学特征与典型的构造意义备受关注，并与钨、锡、铀、铌、钽、稀土等大规模多金属成矿密切相关。扬子板块东缘晚中生代A型花岗岩以高εNd(t)为特征，但对其高εNd(t)的来源问题仍存争议，这就制约了我们对于该地区大陆地壳形成与演化的认识。目前，由于该地区Nd同位素研究仅局限于全岩尺度上，岩浆演化过程中Nd同位素组成的变化不能得到有效的甄别。本课题拟以扬子板块东缘晚中生代A型花岗岩中独居石副矿物作为主要研究对象，利用LA-MC-ICP-MS进行原位微区Nd同位素组成的测定，并结合电子探针和LA-ICP-MS等技术手段获得与之对应的主微量元素含量和年代学等特征，以此反映在A型花岗岩的形成、侵位、演化过程中岩浆体系Nd同位素组成的变化，进而约束高εNd(t)花岗质熔体的源区，为解决A型花岗岩岩石学成因，提供数据支撑以及理论依据。

大陆地壳生长和演化是地球科学领域的核心科学问题。本项目对扬子板块东缘晚中生代A型花岗岩的高εNd(t)属性来源开展研究，以期提高对我们对该地区大陆地壳生长与演化的认识。相比于前人Nd同位素研究局限在全岩尺度上，岩浆演化过程中Nd同位素组成的变化不能得到有效的甄别。我们对选定的河桥、沈中坞、苏州3个A型花岗岩中独居石样品，利用电子显微镜、扫描电镜、电子探针、LA-ICP-MS和LA-MC-ICP-MS等多种技术手段，开展岩相学、年代学、主微量元素、Nd同位素等方面的研究，有效限定独居石成因类型，形成年代和Nd同位素组成。. 主要研究成果如下：（1）确定了河桥和沈中坞A型花岗岩的独居石以岩浆类型为主，而苏州A型花岗岩的独居石为岩浆-热液过渡类型。（2）河桥、沈中坞、苏州A型花岗岩的独居石U-Th-Pb年龄依次为132.5±0.8 Ma、134.8±1.0 Ma和127.4±1.0 Ma，与前人所做的锆石U-Pb年龄在误差范围内一致。（3）河桥、沈中坞、苏州A型花岗岩独居石的Nd同位素相对均一，相应的εNd(t)依次为-6.6~-5.1（均值-6.0）、-5.7~-4.1（均值-5.0）和-9.7~-8.7（均值-9.3），且二阶段Nd模式年龄TDM2(Nd) 依次为1237~1352 Ma、1152~1282 Ma和1530~1605 Ma。（4）结合本次研究和前人已发表的资料数据，我们认为该地区晚中生代A型花岗岩中高εNd(t)地球化学特征为新元古代新生下地壳初始熔融所造成的，而并非后期岩浆混合等演化作用的结果。该项成果为该地区晚中生代高εNd(t)属性的A型花岗岩形成机制及其对大陆地壳生长与演化和成矿作用提供新的视角。