华夏陆块地壳演化的微区原位碎屑独居石U-Th-Pb年龄和Nd-O同位素制约

The mechanisms by which the continental crust grow and rework are one of fundamental questions in the Earth sciences. Large datasets from single zircon U-Pb age and Hf-O isotopes have been increasingly acquired during the past decade due to great advances of in situ microanalytical techniques. While Hf-O isotopes of detrital zircons from sedimentary rocks are widely used in the study of felsic magmatism linking the crustal growth and recycling to the U-Pb crystallization age of the mineral, they seem difficult to reveal metamorphic evolution and fluid alteration characterized by a broad range of temperatures during crustal reworking. Monazite, a LREE phosphate mineral, is commonly formed in peraluminous granitoids, pegmatites and medium- to high- grade metamorphic rocks. It is high in Th and U, thus, can be precisely dated by the U-Th-Pb system. Monazite is also a common detrital mineral in clastic sedimentary rocks. It is more likely to be affected by metamorphism and alteration by fluids than zircon, providing an alternative approach to study crustal rewiring processes at relatively low temperatures. In this project, we will firstly develop SIMS monazite O-isotope analytical techniques and standards. We will carry out integrated in situ analyses of U-Th-Pb age and Nd-O isotopes for detrital zircons from Phanerozoic sedimentary rocks/sediments from the Cathaysia Block, enable new constraints to be placed on the growth and reworking of the continental crust. Our work has significant implications for better understanding (1) whether there exist the Archean basement rocks beneath the Cathsysia Block, and (2) the mechanism of the Indosinian-aged orogenesis in SE China.

大陆地壳的形成与再造演化是地球科学一个重要的基础问题和持续研究的内容。近十几年来全球碎屑锆石U-Pb年龄和Hf-O同位素研究为区域和全球大陆地壳的形成和再造演化提供了重要的记录。碎屑锆石提供了沉积物源区的酸性岩浆岩及其再循环产物的记录，但难以记录变质作用和流体蚀变等地壳再造过程。独居石是LREE磷酸盐矿物，富集Th和U，广泛形成于过铝质花岗岩、伟晶岩和中-高级变质岩，非常适合于同位素地质年代学和地球化学研究。独居石是沉积岩的一种主要碎屑富矿物，对流体蚀变敏感，对地壳变质作用和流体活动的再造示踪有重要作用。本项目拟在研发SIMS独居石微区氧同位素分析方法和标样的基础上，对华夏陆块显生宙沉积岩/沉积物的碎屑独居石进行系统微区原位定年和Nd-O同位素分析，为深入研究华夏是否存在太古代陆壳基底以及印支期造山运动属性等地壳形成和再造演化问题提供进一步的制约。

大陆地壳的形成与演化是地球科学一个重要的基础问题和持续研究的内容。近十几年来全球碎屑锆石U–Pb年龄和Hf–O同位素研究为此提供了重要的记录。锆石主要记录了中酸性岩浆作用，而独居石更容易记录变质作用和流体活动，二者结合能够更全面的示踪区域和全球地壳演化。本项目在完善离子探针（SIMS）独居石O同位素微区分析方法的基础上，综合运用锆石–独居石同位素分析为华夏陆块显生宙地质演化提供新的约束。.本项目主要研究成果包括：.1）RW-1独居石的O–Nd同位素组成在~20 µm的尺度上是均一的，其δ18O推荐值为6.30‰ ± 0.16‰ (2SD)，143Nd/144Nd的推荐值为0.512282 ± 0.000011 (2SD)。.2）独居石的Si含量和Th含量线性相关，可以替代Th含量来校正基体效应。通过单次SIMS实验同时测量独居石28Si–18O–16O同位素，我们实现了“在线”（SIMS直接测定独居石的Si含量）校正基体效应的新方法，该方法显著提高了独居石O同位素分析的效率，且精度和传统方法（电子探针测元素含量）相当（~0.8‰, 2SE）。.3）华夏陆块广东阳江地区的寒武纪变质砂岩中的碎屑锆石年龄分布非常宽（490–3000 Ma），而独居石显示单个年龄峰值235 Ma，表明独居石记录了锆石所没有记录的一次重要的三叠纪变质事件。阳江现代海滨砂中的碎屑锆石年龄谱显示6个峰值：~440、240、155、135、115和100 Ma，而碎屑独居石年龄谱显示主要峰值为237 Ma，次要峰值为435 Ma。海滨砂锆石Hf–O同位素和独居石Nd–O同位素反映早古生代和三叠纪事件以地壳再造为主，侏罗–白垩纪事件有新生地壳物质加入。综合区域上花岗岩的时空分布和地球化学特征来看，侏罗–白垩纪花岗岩产生于活动大陆边缘背景，和古太平洋的俯冲有关；而早古生代和三叠纪岩石都是陆内造山有关的大陆地壳再造，而不是来自增生或碰撞造山带。