华北南缘嵩山地区前寒武纪地壳演化：花岗质岩石的地球化学研究

Archean Craton provide important information to understand crustal evolution processes in the early history of the Earth. The North China Craton (NCC) is one of the oldest cratons in the world, with widespread Archean to Paleoproterozoic basement. A new progress in understanding the evolution of the NCC has resulted in a broad consensus that the NCC comprises at least the independently-developed Eastern and Western Blocks separated by the Trans-North China Orogen. The Precambrian crustal evolution of the NCC has been investigated intensively in the last two decades. These studies mainly focused on the central and the northern margin of the NCC, little has been done on the southern margin of the NCC. The Songshan area is situated in the southern margin of the NCC, where Precambrian granitoids are the focus of this study. Feldspar is an important mineral in granitoid rocks. High common Pb and low U, Th content of feldspar makes radiogenic growth after its formation negligible, which is considered as a powerful isotopic tracer of provenance and petrogenetic processes. Based on petrographical observation, the high-precision in-situ analysis on feldspar Pb isotope, microbeam analyses for trace elements, oxygen and Lu-Hf isotopes, and U-Pb ages will be performed on zircons from these granitoids. In addition, whole-rock major and trace elemental analyses will be carried out for the same set of samples. The results will be used to decipher the petrogenesis of the granitoids and the source nature, and their bearing on crust-mantle interaction. This study finally aims to provide important insights into the Precambrian evolution history of the southern margin of the NCC.

太古代克拉通为研究地球早期地壳演化提供了重要证据。华北克拉通是世界上最古老的克拉通之一，研究表明其至少由独立演化的东部和西部块体，以及经俯冲/碰撞拼合而形成的中部造山带三部分组成。关于华北克拉通前寒武纪地质，现有的研究大部分集中在华北中部及北部地区，对华北克拉通南缘基底构造演化研究相对薄弱。长石是花岗质岩石中的重要矿物，具有高普通Pb和低Th，U含量的特点，使其形成后的放射性增长可以忽略不计，因此长石Pb同位素组成对于示踪岩石成因和物质来源具有重要意义。本项目拟以华北克拉通南缘嵩山地区前寒武纪花岗质岩石为研究对象，在岩相学观察的基础上，通过高精度长石Pb同位素原位微区分析和系统的锆石学(微量元素、氧同位素、Lu-Hf 和 U-Pb 同位素)研究，结合全岩主微量元素分析，探讨其岩石成因和源区物质组成，进一步理解岩石形成过程中发生的壳-幔相互作用，最终探索华北克拉通南缘前寒武纪地壳演化过程。

晚太古代岩浆岩在华北克拉通广泛分布，但是对这些晚太古代岩石的构造背景仍然存在争议。登封杂岩位于华北克拉通南缘，出露一套典型的前寒武纪变质岩，包括大量的晚太古代变质火山岩，记录了华北克拉通早期构造演化和地壳演化过程。本项目通过对登峰杂岩晚太古代变质火山岩(角闪岩、黑云母变粒岩和浅粒岩)的地球化学和锆石U-Pb-Hf-O同位素的研究，制约这些岩石的成因，并进一步认识华北克拉通南缘的年代学框架、构造背景和地壳演化过程。.所研究的角闪岩具有较高的Mg#，富集LREE、Rb、K和Pb的特点。这些样品的锆石显示正的εHf(t)值和主体高的δ18O值(6.0-7.5‰)。所有这些特征都表明，它们极有可能来自一个被低温热液蚀变的洋壳交代的的地幔楔。SIMS锆石U-Pb同位素定年结果表明，角闪岩的原岩年龄在2.76-2.51 Ga，分别在2.48 Ga和1.92 Ga经历了两次变质事件。所研究的长英质变质火山岩具有低的Mg#，富集LREE、Rb、K和Pb，亏损Nb、Ta、Ti，具有不同程度的Eu负异常等特征。它们的锆石显示正的εHf(t)值和与MORB类似的δ18O值。这些特征表明，它们主要来源于新生地壳的部分熔融。SIMS锆石U-Pb同位素定年结果表明，长英质变质火山岩喷发于2.54-2.52 Ga。在后期的变质事件中没有变质锆石生长。在此基础上，我们提出了晚太古代华北克拉通南缘构造演化的新模型，提供了晚太古代构造从俯冲(早于2.83 Ga)到碰撞(2.48 Ga)的时间限制。