花岗岩Nd-Hf同位素研究——以北祁连花岗岩类为例

Zircon has been traditionally considered to be the major host of Hf in granitoids. Zircon Hf isotopic composition is regarded as representing the whole rock Hf isotopic composition. However, volcanic-arc granite Kekeli Batholith and syn-collisional granite Jinfosi Batholith in the North Qilian Orogenic Belt possess significant decoupling between whole rock Hf and zircon Hf isotopes. Besides, whole rock Hf isotopes are largely decoupled from whole rock Nd isotopes but zircon Hf isotopes are not. The significant positive correlation between εHf(t) and TiO2 indicates that the TiO2-rich minerals may be important Hf host apart from zircons and they have the depleted Hf isotopic compositions. The study will combine analysis of geochronology with multiple isotopes for the two batholiths. This project aims to investigate the Nd-Hf isotopic compositions of Ti-rich minerals and mechanisms of decoupling between zircon Hf and whole rock Hf isotopes and between whole rock Hf and Nd isotopes. The ultimate goal is to evaluate the role of Ti-rich minerals in the decoupling and if it can be used as a new source tracer in the petrogenesis of granitoids. The project would also provide a more complete understanding of the tectonic history of the North Qilian Orogenic Belt.

锆石一直以来被认为是花岗岩中Hf的主要载体，其Hf同位素被近似等同于全岩Hf同位素来探讨花岗岩成因与演化。但是，本项目前期研究在北祁连造山带岛弧花岗岩柯柯里岩体和同碰撞花岗岩金佛寺岩体中都发现（1）明显的锆石-全岩Hf同位素脱耦，（2）全岩Nd-Hf同位素脱耦，（3）锆石Hf-全岩Nd同位素不脱耦的现象。柯柯里样品中全岩Hf同位素和TiO2良好的正相关性表明富Ti矿物可能是除锆石之外另一重要的Hf载体，且含有更加亏损的Hf同位素。本项目将对这两个岩体进行系统的年代学和地球化学工作，查明富Ti矿物在花岗岩中的Nd-Hf同位素特征，确定锆石-全岩Hf同位素的脱耦机制和Nd-Hf同位素脱耦、耦合机制。该研究或将首次检验富Ti矿物在花岗岩中示踪的应用前景，对传统的运用锆石Hf同位素示踪花岗岩源区的方法提出重要修正，或将从花岗岩成因的角度为北祁连造山带的构造演化提供更多的证据和制约。

本次研究以同位素在岩浆演化过程中的地球化学性质为切入点，采用系统的地球化学分析方法，研究Nd-Hf同位素在花岗岩成岩过程中的行为，深入解析花岗岩成因及其地球化学过程，深入探讨岩浆的形成演化，具体研究内容包括（1）金佛寺岩体的年代学和地球化学特征；（2）金佛寺岩体Nd-Hf同位素脱耦的原因--石榴子石效应；（3）从金佛寺岩体Nd-Hf同位素脱耦得到的启示---古老TTG岩石应该具有相反的脱耦现象；（4）柯柯里岩体的年代学和地球化学特征；（5）全岩Hf同位素和锆石Hf同位素不一致的原因--矿物结晶温度不同导致结晶顺序不同，锆石不能完整记录岩浆Hf同位素信息，应该综合考虑富Ti矿物或者全岩Hf同位素；（6）柯柯里岩体全岩Nd-Hf同位素脱耦，可能从岩石圈地幔继承而来。