后碰撞花岗岩类的多样性及其构造环境判别的复杂性

造山带中普遍存在着相当数量的后碰撞花岗岩类。它们在时间上晚于碰撞事件形成,在空间上可以不受构造单元的严格控制,而是可以跨构造单元分布,有时可以侵入在蛇绿岩之中。后碰撞花岗岩类的主元素特征属于中—高钾钙碱性系列和钾玄岩系列,但以钙碱性系列为主。按照MISA分类,后碰撞花岗岩类可以有I、S和A等3种类型,有的造山带以发育I型花岗岩类为主,而另一些造山带可以广泛发育S型花岗岩,而碱性A型花岗岩并不是在每个造山带都会出现。在微量元素构造环境判别图解上,后碰撞花岗岩类可以落在多种构造环境的区域。因此,仅仅依靠花岗岩类构造环境的地球化学判别图解会得出似是而非的结果。文中强调时空分布特征及区域地质构造的全面分析可能是厘定后碰撞花岗岩类最重要的依据,而在区域地质研究基础上的高精度锆石U-Pb年代学研究能够建立区域构造演化的年代学框架,进而准确地限定后碰撞花岗岩类岩浆活动的时限。

Diverse post-collisional granitoids and their tectonic setting discrimination

Post-collisional granitoids are common in orogenic belts around the world and are produced during thermal events subsequent to collisional tectonics. They occur not only in an orogenic belt but also in adjacent tectonic units, and locally can intrude ophiolites belts. Compositionally, post-collisional granitoids range from medium- to high-potassium cale-alkaline to shoshonite, but tend to be dominated by the calc-alkaline series. They can be subdivided into I-, S-, and A-types in accordance with the MISA classification. Some orogenic belts are dominated by I-type granitoids, while others are characterized by widespread development of S-type granites. Alkaline A-type granites, however, do not occur in all orogenic belt. It is extremely difficult to recognize the tectonic settings for these granitoids by simply using trace element discrimination diagrams in the absence of sound geological constraints. Therefore, it is emphasized that the recognition of post-collisional granitoids is strongly dependent on their spatial distributions in an orogenic belt and its adjacent units, and on their positions in the geochronological framework, constructed on the basis of high precise zircon U-Pb dating.