大别山黄土岭麻粒岩锆石和共生矿物的微量元素分析及其地质意义

Trace element analysis of zircon and associated minerals in the Huangtuling gran ulite,Dabieshan and its geological implication

对大别山黄土岭麻粒岩中的锆石进行了LA-ICPMS微区微量元素分析。结果表明,黄土岭麻粒岩中锆石的不同区域有不同的微量元素组成,麻粒岩相变质锆石的大多数微量元素含量明显低于岩浆锆石,表明麻粒岩相变质条件下形成的锆石具有较低的微量元素组成。锆石及其共生矿物的微量元素分析结果表明,该麻粒岩中变质锆石Eu负异常是变质锆石形成时长石稳定存在的结果。锆石与石榴子石之间微量元素分配特点表明,变质锆石与石榴子石之间到达了平衡。这些结果表明,该样品的变质锆石形成于麻粒岩相峰期变质阶段,这些变质锆石区域测定的年龄结果对应于麻粒岩相峰期变质作用时间。锆石和共生矿物的微量元素分析对锆石的成因及得到的年龄的解释具有重要的指示意义。锆石与石榴子石之间微量元素的分配特征,不但可以指示锆石与石榴子石之间是否达到平衡,而且可以通过石榴子石这一“桥梁”,为锆石的U-Pb年龄提供合理的p-T条件限定。

Zircons in the Huangtuling granulite were analyzed for trace eleme nts by LA-ICPMS. The granulite-facies metamorphic zircons have lower trace el ement contents than those of the magmatic protolith zircons. The negative Eu ano maly in metamorphic zircon is interpreted by the presence of feldspars. The trac e element partitioning between zircon and garnet indicates that metamorphic zirc on formed in equilibrium with the garnet. Combined with these evidences, it is s uggested that the granulite metamorphic zircons formed at the peak metamorphic c onditions, and thus the U-Pb age of the metamorphic zircons corresponds to the time of the peak metamorphism. The trace element composition of zircon and asso ciated minerals can therefore give effective ways to constrain the zircons' form ation conditions. The trace element partitioning between zircon and garnet can p rovide new constraints on the interpretation of zircon U-Pb age.