锆石形态的定量研究:福州花岗质复式岩体的成岩机制

Quantitative study on zircon morphology: discussion on petrogenesis of the Fuzhou granitic complex.

在浙闽粤沿海地区出露了一系列中生代的钙碱性－碱性侵入杂岩，其中福州花岗质复式岩体为该区最有代表性的岩体。本文试通过该岩体中早期原生锆石的大小与晶型的形态学研究，揭示这类侵入杂岩的岩石学特征及其成岩机制。锆石的形态学研究表明，钙碱性岩系中锆石群的晶体大小及晶型指数随岩体的定位次序而呈现连续变化的趋势，而碱性岩系中锆石群的形态的分布范围与钙碱性岩系的不连续，其Ｉｐｙ值远远大于钙碱性岩系锆石群的Ｉｐｙ值，它们代表了两个相对独立的成岩作用的产物。电子探针分析发现，福州花岗质复式岩体中锆石晶型的演化基本上是由其内部的微量元素含量控制的，表现为锆石的ＨｆＯ２含量越低，它的Ｉｐｒ值越高；而锆石的（ＴｈＯ２＋Ｙ２Ｏ３）含量越高，它的Ｉｐｙ值越高。锆石的形态学和微量元素地球化学特征表明，三个钙碱性岩体各自经历了非常相似的成岩过程，可能属于同一源区多阶段部分熔融作用的产物，而碱性岩体则来源于该区抽取了钙碱性花岗质岩浆以后残留物质再次部分熔融作用而分离出来的熔体，后者的形成显示了该区的构造背景已经由强烈挤压为主转化为引张。

In Zhejiang, Fujian and Guangdong provinces there exist a series of the Mesozoic complex made up of a calc alkaline and an alkaline group, the Fuzhou granitic complex being the most representative one in the area. In this paper we will reveal petrological characteristics and formation mechanism of this kind of granitic complex by means of morphological study on dimension and typology of early zircons. The morphological study indicates that the dimension parameters and the typological indices of zircon populations in the calc alkaline granites vary progressively along with their order of emplacement, but the morphological distribution of zircon population in the alkaline granite differs from that of the calc alkaline granites (cf. I py value of the former is far larger than that of the latter), corresponding to products of two independent diagenesis. With microprobe analysis, it is found that the typology evolution of zircons in the Fuzhou granitic complex is essentially controlled by the contents of trace elements inside the crystals, i.e. the larger the contents of HfO 2 , the smaller the I pr values are, but the larger the contents of (ThO 2+Y 2O 3),the larger the I py values are. According to the morphological and geochemical characteristics of zircon, it is assumed that three calc alkaline granites underwent similar petrogenetic processes and may be considered as products of different stage through partial melting from the same source, but the alkaline magma may be generated from partial melting of residual materials after extraction of the calc alkaline granitic magma, indicating the change in the tectonic environment from a compressive subduction setting to an extensional context.