锆石形态的定量研究: 福州花岗质复式岩体的成岩机制

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 HfO₂, the smaller the I(pr) values are, but the larger the contents of (ThO₂+Y₂O₃), 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.