Petrogenetic and metallogenetic characteristics of the Xikeng granitic pegmatites,Fujian Province

The Xikeng pegmatite field lies on the eastern margin of the south China fold system in Fujian Province, and it is located at the junction of three major tectonic units. The distribution of pegmatites is obviously controlled by the fold system. There exists apparent injection relationship between the pegmatites and the surrounding Sinian schist and granulitite. The granitoids extensively distributed in the field belong either to the Variscan or to the Yenshanian cycle, and it is evident that the pegmatites are genetically related to Variscan migmatitic granites. The pegmatites can be grouped into four types: muscovite-orthoclase-albite pegmatite (I), muscovite albite-orthoclase pegmatite (II), muscovite-orthoclase-albite pegmatite (III), and muscovite-albite-spodumene pegmatite (IV). Owing to strong metasomatism and multi-stage emplacement of pegmatitic meltsolution, the sequence of interior assemblage zones in the pegmatites does not always represent the sequence of original crystallization. The mineral composition of the pegmatites is extremely complicated. 81 kinds of minerals have so far been found. From type I to type IV, the mineral assemblage tend to get increasingly complex, together with the synchronous intensification of rare-metal and Sn mineralizations. Most of the type-IV pegmatites are of economic value. The features of fluid inclusions in the minerals are significantly different not only in different types of pegmatite, but also in different parts of a single pegmatite vein. Theδ ¹⁸O values of migmatitic granite and pegmatites are comparatively low (9.3–10.4‰), and those of rock-forming fluids are higher than 9.5‰. Isotopic ages of the pegmatites range from 235 to 328 Ma with initial⁸⁷Sr/⁸⁶Sr ratios being 0.715–0.746. According to the temporal and spatial relationships between the pegmatites and the migmatitic granite, combined with the features of the pegmatites themselves, it can be concluded that the Xikeng pegmatites are the product of differentiation closely related to the migmatitic granite.