| Abstract:The Xikeng pegmatite field lies in the eastern margin of South China Fold System; in Fujian Province, it is located at the connection of three major tectonic cycles. Although folds and faults are both well developed in the area, the distribution of pegmatites is obviously controlled by the former which, according to the axial trend, can be assigned to NEE group and NNE group, with the former group having most intimate affinity with the pegmatites. There exists apparent injection relationship between the pegmatites and the surrounding Smian schist and granulitite. The granitoids extensively distributed in the field belong either to Variscan or to Yenshuiian cycle, and it is evident that the pegmatites are genetically related to Variscan migmatitic granites. The pegmatites can be grouped into form- types: muscovite-orthoclase-early albite pegmatite (I), muscovite-albite-orthoclase pegmatite (II), muscovite-or-thoclase-ajbite pegmatite (III), and muscovite-albite-spodumene pegmatite (W) . From the contact zone of Variscan migmatitic granites outwards, pegmatites of Type I to Type IV appear in turn, with their form, size and interior zoning gradually becoming larger and more complex. Owing to strong metasomatism and multi-step emplacement of pegmatitic melt-solution, the sequence of interior assemblage zones in the pegmatites does not always represent the sequence of original crystallization. Mineral composition of the pegruatites is extremely complicated. 81 sorts of minerals have so far been found, including 5 species of Li-minerals, 6 species of Be-minerals, 3 species of Cs-minerals, 6 families of Nb, Ta-minerals and 18 species of phosphates. From Type I to Type IV, the mineral assemblages tend to grow complex, together with the synchronous intensification of rare metal and Sn mineralizations. Most of Type IV pegmatites have economic value. Diffusion halos of Li, Rb and Cs are distinct in the altered wall rocks. The features of fluid inclusions in the minerals are significantly different either in different types of pegmatites or in different parts of a single pegmatite vein. Oxygen isotope ratios of migmatitic granites and pegmatites are comparatively low (δ18O=9.3-10.4‰), and also than 9.5‰. Isotope ages of pegmatites are 235-328 Ma with their initial 87Sr/86Sr values of rock-forming fluids are higher ratio being 0.715-0.746. According to the temporal and spatial relationship between the pegmatites and the migmatitic granites, combined with the features of pegmatites themselves, it can be thought that the Xikeng granitic pegmatites are products of differentiation related closely to migmatitic granites. |
