赣东南富城杂岩体的地球化学和年代学研究

The Fucheng granitic complex is located in the southeastern Jiangxi province, which is combined with the Hongshan granite in the southwestern Fujian province to be the Fucheng-Hongshan Complex. The Fucheng Complex may be divided to three units, the Fucheng megacrystic porphyrotic biotite monzogranite, the Cushiba middle-grained two-mica granite and the Zhuchangdong andalusite-bearing fine-grained granite. The Fucheng biotite granites have relatively low SiO₂, Rb and Nb concentrations and high Al₂O₃, Ba, Sr, Zr, REE concentrations. Their ACNK are > 1.10 and K₂O/Na₂,O > 1.60. The Cushiba granites are characterized by high SiO₂, K₂O and Al₂O₃ (ACNK = 1.13∼1.20), and low CaO and P₂,O₅. The rocks are rich in Rb and Nb, and poor in Ba, Sr, Zr and REE. They all have relatively low K/Rb and Eu/Eu ° ratios. The Zhuchangdong andalusite-bearing granites are similar to the Fucheng granites in geochemistry, except for higher ACNK (1.22∼1.36), Cs, Rb, Nb, Ta and Sn concentrations and lower Sr, Ba, Zr, Hf and REE contents. The Fucheng granites have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7135∼0.7196 and ε_Nd (t) of -9.4∼10.2 with the Nd model ages ranging from 1.84Ga to 1.78Ga. The zircons in the Fucheng granite have Hf model ages of 1.70 ∼ 1.89Ga, consistent with bulk Nd model ages. The Cushiba granites have bulk Nd isotope and zircon Hf isotope similar to the Fucheng granites, The Zhuchangdong granite has higher initial ⁸⁷Sr/⁸⁶Sr ratio (0.7214) and lower ε_Nd (t) value (-16.9) with older Nd model age (2.37Ga). These geochemical data indicate that the protoliths of granites in the Fucheng Complex are sedimentary rocks consisting of ancient crustal materials. The original magma of the Fucheng and Cushiba granites probably originated from late Paleoproterozoic basement, and the source of the Zhuchangdong granites is early Paleoproterozoic basement. A variety of granites from the three units in the complex do not exhibit coherent geochemical change, suggesting they are not the results evolved from an original magma, which also is supported by field observation and geochronological studies. The difference in geochemistry among granites from three units is likely caused by the source compositions. LA ICPMS zircon U-Pb dating indicates that the Fucheng granite formed at ca. 239Ma, and the Cushiba and Zhuchangdong granites formed at 231 ∼ 229 Ma, which are in accordance with peak time of early Indosinian tectonic-magmatic activity in south China. Coupled with the geochemical characteristics and assemblage of Indosinian rocks, it is suggested that the Fucheng-Hongshan granitic complex was probably formed in the syn-orogenic tectonic setting.