福建金山花岗质复式岩体的元素和同位素地球化学及其成因研究

The Jinshan composite pluton, a large Late Yanshanian granitic batholith, is located in the borderland between Hua'an and Nanjing Counties, southern Fujian Province. It can be divided into two units, the Yangzhujing (YZJ) unit in Hua'an and the Longshan (LS) unit in Nanjing. Zircon LA-ICP-MS U-Pb dating yields an age of 140. 3 ± 1. 2 Ma for YZJ unit and an age of 105. 1 ± 0. 8 Ma for LS unit. Lithologically, the YZJ unit in the east part of the composite pluton consists mainly of medium-grained moyites and small biotite granites in its central part. Geochemically, the granites of the YZJ unit have high SiO₂(=73. 72% ∼ 76. 06%) and moderate Al₂O ₃(=12. 70% ∼13. 52%) contents. They are weakly peraluminous with A/NKC values ranging from 1. 00 to 1. 05, and have moderate alkaline contents (K₂O+ Na₂O=8. 30% 8.84%, AKI = 0. 80∼0. 90) but high potassium abundances (K₂O=4. 59%∼5. 58%, K₂O/Na ₂O=1. 18∼1. 95). The rocks are also rich in LREE and LILE (e. g., Rh, Th) and poor in Ba, Sr, Ti, P, and show high Rb/Sr ratios (1. 55∼12. 24). Chondrite-normalized REE distribution patterns of them display moderate to strong negative europium anomalies (δEu = 0. 11∼0. 52). The LS unit in the west part of the composite pluton is homogeneous and comprises predominantly of biotite adamellites. Compared with the YZJ unit, the LS biotite adamellites show lower contents of SiO₂ and K₂O+Na ₂O, lower Rb/Sr ratios,but higher LREE/HREE ratios, and display weak negative europium anomalies. In comparison with the representative A-type granites, all the granites of the Jinshan composite pluton have lower Ga/Al ratios and HFSE (e. g., Zr, Nb, Ce, Y) concentrations. Integrated geological and geochemical data suggest that the rocks are genetically of high-K calc-alkaline I-type granites. The biotite granites and moyites in the YZJ unit display consistent Nd isotopic compositions, with ε_Nd(i) values of-1. 13 and - 1. 16∼ - 1. 94 respectively, indicating that they were originated from a common source via fractional crystallization. The LS granites, emplaced in a rather late stage with a time interval of about 35 Ma, show distinctly different petrological and geochemical characteristics relative to the YZJ granites. Chemical variation trends suggest that the two units could not be generated by fractional crystallization from the same primary magma rather they should be originated via different magmatic events. The Jinshan compsosite pluton was most likely produced by multiple-stage partial melting of a mixed crustal protolith with both juvenile crustal materials and pre-existing ancient basement crust which is induced by continuously underplating of mantle-derived magma under an extensional back-arc setting.