浙江瑶坑碱性花岗岩体的年代学、地球化学及其成因与构造指示意义

Located at the coastal juncture of Zhejiang and Fujian provinces, the Yaokeng pluton is a typical alkaline granitic intrusion in this Late Mesozoic A-type granite belt. Zircon LA-ICP-MS U-Pb dating yields an age of 91.3±2.5 Ma (MSWD = 3.7, 2σ), indicating that it was formed in Late Cretaceous. The granites of the Yaokeng pluton develop characteristic micrographic texture and miarolitic structure, and contain typical alkaline ferromagnesian minerals, such as arfvedsonite. Geochemically, the granites are rich in silicon, alkalis, poor in calcium and magnesium, and show enrichments of K, Rb, Cs, Th, U and depletions of Sr, Ba, P, Ti, Sc. They also have higher concentrations of Nb, Ta, Zr, Hf, Ga, and display high (Ga/Al) × 10⁴ values (= 3.64∼4.41) and strong europium depletions (δEu = 0.15∼0.20). Compared with the other A-type granitic plutons in the coastal areas of Zhejiang and Fujian provinces, the Yaokeng granites have higher ε_Nd (t) values (= -0.7 ∼ -1.4) and younger two-stage Nd model ages (t_2DM = 0.95Ga ∼ 1.01Ga), indicating that mantle-derived components have made greater contributions to the magma genesis. The rocks also show lower Y/Nb and higher Nb/Ta ratios, suggesting that they belong to the A₁ type proposed by Eby (1992), whereas the main A-type granites in this belt are of A₂ type. By integrated study of the tectonic setting and geochemical characteristics, we suggest that the elemental and isotopic differences between the Yaokeng and the other A-type granites in this belt were mainly resulted from more mantle material involvements in the magma generation, which was induced by intersections of multi-oriented deep faults in the Yaokeng pluton.