胶东地区郭家岭花岗闪长岩的地球化学特征及成因

Guojialing granodiorites, the main host rocks of gold ore deposits, were formed during late Yanshanian in the northwest of the Jiaodong Peninsula, eastern China. The data of occurrence, petrography, major and trace elements, and Sr and Nd isotopic components are reported, and the source and geological significance of the granodiorites are discussed in this paper. The granodiorites, with the characteristics of high SiO₂ (62% ∼ 72 % ) , Na₂ O/K₂O = 0.64 ∼ 1.8 (most samples are more than 1.0) , alkaline concentrations ALK=7.34% ∼ 8.49 % , and the aluminium index A/CNK) = 0.82 ∼ 1.1, are enriched in Sr and Ba (Sr, Ba > 500μg/g) and depleted in Y (Y = 2.85 ∼ 9.16μg/g) , Yb (Yb = 0.27 ∼ 0.71 μg/g) and high field strength elements (HFSE) such as Nb, Ta, Zr and Hf. These geochemical features are similar to those of adakites, Archean TTG, Na-rich granites and high-Ba, Sr granites (HiSrBa) but different from those dacite of BADR in the arc background. They have high (La/Yb _N and Sr/Y ratios and δSr_N values (the δSr_N of high Mg^# samples are more than 2), indicating the garnet and/or amphibole without plagioclase in the residue during magma genesis. δ_Nd (130 Ma) values of Guojialing granodiorites are highly negative [δ_Nd(130 Ma) = - 11 to 17]. Compared with the Jiaodong metamorphic rocks and Linglong granites, they were not derived from the Precambrian basement. The geochemical features, similar to those of TTG and Na-rich granites and the mafic enclaves of Guojialing granodiorites containing acicular apatite indicate that the magma was derived from the lower mafic crust formed by previously ancient enriched mantle-derived magma underplating, but assimilated by an upper crustal component. They are different from adakites which were derived from partial melting of subducted oceanic slab with or without reaction with mantle peridotite and HiBaSr granites which were directly derived from crystal fractionation of mantle-derived magmas. Guojialing granodiorites were derived from dehydrational partial melting of mafic lower crust with small-degree crustal contamination during magma evolution. The low Y and Yb contents and high Sr/Y and La/Yb ratios indicate that there exist garnets ( ± amphibole without plagioclase) in the residue, which induces delamination of lithosphere and gives an indication to the mechanism and process of lithosphere thinning during the Late Mesozoic in the Jiaodong Peninsula.