Geochronological, geochemical and Nd–Hf isotopic constraints on the petrogenesis of Late Cretaceous A-type granites from the southeastern coast of Fujian Province, South China

Abstract

We present comprehensive petrological, geochronological, major and trace element, and Nd–Hf isotopic data for the Baishishan, Jingangshan, and Wushan granitic plutons on the southeastern coast of Fujian Province, South China, with the aims of elucidating their origin and gaining new insights into the petrogenesis of aluminous A-type granites. Zircon U–Pb ages obtained by laser ablation–inductively coupled plasma–mass spectrometry show that the three investigated plutons were emplaced at 92–86Ma, indicating that they were generated during a Late Cretaceous magmatic event. The granites from the three plutons are composed mainly of perthite, quartz, plagioclase, and minor biotite; they have high SiO2 contents, and low CaO, MnO, Fe2O3tot, and MgO contents, and show a metaluminous to slightly peraluminous signature. The granites are enriched in some large ion lithophile elements (e.g., Rb, Th, and U) and high field strength elements (e.g., Nb and Ta) with elevated Ga/Al ratios, and spidergrams show strong depletions in Ba, Sr, Ti, and P. Chondrite-normalized REE patterns show relative enrichments in light rare earth elements, flat heavy rare earth element profiles, and strongly negative Eu anomalies. These mineralogical and geochemical characteristics suggest that all three plutons can be classified as aluminous A-type granites. The plutons exhibit nearly identical whole-rock Nd and zircon Hf isotopic compositions, and yield Mesoproterozoic two-stage model ages (1.4–1.1Ga) for both Nd and Hf isotopes. Based on a synthesis of the geochemical and isotopic data and petrogenetic modelling, we suggest that these A-type granitic rocks were most likely formed by variable degrees of fractional crystallization of magmas produced by the partial melting of a tonalitic to granodioritic source, with plagioclase-rich residual phases in the middle to lower crust, and emplaced at shallow crustal levels along the Changle–Nan’ao Fault. Our data on the Baishishan, Jingangshan, and Wushan granites, coupled with previous studies of Cretaceous magmatism in coastal areas of SE China, indicate that a gradually intensifying regional extension took place in the region during the late Yanshanian (Cretaceous), which peaked during the Late Cretaceous in response to the roll-back of the Palaeo-Pacific plate. Consequently, we conclude that the generation of A-type granites was related to intensive tectonic extension along the Changle–Nan’ao Fault and the induced underplating of mantle-derived magma.