A hybrid origin for the Qianshan A-type granite, northeast China: Geochemical and Sr–Nd–Hf isotopic evidence

Abstract

Major and trace element, whole rock Sr and Nd isotope and zircon Hf isotope data are reported for a suite of A-type granites and mafic microgranular enclaves from the Early Cretaceous (126 ± 2 Ma) Qianshan pluton, Liaodong Peninsula, northeast China, with the aim of investigating the sources and petrogenesis of A-type granites. The Qianshan pluton includes hornblende alkali-feldspar granite, graphic biotite granite and mafic microgranular enclaves. The hornblende alkali-feldspar granites have high SiO 2, Fe 2O 3T / MgO, K 2O + Na 2O, Rb, Zr and LREE contents and low Ba and Sr concentrations with strongly negative Eu anomalies. Their high Rb / Sr ( 87Rb / 86Sr = 16.76–24.15) and initial 87Sr / 86Sr ratios (0.7215 to 0.7283), negative ε Nd( t) values (− 14.1 to − 16.5) and zircon ε Hf( t) values (− 18.9 to − 11.5) indicate they were mainly derived from a crustal source, but with involvement of high ε Nd( t) and ε Hf( t) materials. Graphic biotite granites have similar geochemical features and Sr–Nd–Hf isotopic compositions to enclaves, indicating they were the result of crystal fractionation of evolved mafic magmas, but with involvement of low ε Nd( t) and ε Hf( t) materials. The mafic enclaves have an igneous texture and contain acicular apatite, suggesting quenching of mafic magmas that have co-mingled with the host granites. They have low initial 87Sr / 86Sr ratios (0.7097–0.7148), negative ε Nd( t) (− 14.5 to − 11.9) and zircon ε Hf( t) (− 17.1 to − 6.9) values, and are enriched in LILEs and LREEs and depleted in HFSEs. When coupled with the high MgO (Mg# up to 54), this indicates derivation from an enriched lithospheric mantle source, but contaminated by crustal materials. Geochemical and Sr-, Nd- and zircon Hf-isotopic compositions rule out simple crystal–liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, magma mixing of mantle-derived mafic and crustal-derived magmas, coupled with crystal fractionation, is compatible with the data. This example shows that at least some A-type granites formed through a complex process involving mantle- and crustal-derived magma mixing, crystal fractionation and infracrustal melting.