Important role of magma mixing in generating the Late Cretaceous Shima intrusion along the Middle-Lower Yangtze River belt: Evidence from petrology, geochemistry, and zircon U-Pb-Hf isotopes

Abstract

Mafic microgranular enclaves (MMEs) are common in the granodiorites from the Shima intrusion in the Ningzhen region of the Middle-Lower Yangtze River belt (MLYRB). In this paper, we report new zircon U-Pb ages, mineral and whole rock geochemical and Sr–Nd–Hf isotopic data for the Shima MMEs and their host granodiorites to constrain their petrogenesis and tectonic implication. Zircon LA-ICP-MS U-Pb dating yields ages of 101.6 ± 1.1 Ma for the host granodiorites, and 101.8 ± 1.0 Ma for the MMEs, indicating formation from coeval magmas. This magmatism was contemporaneous with the latest stage (109–100 Ma) of magmatic activity within the MLYRB. Geochemically, the Shima granitoids have geochemical signatures resembling adakites, such as high Al2O3 (14.78–16.48 wt%), Sr (442–685 ppm), Na2O (>3.5 wt%), negative Nb–Ta–Ti anomalies, low Y (10.9–18.0 ppm), Yb (0.95–1.72 ppm), Sc (6.7–11.5 ppm), and resultant high Sr/Y (24.6–57.6) and La/Yb (28.3–46.6) ratios. The Shima host granodiorites are sodic, metaluminous, high-K calc-alkaline, enriched in large ion lithophile elements (LILEs; e.g., Rb, Sr, and Ba), but depleted in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti), with slightly negative to positive Eu anomalies of 0.99–1.12, (87Sr/86Sr)i of 0.7060 to 0.7061, εNd(t) values of −10.2 to −11.7 and εHf(t) of −14.1 to −16.0. The Shima MMEs have lower SiO2 (59.81–63.81 wt%), and they are also enriched in large ion lithophile elements, depleted in high field strength elements, with slightly negative to positive Eu anomalies of 0.91–1.16, (87Sr/86Sr)i of 0.7050 to 0.7061, εNd(t) values of −3.9 to −10.6 and εHf(t) of −13.0 to −15.7. Hornblende thermobarometry show that the Shima intrusion crystallised at temperatures of 769–832 °C and depths less than 11 km. All these petrographical, geochronological and geochemical results demonstrate that the Shima intrusion was likely formed through mixing of felsic melts derived from an ancient lower crust with mafic magmas derived from a metasomatized lithospheric mantle, coupled with fractional crystallization and minor wall-rock contamination. Our new data, combined with the regional tectonics and the temporal-spatial variation of the Mesozoic magmatism in the MLYRB, indicate that the Shima intrusion could be related with the back-arc extensional regime triggered by slab rollback of the paleo-Pacific plate.