Late Cretaceous Mafic–Intermediate Magmatism in Eastern Guangdong, SE China: Evidence for a Subduction-Modified Enriched Mantle Source

Mafic–intermediate dikes provide useful constraints on mantle source characteristics, crust–mantle interaction, and regional tectono-magmatic evolution. Here we present field, petrographic, whole-rock geochemical, zircon U–Pb–Hf isotopic, and whole-rock Sr–Nd isotopic data for the Haojiang mafic–intermediate dike suite and its host granitoids in eastern Guangdong, SE China. Field observations show that dark, fine-grained dikes sharply intrude light-colored granitoids along fractures or joints. Zircon U–Pb dating of the host granitoid sample HJ-7-3 yields a crystallization age of 137.56 ± 0.76 Ma, whereas the representative dike sample HJ-1 yields a magmatic zircon age of 94.25 ± 0.64 Ma. These ages indicate that at least part of the Haojiang dike suite represents a Late Cretaceous mafic–intermediate magmatic event that postdated Early Cretaceous granitoid emplacement. An older zircon population in HJ-1 yielded an age of 135.8 ± 1.1 Ma and is interpreted as xenocrystic or inherited zircon probably related to interaction with Early Cretaceous granitoid crust or a coeval crustal component during magma ascent. The dike samples have SiO2 contents of 50.10–58.52 wt.% and plot mainly in the basaltic andesite to andesite fields in the TAS diagram. Major-element variations, compatible trace elements, and weak Eu anomalies are consistent with variable fractional crystallization, although open-system processes cannot be excluded. The dikes are enriched in LILEs and LREEs and depleted in Nb, Ta, and Ti. Their initial 87Sr/86Sr ratios of 0.705232–0.705772, εNd(t) values of −3.10 to −0.12, and zircon εHf(t) values of −1.61 to +1.30 indicate weakly enriched isotopic compositions. Taken together, these features favor involvement of a subduction-modified enriched mantle source, although contributions from metasomatized lithospheric mantle and enriched asthenospheric mantle cannot be uniquely distinguished. Possible limited crustal interaction during magma ascent is also allowed. The ca. 94 Ma Haojiang dike therefore provides evidence for Late Cretaceous mantle-derived or mantle-influenced magmatism in eastern Guangdong, consistent with a regional extensional regime commonly associated with Paleo-Pacific slab rollback.