Ancient Crustal Signatures in Mesozoic Magmatic Rocks of the Gan–Hang Belt: Zircon U-Pb Age and Trace Element Evidence from a Microdiorite Dike in Jiangshan, South China

Geochronological investigation and geochemical tracing of xenocrystic zircons entrained in magmatic rocks serve as effective approaches for directly unraveling the material composition and architectural framework of the deep-seated ancient continental crust. Here, we present an integrated petrological, zircon petrographic, U-Pb geochronological, and trace elemental study of a microdiorite dike from the Jiangshan area within the Gan–Hang tectonic belt, which is a key suture zone between the Yangtze and Cathaysia Blocks. Four distinct zircon types are recognized. Type I (2528 ± 21 Ma) is a metamorphic zircon that is derived from eclogite-facies metamorphism of a Neoarchean basement, revealing the existence of a Neoarchean continental crust in the deep South China Block. Types II (848 ± 53 Ma) and III (481 ± 93 Ma) are magmatic zircons representing captured Neoproterozoic and Early Paleozoic magmatic rocks, respectively. Type IV (137 ± 8.4 Ma) occurs as an overgrowth on its inherited cores and records the emplacement age of the microdiorite dike. Trace element compositions of Early Cretaceous zircons (type IV) show low Nb/Hf and elevated Th/Nb ratios, consistent with a magmatic arc setting. Combined with petrographic evidence of quartz xenocrysts and the diversity of captured zircons, we propose that the parental magma originated from partial melting of the subcontinental lithospheric mantle induced by slab-derived fluid flux, with the subsequent assimilation of diverse crustal materials during its ascent. This study not only constrains the Early Cretaceous arc magmatism in response to Paleo-Pacific Plate subduction but also provides evidence for Neoarchean crustal relics in the deep subsurface of the South China Block, with important implications for the deep compositional architecture and tectono-magmatic evolution of South China.