Decoding the componential transition of Early Cretaceous arc-like magmatism in the Middle to Lower Yangtze River Belt, Eastern China: Insights from multi-isotopes (Hf–O–Sr–Nd–Mg–K)

Abstract Arc-like magmatism, beyond its occurrence at contemporary active continental margins, can also manifest within continental interiors at fossil orogenic margins. Located on the northeastern margin of the fossil Jiangnan Orogen, the Middle to Lower Yangtze River Belt (MLYRB) is characterized by a temporal transition from K-rich to Na-rich arc-like magmatism during the Mesozoic. Nonetheless, there remains ongoing debate regarding whether the compositional and structural characteristics of these magmatic rocks are inherited from the fossil Jiangnan Orogen or the contemporaneous subducting Paleo-Pacific plate. The present study focuses on the Na-rich Jiangmiao intrusion from the MLYRB, comparing it with previously published data on shoshonitic intermediate-mafic volcanics, to investigate their petrogenetic relationships and underlying deep geological processes. Zircon U–Pb geochronology indicates slightly later formation of the Na-rich Jiangmiao gabbros (122 Ma) compared with the shoshonitic intermediate-mafic volcanics (133–122 Ma). While the Jiangmiao intrusion, like the shoshonitic volcanics, exhibits trace element patterns akin to arc magmas, it displays more depleted Sr, Nd, and Hf isotopic compositions, coupled with highly variable oxygen isotopic signatures. The geochemical characteristics indicate that the shoshonitic volcanics originated from lithospheric mantle that had been metasomatized by the Neoproterozoic Cathaysia plate, whereas the Jiangmiao intrusion was derived from asthenospheric mantle that had been metasomatized by the Mesozoic Paleo-Pacific plate. The in situ clinopyroxene elemental analyses and heavy whole-rock Mg–K isotopic signatures of the Jiangmiao intrusion suggest that the metasomatic agent comprised predominantly serpentinite-derived fluids with minor sediment-derived melts. Our findings reveal contrasting source mineralogy and depth signatures between Na-rich and K-rich rocks in the MLYRB, suggesting that the rollback of the Paleo-Pacific slab facilitated the thinning of the preexisting orogenic lithosphere during the late Mesozoic, accounting for the absence of fossil orogen signatures in Na-rich rocks. Overall, the petrochemical transition from K-rich to Na-rich compositions in arc-like rocks at fossil orogenic margins provides a key indicator of the preexisting orogenic lithosphere thinning.