Latest Early Cretaceous tectonic transition from contraction to large-scale extension in Southeast China: Insights from the magmatism along the Changle-Nan’ao Fault Zone

Long-lived subduction and variable plate geometry of the Paleo-Pacific plate resulted in multiple episodes of both extensional and contractional deformation. While tectono-magmatic evidence provides critical insights into the tectonic evolution of the South China Block (SCB), the timing and dynamic mechanisms governing the transition from late Early Cretaceous contraction to large-scale intraplate extension remain contentious. Here, we present zircon U-Pb ages, geochemical data, and Hf isotopic data for 5 samples from the NNE-trending dioritic dikes and granodiorites in the Changle-Nan’ao Fault Zone, Southeast China, as well as crustal thickness reconstructions based on Sr/Y and (La/Yb)N geochemical proxies to constrain the crucial tectonic transition in the late Early Cretaceous. Our samples exhibit comparable zircon U-Pb ages (105–103 Ma) and are characterized by enrichment in LILEs, pronounced depletion in HFSEs, and variable εHf(t) values. These features suggest that they derived from the mixing of felsic and mantle-derived mafic melts, differentiated at successive stages through fractional crystallization. Notably, the intrusion of the dioritic dikes along the fault zone highlights a phase of extensional activity. Integrated magmatic geochemistry and sedimentary stratigraphic records, coupled with crustal thickness data indicating a reduction to ~40 km before ~120 Ma followed by significant thickening to a peak of ~60–70 km at ~105 Ma, suggest two tectonic transitions at ~120 Ma and ~105 Ma, respectively. It is noteworthy that the extension observed in the coastal region due to slab break-off was manifested as coeval contraction in the inland region at ~105 Ma, which was subsequently followed by widespread extension across SE China. We propose that the tectonic transitions between the compression and extension were driven by changes in subduction angle of Paleo-Pacific slab, specifically the evolution from flat-slab subduction to slab roll-back and steepening.