Volcanism transition from low-angle subduction to slab rollback: a case study of Late Mesozoic volcanism in the Fengning-Duolun area, North China Craton

Abstract The forward low-angle subduction and subsequent slab rollback of the Paleo-Pacific plate are commonly invoked to explain the spatiotemporal distribution, migration, and chemical signatures of Late Mesozoic intracontinental magmatism in the eastern North China Craton (NCC). However, the volcanic response to this process in the hinterland of the eastern NCC remains poorly understood, largely due to limited constraints on its timing and petrogenesis. These issues were addressed here through integrated zircon U-Pb geochronology and whole rock and mineral geochemical studies of Late Mesozoic volcanic rocks from the Fengning-Duolun area. Two magmatic episodes were identified, represented by the Late Jurassic Baiqi Formation (BF; ~149 Ma) and the Early Cretaceous Zhangjiakou (ZF; 142.5–135.1 Ma) and Huajiying (HyF; 134.4–133.5 Ma) Formation volcanic rocks. The BF volcanic rocks have high SiO2 (~71.9 wt.%), enriched Sr-Nd-Hf isotopic ratios ((87Sr/86Sr)t=0.70734, εNd(t)=−10.0, and εHf(t)=−14.6), and restricted zircon Hf-O isotopic compositions (εHf(t)=−15.8±1.5 and δ18O=6.4±0.2‰). Combined with regional coeval magmatism, it is suggested that these rocks were sourced from of lower continental crust (LCC) materials, likely including both the ancient and newly accreted components. Features such as limited outcrop distribution and consistency in Hf isotope compositions between whole rock and zircons are likely attributable to a low magma flux under a low-angle subduction regime, which inhibited extensive magma mixing. In contrast, the ZF and HyF volcanic rocks (SiO2>75wt.% and 56.7–69.7 wt.%, respectively) define coherent compositional trends, with the ZF rocks representing high evolved (>75% differentiation) derivatives of the HyF magmatic system. This, along with variable whole rock Sr-Nd-Hf ((87Sr/86Sr)t=0.70638–0.70814, εNd(t)=−14.6–−9.3, and εHf(t)=−20.0–−9.2) and zircon Hf-O (εHf(t)=−23.3–+4.2 and δ18O=4.5–7.4‰) isotopic compositions, is interpreted to reflect superimposed processes of assimilation, fractional crystallization, and magma mixing among three magma components from ancient LCC, enriched SCLM, and depleted asthenosphere mantle, with the first two components predominating. A binary mixing calculation indicates a significant mantle-derived contribution (~0.4) to the erupted magmas. These signatures are compatible with elevated mantle-derived magma flux resulting from slab rollback, which, possibly in combination with a thickened crust, provided favorable conditions for the development of a transcrustal magmatic system and contributed to voluminous ZF volcanism. Such a system may have promoted entrainment of crystals from diverse magmas, accounting for the observed crystal-melt disequilibrium. The study provides an example of the role of subduction processes in modulating the scale, geochemical characteristics, and source of continental volcanism.