Petrogenesis of mafic-ultramafic rocks in the Eastern Hebei Complex of the North China Craton: implications for the Neoarchean tectonic regime

The Neoarchean era represents a pivotal transition in early Earth evolution and tectonic regime transformation, serving as a critical window for investigating continental origin and early geodynamic processes. The tectonic setting of the Neoarchean Eastern Hebei Complex in the North China Craton remains intensely debated. Ultramafic rocks provide key insights into deep Earth material circulation and dynamics, serving as petrological probes into early crust-mantle evolution, the onset of plate tectonics, crustal recycling, and deep geodynamic processes. We present an integrated study incorporating field investigations, petrology, whole-rock and mineral geochemistry, and zircon U-Pb geochronology on serpentinized lherzolites, pyroxenites, and metamafic rocks from the Zunhua-Shangying ophiolitic mélange belt. The field geological mapping shows that the ultramafic rock is mainly exposed in the Songling, Longwan, and Shangying regions. Serpentinized lherzolites exhibit refractory mantle characteristics with high Mg# (~85) and relatively flat to slightly depleted rare earth element patterns. High-Mg# (75.60-80.78) Songling-Longwan pyroxenites represent high-pressure cumulates derived from hydrous, subduction-modified basaltic magmas in the lower arc crust, whereas low-Mg# (24.6-41.6), high-Al₂O₃ (13.7-19.7 wt%) metamafic rocks constitute complementary evolved melts. Magmatic zircons from metamafic rocks yield a weighted average age of 2.52 Ga, interpreted as the crystallization age of the Songling-Longwan mafic-ultramafic suite. The Shangying garnet pyroxenites, showing typical N-MORB geochemical affinities, are identified as Archean oceanic crust remnants. Coexisting metagabbro yields an identical magmatic zircon age of 2.52 Ga, constraining the crystallization of the Shangying garnet clinopyroxenite. Metamorphic zircons in pyroxenites and granulites record subsequent tectonothermal events at 2.48 Ga and 1.85 Ga. Integrated geochronological and geochemical data demonstrate that the Eastern Hebei mafic-ultramafic suite developed in an intra-oceanic arc system, with distinct formation (2.55-2.52 Ga) and emplacement (2.52-2.47 Ga) stages. During the formation stage, the Songling-Longwan rocks originated at the arc root crust-mantle transition zone, while the Shangying garnet pyroxenites formed at greater depths within the subducted slab. Throughout the emplacement stage, both the N-MORB-type Shangying garnet clinopyroxenites and metagabbros and the arc-related Songling-Longwan mafic-ultramafic blocks were incorporated as coherent and elongated tectonic fragments into the Zunhua-Shangying forearc ophiolitic mélange. The late Neoarchean to early Paleoproterozoic metamorphism coincided with subduction of the Eastern Block beneath the Wutai/Fuping arc terrane and subsequent arc-continent collision, leading to the stabilization of the Eastern Block and the final accretion of the Zunhua-Shangying ophiolitic mélange belt.