The coexistence of Neoarchean SSZ and MOR ophiolitic mélanges in the North China Craton: dynamics of an Archean paleo-subduction zone

Ophiolitic mélange is a key geological component in ancient convergent plate margins. Understanding the block and matrix types, their igneous, sedimentary, and metamorphic P-T histories, and the structural and tectonic processes from formation to emplacement, is essential for deciphering paleo-subduction zone dynamics. Subduction channels, where some ophiolitic mélanges develop, are mechanically weak shear zones between the upper and lower plates, containing stacked oceanic sequences offscraped from the subducted plate, mixed with eroded crust and mantle materials from the forearc in arc-continent accretionary/collision zones. In this study, we present the results of detailed field mapping, structural analysis, geochemical, and geochronological investigations of the Neoarchean Zunhua and Shangying ophiolitic mélanges from the Eastern Hebei Complex in the Central Orogenic Belt, North China Craton. The Zunhua ophiolitic mélange consists of forearc-affinity ultramafic-mafic blocks (peridotite, podiform-chromite-bearing dunite, pyroxenite, metagabbro, metadiabase, metabasalt), with podiform chromitites containing inclusions of UHP TiO2(II) and remnants of UHP chenminigite, suggesting derivation from depths of 270–410 km prior to mélange formation. In contrast, the Shangying ophiolitic mélange to the east contains layered and isotropic N-MORB affinity metagabbro and garnet clinopyroxenite, metamorphosed under HP eclogite-facies conditions at 65–70 km. Both mélanges are characterized by a strongly sheared metasedimentary matrix. Zircon U-Pb dating of blocks and crosscutting dikes indicates that the ultramafic-mafic blocks in the Zunhua ophiolitic mélange formed at 2.55–2.52 Ga and were incorporated into the mélange between 2.52 and 2.50 Ga. The Shangying ophiolitic mélange formed between 2.53 and 2.52 Ga and was emplaced in the Eastern Block between 2.52 and 2.47 Ga. The Zunhua mélange preserves a nearly complete ophiolite sequence and records a Neoarchean subduction initiation and arc-continent collision event in a forearc supra-subduction zone (SSZ) setting. The Shangying mélange, composed of meta-basalts, gabbros, and garnet clinopyroxenites, formed at a mid-ocean ridge (MOR). The Zunhua SSZ ophiolitic mélange was emplaced over the forearc by accretionary thrusts, whereas the Shangying MOR ophiolitic mélange formed through subduction, mixing, and exhumation within a subduction channel. The juxtaposition of these two mélange belts with different tectonic affinities and emplacement styles during the Neoarchean arc-continent collision reveals a west-northwest-dipping subduction polarity, consistent with kinematic fabrics in the mélanges and their correlatives along the ~1,800 km paleo-subduction zone of the Central Orogenic Belt. The co-existence of SSZ and MOR ophiolitic mélanges in the Eastern Hebei Complex suggests that large-scale subduction/accretionary zones were active in the late Neoarchean. Geochronological data show that the processes of seafloor spreading, subduction initiation, forearc thrusting, and exhumation of subduction materials occurred over <80 million years, similar in duration to many Phanerozoic subduction-collision zones. These findings suggest that tectonic processes in the late Neoarchean were comparable to those of modern Earth.