Neoarchean SSZ and MOR ultra-/high-pressure ophiolitic mélanges of the Eastern Hebei Complex, North China Craton: Dynamics of an Archean paleo-subduction zone

Ophiolitic mélange is a significant geological component in ancient convergent plate margins. Characterization of block and matrix types, their igneous, sedimentary and metamorphic P-T histories, as well as the structural and tectonic sequence from origin to final emplacement are crucial to our understanding of paleo-subduction zone dynamics. Subduction channels, where some ophiolitic mélanges form, are the mechanically weak shear zone between the upper and lower plate, which consists of stacked oceanic sequences off-scraped from the subducted plate, and mixed with eroded crust and mantle material from the forearc in the arc-continent accretionary/collision zone. Here, we present results of comprehensive field mapping, structural analysis, geochemical and geochronological studies of the Neoarchean Zunhua and Shangying ophiolitic mélanges in the Eastern Hebei Complex, Central Orogenic Belt, North China Craton. The Zunhua ophiolitic mélange is composed of a series of petrogenetically linked forearc affinity ultramafic-mafic blocks (peridotite, podiform-chromite-bearing dunite, pyroxenite, metagabbro, metadiabase, metabasalt), and the podiform chromitites locally have inclusion of UHP TiO2(II) and remnants of UHP chenminigite phase, indicating some grains were derived from 270 to 410 km depth and incorporated in the forearc ophiolite before mélange formation. In contrast, the Shangying ophiolitic mélange to the east consists of layered and isotropic N-MORB affinity metagabbro, and garnet clinopyroxenite, metamorphosed from oceanic gabbro under eclogite-facies conditions at 65–70 km. Both mélanges have a strongly sheared metasedimentary matrix. Zircon UPb dating of blocks and crosscutting dikes shows that 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, whereas the Shangying ophiolitic mélange formed between 2.53 and 2.52 Ga, and was emplaced in the Eastern Block at 2.52–2.47 Ga. The Zunhua ophiolitic mélange contains a nearly complete ophiolite sequence as tectonic blocks, which formed in a forearc supra-subduction zone (SSZ) setting, and records a Neoarchean subduction initiation and arc-continent collision event. In contrast, the Shangying ophiolitic mélange includes meta-basalts, gabbros and garnet clinopyroxenites, which formed at a mid-ocean ridge (MOR). The Zunhua SSZ ophiolitic mélange was emplaced over the forearc region through accretionary thrusts, whereas the Shangying MOR ophiolitic mélange contains subducted masses mixed and exhumed in a subduction channel. The two ophiolitic mélanges with different tectonic attributes and contrasting emplacement modes were juxtaposed in a forearc and subduction channel complex during a Neoarchean arc-continent collision, and their spatial configuration reveals a west-northwest-dipping subduction polarity, consistent with the kinematic fabrics in the ophiolitic mélanges, and their correlatives along the ∼ 1800 km long paleo-subduction zone marked by the Central Orogenic Belt. Documentation of the coexisting SSZ and MOR ophiolitic mélanges in the Eastern Hebei Complex suggests that the large-scale subduction/accretionary zone was active in the late Neoarchean, and the geochronological data reveals that the geological process involving seafloor spreading, subduction initiation, forearc thrusting and exhumation of subduction materials occurred within < 80 Myr (million years), which is similar in duration to many Phanerozoic subduction-collision zones. These features indicate that the tectonic paradigm for the late Neoarchean has evolved to be similar to the modern Earth.