Identification of the Neoarchean Jianping pyroxenite-mélange in the Central Orogenic Belt, North China Craton: A fore-arc accretional assemblage

Abstract

Mélanges are widespread in accretionary and collisional orogens at convergent plate margins. We document a Neoarchean mélange belt exposed in the Jianping Complex, North China Craton (NCC). The mélange belt contains a structurally chaotic tectonic mixture of paragneiss, metapelites mixed with exotic tectonic blocks of mafic and ultramafic rocks including pyroxenites, metagabbros, metadiabases, metabasalts, and TTG gneisses. The blocks have formation ages of ca. 2.53–2.51 Ga and all units in the mélange have been intruded by undeformed pegmatite dykes (ca. 2.49 Ga). Geochemical studies show that the pyroxenites in the mélange have high contents of MgO (15.84–20.87%), Mg# (68–80), Ni (211–446 ppm) and Cr (963–2491 ppm). They are characterized by slightly negative Eu anomalies, enrichment in light rare earth elements (LREE) and pronounced negative high field strength elements (HFSE) anomalies on chondrite- and primitive mantle- normalized diagrams. We suggest that the pyroxenites are cumulates derived from an arc-related lithospheric mantle source. The pyroxenites in the mélange are suggested to have formed in a subduction-related fore-arc environment whose mantle source was metasomatized by fluids derived from the subducted slab. The recognition of the Neoarchean Jianping pyroxenite-mélange and regional tectonic analysis of the NCC indicate that the mélange belt in the Jianping Complex is one segment of a 2.5 Ga suture zone that crosses the NCC, stitching a forearc segment of an oceanic arc terrane in the Central Orogenic Belt (COB) with the Eastern Block. The Jianping pyroxenite-mélange is therefore correlative with the Zunhua, Zanhuang, and Dengfeng mélanges in more southern segments of the Central Orogenic Belt, defining a Neoarchean suture that extends for approximately 1600 km across the craton. The geochemistry of the arc-derived magmas, and the tectonic association with a subduction-related mélange is similar in petrology, chemistry, structural style, and size to modern-day forearc settings, negating any calls for non-plate tectonic settings for arc-like magmas in the Archean.