A 2.5 Ga fore-arc subduction-accretion complex in the Dengfeng Granite-Greenstone Belt, Southern North China Craton

Abstract

The Dengfeng granite-greenstone belt (DGGB), located in the southern segment of the Central Orogenic Belt (COB) of the North China Craton (NCC), consists of a volcano-sedimentary assemblage, intruded by tonalite, trondhjemite, granodiorite (TTG suite), diorite, granites and late mafic dikes. The volcano-sedimentary assemblage in the DGGB mainly consists of tectonically imbricated basaltic amphibolites, meta-gabbroic rocks with minor ultramafic rocks, and metagreywacke, marble and quartzite, consistent with characterisitics of typical Phanerozoic subduction-accretion complexes. The basaltic amphibolites yield a metamorphic zircon 207Pb/206Pb age of 2507±26 Ma, interpreted to represent the peak age of amphibolite facies metamorphism that took place during subduction/accretion of the basaltic protolith. The basaltic amphibolites are characterized by a tholeiitic affinity, and flat LREE patterns with minor negative Nb and Zr anomalies. Based on mixed MORB- and arc-affinities, the basaltic amphibolites in the DGGB are interpreted to have formed in a fore-arc tectonic setting. One late potassic granite dike cutting across the fabrics of the volcano-sedimentary assemblage yields an intrusion age of 2492±35 Ma, constraining the minimum deformation age for tectonic assembly of the package. The TTG gneisses and diorites intruded the western margin and the center of the subduction-accretion complex, respectively. One TTG sample and one diorite sample yield igneous zircon 207Pb/206Pb ages of 2514±26 Ma and 2518±36 Ma, respectively, constraining their intrusion ages. The TTG gneisses display high ratios of (La/Yb)cn and Sr/Y, and depletion in HFSE with negative Nb, Ta and Ti anomalies, consistent with those of typical Archean TTGs. The TTG gneisses are therefore considered to be generated from partial melting of a shallowly subducting oceanic slab and/or accreted arc amphibolites. The diorites have high concentrations of MgO (2.89–6.05wt.%), Ni (148–178ppm) and Cr (85.7–120.6ppm), and highly fractionated REE patterns and are depleted in HFSEs with negative Nb, Ta, Zr, Hf and Ti anomalies. We suggest that the high-Mg diorites in the DGGB may have been derived from a hydrated mantle wedge which was previously metasomatized by subduction-derived melts and/or fluids. Collectively, a Neoarchean subduction-accretion-collision event is therefore proposed to have generated the DGGB. The volcano-sedimentary assemblage in the DGGB represents a fore-arc subduction-accretion complex, which is interpreted to be related to the suture zone of ca. 2.5 Ga arc-continent collision between a TTG-dominated arc terrane in the COB and the Eastern Block of the NCC. We further propose a long N-S striking Neoarchean suture zone occurring in the eastern margin of the COB mainly consists of the ca. 2.5 Ga subduction-accretion complex in the DGGB to the south, the ca. 2.5 Ga mélange belt in the Zanhuang Complex in the central, and the ca. 2.5 Ga ophiolitic mélange belt in the Zunhua-Dongwanzi structural belt to the north, which separate an arc terrane in the COB from the Eastern Block of the NCC.