A 2.5 Ga fore-arc subduction-accretion complex in the DengfengGranite-Greenstone Belt, Southern North China Craton
The Dengfeng granite-greenstone belt (DGGB), located in the southern segment of the Central OrogenicBelt (COB) of the North China Craton (NCC), consists of a volcano-sedimentary assemblage, intrudedby 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, consistentwith characterisitics of typical Phanerozoic subduction-accretion complexes. The basaltic amphibolitesyield a metamorphic zircon207Pb/206Pb age of 2507 ± 26 Ma, interpreted to represent the peak age ofamphibolite 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 nega-tive Nb and Zr anomalies. Based on mixed MORB- and arc-affinities, the basaltic amphibolites in the DGGBare interpreted to have formed in a fore-arc tectonic setting. One late potassic granite dike cutting acrossthe fabrics of the volcano-sedimentary assemblage yields an intrusion age of 2492 ± 35 Ma, constrain-ing the minimum deformation age for tectonic assembly of the package. The TTG gneisses and dioritesintruded the western margin and the center of the subduction-accretion complex, respectively. One TTGsample and one diorite sample yield igneous zircon207Pb/206Pb ages of 2514 ± 26 Ma and 2518 ± 36 Ma,respectively, constraining their intrusion ages. The TTG gneisses display high ratios of (La/Yb)cnand Sr/Y,and depletion in HFSE with negative Nb, Ta and Ti anomalies, consistent with those of typical ArcheanTTGs. The TTG gneisses are therefore considered to be generated from partial melting of a shallowlysubducting oceanic slab and/or accreted arc amphibolites. The diorites have high concentrations of MgO(2.89–6.05 wt.%), Ni (148–178 ppm) and Cr (85.7–120.6 ppm), and highly fractionated REE patterns andare depleted in HFSEs with negative Nb, Ta, Zr, Hf and Ti anomalies. We suggest that the high-Mg diorites inthe DGGB may have been derived from a hydrated mantle wedge which was previously metasomatized bysubduction-derived melts and/or fluids. Collectively, a Neoarchean subduction-accretion-collision eventis therefore proposed to have generated the DGGB. The volcano-sedimentary assemblage in the DGGBrepresents a fore-arc subduction-accretion complex, which is interpreted to be related to the suture zoneof ca. 2.5 Ga arc-continent collision between a TTG-dominated arc terrane in the COB and the EasternBlock of the NCC. We further propose a long N-S striking Neoarchean suture zone occurring in the easternmargin of the COB mainly consists of the ca. 2.5 Ga subduction-accretion complex in the DGGB to thesouth, the ca. 2.5 Ga mélange belt in the Zanhuang Complex in the central, and the ca. 2.5 Ga ophioliticmélange belt in the Zunhua-Dongwanzi structural belt to the north, which separate an arc terrane in theCOB from the Eastern Block of the NCC.