Late Neoarchean magmatism and tectonic evolution recorded in the Dengfeng Complex in the southern segment of the Trans-North China Orogen

Abstract

The Dengfeng Complex exposed in Songshan area in the southern segment of the Trans-North China Orogen (TNCO) mainly consists of Late Neoarchean tonalite-trondhjemite-granodiorite (TTG) gneiss, metadiorite and amphibolite. The Sanhuangzhai amphibolite records the earliest mafic magmatism (2658±5Ma) in the region, which is enriched in LREE and LILE with negative Nb, Ta and Ti anomalies and positive initial εNd(t) and εHf(t) values, suggested to be the remnant of an arc-related juvenile mafic crust. The Shipaihe amphibolite at the end of Neoarchean (2492±7Ma) is characterized by flat REE pattern, slightly negative Nb, Ta and Ti anomalies and positive initial εNd(t) and εHf(t) values, which is interpreted as large-degree partial melt of metasomatised mantle source in a back-arc basin. Zircon U-Pb dating shows two episodes of TTG magmatism (2.57–2.55Ga and 2.55–2.50Ga) within the Dengfeng Complex. All TTG gneisses have high SiO2 and Na2O, but low Mg#, Cr, Ni, [Yb]N and Y with high [La/Yb]N and Sr/Y. The early TTG gneiss (2.57–2.55Ga) in the eastern Songshan area has variable whole-rock Hf-Nd isotopes (εHf(t)=−2.14–4.64; εNd(t)=0.11–3.35; respectively), and its protolith is interpreted as the result of partial melting of a thickened lower crust with residual garnet and amphibole in the source. But the later TTG gneiss (2.55–2.50Ga) in the western Songshan area has depleted whole-rock Hf-Nd isotopes (εHf(t)=5.59–6.08; εNd(t)=2.42–3.86; respectively), suggesting an overall juvenile source, and its protolith is interpreted as the partial melt of flat-subducting oceanic crust. The Shipaihe metadiorite is characterized by high Mg#, Cr, Ni, Sr, Ba, highly fractionated REE patterns and strong negative Nb, Ta and Ti anomalies, resembling to sanukitoid suite, which is interpreted as the partial melt of a mantle wedge metasomatised by subduction-derived fluids/melts. Integrated geochronology and geochemical data of the Dengfeng Complex in Songshan area reveal the Late Neoarchean crustal accretion process from subduction to arc-continental collision (accretionary orogeny) followed by post-collisional extension (back-arc basin), implying an active continental margin arc environment in the southern segment of the North China Craton.