Geochronology, geochemistry and petrogenesis of the Neoarchean magmatism in the Jiefangyingzi area, northern North China Craton: Implications for crustal growth and tectonic affinity

Abstract

The Bayan Obo-Duolun-Chifeng-Kaiyuan fault is considered as the craton-orogen boundary between the northern North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB). However, locations of this boundary from Duolun, Chifeng to Kaiyuan have not been well constrained. The Jiefangyingzi area northeast to Chifeng is located at the junction of the Bainaimiao arc belt and northern NCC, and has long been regarded as part of the Bainaimiao arc belt of the CAOB. This paper presents new LA-ICP-MS zircon U-Pb geochronological, whole-rocks geochemical and Nd-Hf isotopic data of some recently identified Neoarchean magmatic rocks from the Jiefangyingzi area. These rocks consist mainly of amphibolites, dioritic gneisses, quartz dioritic gneisses, tonalitic gneisses and trondhjemitic gneisses formed at 2569–2536 Ma and minor amphibole plagioclase gneisses formed at ca. 2640 Ma, and are distributed in an east–west belt that is about 13 km long and 1.5 km wide. Geochemical and Nd-Hf isotopic compositions suggest that the amphibolites were derived from partial melting of a depleted mantle that was metasomatized by subduction-derived fluids with significant crustal contamination, and the dioritic gneisses were generated by partial melting of mantle peridotite metasomatized by slab-melts. The quartz dioritic and tonalitic gneisses originated from high degree partial melting of the subducted oceanic crust with garnet-bearing amphibolite facies in the residue, while the trondhjemitic gneisses originated from low degree partial melting of the thickened mafic lower crust with eclogite facies in the residue due to underplating of mafic magmas. The occurrences of Neoarchean basement rocks, combined with previous Nd-Hf isotopic data of the late Paleozoic magmatic rocks, suggest that the Jiefangyingzi area has a tectonic affinity with the Eastern Block of the NCC rather than the CAOB, and the craton-orogen boundary between the northern NCC and the CAOB should be situated in places north to the Jiefangyingzi area. Our results support that the late Neoarchean (2.6–2.5 Ga) is also a period of crustal growth as well as extensive reworking of the 2.9–2.7 Ga ancient crust in the northwestern margin of the Eastern Block of the NCC. In addition, the transition from tonalite-trondhjemite-granodiorite (TTG) and other potassium-poor granitoid gneisses to crustal-derived potassium-rich granitoid gneisses at the end of Neoarchean (ca. 2.53 Ga) marked the initial cratonization of the NCC and ca. 2.53 Ga is a critical transformation period of maturation of the continental crust in the NCC.