Geochemical constraints on the nature of Late Archean basaltic-andesitic magmatism in the North China Craton

The North China Craton (NCC) is distinguished from the majority of Archean cratons in the world by its extensive records of late Neoarchean (2.6–2.5 Ga) mafic magmatism but its rare records of early Neoarchean (ca. 2.8–2.7 Ga) magmatism. Controversial issues have long existed on the tectonic setting of these mafic magmatisms, and it has been enigmatic about the role that the NCC has played in the Archean global tectonics. These issues are addressed by an integrated study of whole-rock major and trace elements as well as zircon Hf and whole-rock Nd isotopes in late Archean basaltic-andesitic rocks from the NCC. There are three episodes of basaltic-andesitic magmatism at ca. 2.88–2.73 Ga, ca. 2.65–2.60 Ga, and ca. 2.56–2.50 Ga, respectively. The first two phases are almost entirely tholeiitic basalts, whereas the last phase is composed of tholeiitic basaltic rocks, tholeiitic andesitic rocks and calc-alkaline rocks. The tholeiitic suites can be subdivided into LREE-depleted and LREE-enriched types in terms of their (La/Sm)N ratios, and the calc-alkaline suites are categorized into conventional andesitic rocks and sanukitoids according to their La/Yb and Sr/Y ratios as well as Sr and Ba concentrations. These rocks show island arc basalts (IAB)-like trace element distribution patterns and high water contents, indicating their formation through subduction zone magmatism. The LREE-depleted basaltic rocks were derived from partial melting of a mantle source that was weakly metasomatized by subduction zone fluids (mainly aqueous solutions), whereas the LREE-enriched basaltic rocks were originated from a mantle source that was significantly metasomatized by subduction zone fluids (including both aqueous solutions and hydrous melts). The tholeiitic andesitic rocks were generated by magma differentiation of the tholeiitic basaltic rocks. The coexistence of LREE-depleted and LREE-enriched tholeiitic rocks resembles the bimodal rock assemblage in backarc basins above modern oceanic subduction zones. The calc-alkaline andesitic rocks and sanukitoids were produced by partial melting of metasomatic domains that were generated by reaction of the mantle wedge peridotite with the hydrous melts, but the mantle source of sanukitoids contain more crustal components with higher melt/peridotite ratios of >0.1, in which the metasomatic agent would contain variable amounts of the low-degree partial melt from dehydrated melting of the subducting basaltic crust. The dominance of tholeiitic basaltic rocks and the lack of >2.8 Ga relict zircons in the basaltic-andesitic rocks indicate that the Archean NCC was characterized by development of a series of oceanic arcs around the continental nuclei. The common occurrence of bimodal basaltic rocks suggests the widespread development of backarc basins in the Neoarchean NCC with respect to the Archean plate tectonics. From the early to late Neoarchean, the basaltic-andesitic rocks became more enriched in alkali contents, higher in K2O/Na2O ratios, and more enriched in Nd and Hf isotope compositions. This indicates that more and more crustal materials were recycled into their mantle sources through oceanic subduction in this period. Compared to the other Archean cratons elsewhere on Earth, the NCC is significantly deficient in komatiites and komatiite-related rocks, but it contains much higher proportions of the LREE-enriched basaltic rocks and calc-alkaline andesitic rocks. This difference suggests that the formation of the NCC was primarily driven by oceanic subduction though the mantle plume would locally operate at ca. 2.8 Ga in the western Shandong region. Large-scale subduction of the oceanic slab in the NCC would be initiated at ca. 2.65–2.60 Ga for mafic arc magmatism due to the convergence of heterogeneous oceanic plates.