A MORB-arc basalt–adakite association in the 2.5 Ga Wutai greenstone belt: late Archean magmatism and crustal growth in the North China Craton

Abstract

The Wutai, Fuping and Hengshan Complexes form part of a major 2.5Ga late Archean arc system in the northern part of the North China Craton. The Wutai Complex is a well-preserved greenstone belt dominated by mafic, intermediate and felsic volcanic rocks, with a mid-ocean ridge basalt (MORB)-arc-back arc, basalt–adakite association. Multi-element plots for MORB-type mafic volcanic rocks are characterised by depletion of Th, Nb (Ta) and La, nearly flat or LREE-depleted REE patterns, and Nb/U and Nb/Th ratios greater than primitive mantle values. Back arc basin basalt (BABB)-type rocks have nearly flat trace element patterns with Nb, Ti and Y troughs, and LREE-depleted or near-flat REE patterns. Island arc basalt (IAB)-type rocks are characterized by negative anomalies of Nb (Ta), Zr (Hf), Ti and Y, and by LREE-enrichment. The felsic volcanic rocks conform to most compositional criteria for Cenozoic adakites, including low Y and Yb, high La/Smn, La/Yb and Zr/Sm ratios, and similar trace element and REE patterns. Trends defined by the association are consistent with magma mixing between MORB-like melts and subduction (slab)-related melts. Intra-oceanic subduction, coupled with contemporary MORB-type mantle upwelling related to back arc basin extension, can account for the interaction between the two components. Subduction–accretion or back arc basin collapse led to accretion of the Wutai intra-oceanic arc-back arc complexes with the magmatic arcs of the Fuping and Hengshan terranes, that are dominated by high-grade tonalite-trondhjemite-granodiorite (TTG) gneisses. This geodynamic interpretation is different from many tectonic models for Archean greenstone belts in which magmatism and crustal growth were dominated by mantle plumes, or interaction between subduction zones and mantle plumes. The final disposition of the arc complexes was controlled by the collision of the eastern and western blocks of the North China Craton during the Paleoproterozoic, resulting in extensive tectonic interleaving within a major orogenic belt that extended throughout the central portion of the North China Craton.