Geochronology, mantle source composition and geodynamic constraints on the origin of Neoarchean mafic dikes in the Zanhuang Complex, Central Orogenic Belt, North China Craton

Mafic granulitic and amphibolitic boudins dispersed in Archean felsic gneisses are widely distributed in the Central Orogenic Belt (COB) and the Eastern Block of the North China Craton (NCC) and are considered to constitute deformed mafic dike swarms. Previous studies have demonstrated that the mafic dikes in the Zanhuang Complex of the NCC intruded the fabrics of an Archean mélange belt and were boudinaged during younger deformation. Igneous zircons from an undeformed mafic dike yield a 207Pb/206Pb age of 2535±30Ma, which is interpreted as the crystallization age. In addition, pegmatites cutting across the mafic dikes in the field also yield an igneous zircon 207Pb/206Pb age of 2504±16Ma, providing strong evidence that the mafic dikes in the NCC intruded during the Neoarchean. Metamorphic zircons from one deformed mafic dike sample yield a metamorphic 207Pb/206Pb age of 2090±83Ma, and another four samples from deformed mafic dikes have a consistent metamorphic zircon age of ca. 1850Ma, indicating that the mafic dikes underwent at least two generations of Paleoproterozoic metamorphism of ca. 2.1Ga and ca. 1.85Ga. Previously reported trace element systematics of the mafic dikes are consistent with an arc-related lithospheric mantle source region, rather than an ocean island basalt (OIB)-like source region. The new whole rock Nd isotopic composition (ɛNd(t)=+0.71 to +3.70) is relatively more evolved compared to that of the depleted mantle at 2.5Ga, indicating an enriched lithospheric mantle source. Accordingly, the mafic dikes are proposed to have been formed in a subduction-related environment and their enriched mantle source was metasomatized by the melts and fluids derived from the subducted slab. Based on previous studies of the NCC and new geochronological and isotopic data in this contribution, a new comprehensive tectonic model is proposed for the evolution of the NCC between 2.7Ga and 1.85Ga: (1) from 2.7 to 2.5Ga, an oceanic arc terrane belt developed in what is now the COB of the NCC while a passive margin sequence was deposited on the western edge of the Eastern Block on the margin of the intervening ocean; (2) the oceanic arc terrane collided with the passive margin sequence at 2.5Ga above a west-dipping subduction zone; (3) the subduction polarity was reversed at ca. 2.5Ga and a new east-dipping subduction zone gave rise to the intrusion of the 2.5Ga mafic dikes of the NCC; (4) the final collision between the Western Block and the Eastern Block occurred at ca. 2.1Ga; (5) the whole NCC then collided with the Columbia Supercontinent along the north margin of the craton at 1.9–1.8Ga.