Tectonic setting and significance of 2.3–2.1 Ga magmatic events in the Trans-North China Orogen: New constraints from the Yanmenguan mafic–ultramafic intrusion in the Hengshan–Wutai–Fuping area

A detailed study of the ∼2.2 Ga Yanmenguan mafic–ultramafic intrusion in the Hengshan area is presented with the aim of clarifying the tectonic history of the Trans-North China Orogen in the North China Craton during the period 2.3–2.1 Ga. SHRIMP U–Pb dating of magmatic zircons from a gabbro indicates emplacement at ∼2193 Ma. The whole rock and mineral geochemistry of this intrusion are akin to Alaskan-type complexes, and are typical of subduction-related magmas. The parental magma likely originated from the depleted mantle wedge above a subduction zone, and experienced fractional crystallization of olivine and clinopyroxene. The continental arc, represented by the mafic–ultramafic intrusions and contemporary granitic and volcanic rocks, together with the Lüliang back-arc basin constitute a marginal arc–back-arc basin system that evolved between 2.3 and 2.1 Ga. Based on geochronological datasets and available data on magmatism, metamorphism and structural deformation, a comprehensive new tectonic model for the evolution of the TNCO during the Neoarchean to Paleoproterozoic is constructed: (1) ∼2.5 Ga intra-oceanic arc–back-arc basin system and its accretion at the end of the Neoarchean; (2) a long hiatus in magmatism and tectonism between ∼2.5 and 2.3 Ga; (3) ∼2.3–2.1 Ga marginal arc–back-arc basin magmatism developed in the accreted Hengshan–Wutai–Fuping arc terranes and the Lüliang terrane; (4) ∼2.1 Ga collision, resulting in closure of the ∼2.3–2.1 Ga Lüliang back-arc basin, accretion of the Lüliang–Hengshan–Wutai–Fuping arc terranes, and their attachment to the Western Block of the North China Craton; (5) the intrusion of ∼1970–1910 Ma post-collisional mafic dykes in an extensional event; (6) final collision between the Western and Eastern Blocks between ∼1.9 and 1.8 Ga, resulting in extensive magmatism, tectonism and metamorphism; and lastly (7) post-1.8 Ga crustal extension.