Paleoproterozoic Alaskan-type ultramafic-mafic intrusions in the Zhongtiao mountain region, North China Craton: Petrogenesis and tectonic implications

Neoarchean to Paleoproterozoic Alaskan-type ultramafic-mafic complexes constitute a rare but genetically important occurrence within the cratonic provinces on Earth and their genesis holds key information for monitoring the evolution of mantle source and the geodynamic processes along ancient convergent plate margins. This geochronological and geochemical study presents one episode of Paleoproterozoic Alaskan-type ultramafic-mafic intrusions from the Zhongtiao Mountain (ZTM) region, North China Craton (NCC), with an emplacement age of ca. 2315 Ma. The intrusions consist of a metamorphosed rock assemblage of peridotite, pyroxenite, gabbro, gabbroic diorite, hornblendite and doleritic dykes. They exhibit a compact elliptical geometry as single entity, linear array of stock clusters and crude lithological zonation from cores of peridotites to margins of hornblendites and gabbroic rocks. They are tholeiitic in composition with an SiO2 range from 39.4 to 57.4%, modest enrichment in light rare earth elements (LREE) and depletion in high field strength elements (HFSE). They possess variable whole-rock εNd(t) values from −3.41 to +2.68 and εHf(t) values from −7.23 to +5.11, as well as zircon εHf(t) values from −2.06 to +1.82. These structural and compositional traits bear a close resemblance to those of classic Phanerozoic Alaskan-type complexes. Geochemical and isotopic data indicate that the formation of the present ultramafic-mafic intrusions most likely involved a Mg-rich hydrous parental magma from a subduction fluid-metasomatized lithospheric mantle source and subsequent crystal fractionation-accumulation processes. Besides representing the first recognition of mantle melting events in the ZTM region during the early Paleoproterozoic Siderian interval of ca. 2.45-2.20 Ga, this unusual magmatic episode presents a temporal hallmark for indicating the prevalence of continental arc magmatism. Synthesizing these intrusions with the ca. 2.36-2.10 igneous episodes as well as concomitant episodic metamorphic events in the Trans-North China Orogen can lead to the characterization of a possible divergent double subduction system in the final amalgamation of the NCC during the Paleoproterozic.