Geochronology and geochemistry of the polyphase mafic rocks in the North Liaohe Group, Jiao-Liao-Ji Belt, North China Craton: implications for petrogenesis and tectonic evolution

Abstract

The Jiao-Liao-Ji Belt (JLJB) is one of the Paleoproterozoic orogenic belts within the North China Craton (NCC), whose tectonic evolution is still controversial due to multiple magmatic/metamorphic events after its formation. To tackle this controversy, we conducted petrological, geochronological, geochemical and zircon Hf isotopic studies for the meta-mafic rocks from the North Liaohe Group (NLG) in the central JLJB. ²⁰⁷Pb/²⁰⁶Pb weighted average ages of 1849 ± 36 Ma and 1853 ± 13 Ma indicate that the protoliths of the amphibolites in the Shisixian area formed at ca. 1.85 Ga. Geochemically, the amphibolites belong to tholeiitic series; while the meta-gabbros can be divided into the low-Ti and high-Ti types: the former belongs to calc-alkaline series, whereas the latter belongs to tholeiitic series. The high-Ti rocks are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs; e.g., Ba, K and Pb), and depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs; e.g., Nb, Ta, P and Ti). These rocks display slightly positive εHf(t) values (+1.40 to +2.02). The low-Ti meta-gabbros were most likely derived from the partial melting of the enriched lithospheric mantle in the spinel stability field, which was metasomatized by subduction-related fluids and/or melts with significant contamination of crustal material, while the amphibolites and high-Ti meta-gabbros were derived from partial melting of depleted asthenospheric mantle in the spinel stability field, which was metasomatized by limited subduction-related fluids and/or melts, coupled with fractional crystallization. The lithological and geochemical characteristics show that the low-Ti meta-gabbros formed in a magmatic arc environment, the high-Ti rocks formed in the later stage of a back-arc basin; while the amphibolites formed in a post-collisional extensional environment. Combined with literature data, we suggest that the JLJB experienced a process from subduction, collision, to extension: At ca. 2.2–2.1 Ga back-arc basin opened via southward subduction of an oceanic plate followed by northward subduction at ca. 2.1–1.91 Ga, and subsequently closed to form the JLJB at ca. 1.91 Ga, which led to the Longgang-Nangrim continental collision, and post-collision extension occurred at ca. 1.85 Ga along with the collapse of the collisional orogen.