Source and petrogenesis of Paleoproterozoic meta-mafic rocks intruding into the North Liaohe Group: Implications for back-arc extension prior to the formation of the Jiao-Liao-Ji Belt, North China Craton

Meta-mafic rocks as sills, dykes and veins are widely distributed within the North Liaohe Group, Liaodong Peninsula, which consist mainly of meta-diabase/gabbro and (garnet) amphibolite with minor felsic clastic rocks as enclaves. Although a long-standing debate existed about the petrogenesis for the meta-mafic rocks, it is a critical judgement for using geochemistry and geochronology of these rocks to reveal the complicated tectonic evolution of the Paleoproterozoic Jiao-Liao-Ji Belt (JLJB). In this paper, a combined study of zircon U-Pb isotopic data, whole-rock geochemistry and Sm-Nd isotope analyses are reported for these rocks to investigate their sources, petrogenesis and tectonic settings. Abundant zircon U-Pb LA-ICP-MS age data reveal that the meta-mafic rocks have a magmatic emplacement age of ca. 2130 Ma with a metamorphic age of ca. 1878 Ma. These meta-mafic rocks have characteristic geochemical compositions with moderate SiO2 (46.68-53.90 wt%), low TiO2 (0.56-2.33 wt%) and variable total Fe2O3 (9.65-17.58 wt%), MgO (2.77-11.49 wt%), Cr (1.44-1580 ppm) and Ni (6.4-168 ppm) similar to evolved tholeiitic basalt. They exhibit enriched mid-ocean ridge basalt (E-MORB)-like geochemical features (e.g., trace element patterns) with highly variable εNd(t) values from −1.5 to +3.1, relative enrichment of light rare earth elements (LREEs) and depletion of high field strength elements (HFSEs; e.g., Nb, Ta, Zr, Hf, and Ti). The geochemical trends of major and trace elements and Nd isotopes suggest that the primary magma for these meta-mafic rocks were probably derived from partial melting of a depleted asthenospheric mantle in the spinel stability field, with extensive fractional crystallization of a three-phase assemblage of olivine, clinopyroxene and plagioclase, crustal assimilation (e.g., continental detritus), and limited metasomatism from subduction-related fluids and/or melts. In view of the widespread distribution of Paleoproterozoic magmatism, including the Liaoji granites, and the voluminous sedimentary rocks in the Liaodong Peninsula, a newly tectonic model has been proposed that a ca. 2.1 Ga oceanic plate subduction event induced a back-arc basin opening, which closed to form the JLJB in the period of ca. 1.9 Ga.