Constraints of mafic rocks on a Paleoproterozoic back-arc in the Jiao-Liao-Ji Belt, North China Craton

Abstract

The Paleoproterozoic Jiao-Liao-Ji Belt (JLJB) in the North China Craton, a major terrane boundary, has remained controversial with respect to its tectonic history. Here we report LA–ICP–MS U–Pb zircon ages, and whole-rock elemental and isotopic compositions of mafic rocks intruding into the North and South Liaohe Groups, with the aim of evaluating their petrogenetic affinities and metamorphic history, and their implications for the tectonic evolution of the JLJB. The mafic rocks in the North Liaohe Group dominantly composed of meta-diabase and meta-gabbro, whereas mafic rocks in the South Liaohe Group are mainly (garnet-) amphibolite. LA–ICP–MS zircon U–Pb analyses yield crystallization and metamorphic ages of ca. 2130 Ma and ca. 1880 Ma, respectively, suggesting that they are coeval, and were metamorphosed at the same time. Whole-rock elemental and isotopic compositions suggest tholeiitic affinity for the protoliths of both of them, with variable SiO₂ and MgO concentrations and Mg# and εNd(t) values. They both display enriched mid-oceanic-ridge basalt (E-MORB)-like trace element patterns with enrichment in light rare earth elements and depletion in some high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). The data suggest derivation of the melts from a depleted asthenospheric mantle, coupled with fractional crystallization and crustal contamination. Some of the mafic rocks in the South Liaohe Group have lower HFSE (e.g., Nb, Ta, Zr, Hf, and Ti) concentrations, and La/Sm and Th/La ratios, and higher U/Th ratios than their counterparts in the North Liaohe Group, possibly suggesting enhanced metasomatism by subduction-related fluids and/or melts. The geochemical features of the mafic rocks from JLJB contrast with those of typical intra-continental rifts and volcanic arcs, and may have formed in a back-arc basin. Our study supports the presence of a ca. 2.2–2.1 Ga back-arc basin that formed due to northward subduction and subsequent closure of an oceanic plate, resulting in arc–continent collision and formation of the JLJB at ca. 1.9 Ga.