Geochemistry and petrogenesis of ca. 2.1 Ga meta-mafic rocks in the central Jiao–Liao–Ji Belt, North China Craton: A consequence of intracontinental rifting or subduction?

Abstract

As one of the global 2.0–1.8 Ga continent-to-continent collisional orogens that welded the Supercontinent Columbia or Nuna, the Jiao–Liao–Ji Belt (JLJB) in the North China Craton was formed through ca. 1.90–1.85 Ga collision between the Longgang and Nangrim Blocks. However, the tectonic process before the final collision remains highly controversial. Some people insisted that during 2.2–2.1 Ga the JLJB was in a back-arc area of a subduction regime. In contrast, some other people proposed that ca. 2.2–2.1 Ga represented a continental rifting stage instead of subduction. To tackle this controversy, the coeval mafic rocks and their petrogenesis become the key hints. Therefore, we implemented synthetic petrological, geochronological, geochemical, zircon Hf and the whole rock Sr–Nd isotopic studies for the newly mapped meta-mafic rocks from the central JLJB. Zircon U–Pb dating results revealed that they were emplaced at ca. 2133–2099 Ma and experienced subsequent collision-related regional metamorphism at ca. 1879–1852 Ma. Geochemically, they are sub-alkaline basalts and contain moderate SiO2 and high concentrations of MgO, total Fe2O3, Ni, and V. They are enriched in light rare earth elements (LREE) and large-ion lithophile elements (LILE; e.g., Ba, Sr and K), and are depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE; e.g., Nb and Ta). Geochemical features show largely affinity to the enriched mid-ocean ridge basalts. Isotopic results show low 87Sr/86Srt ratios (low to 0.676869), positive εNd(t) values (+0.50 to +2.45) and εHf(t) values (+0.04 to +9.81). The parental magmas of ca. 2.1 Ga meta-mafic rocks were most likely derived from the partial melting of the depleted asthenospheric mantle with contamination of crustal material. Associated with the regionally developed A-type and adakitic granites, as well as the bimodal volcanic rocks throughout the JLJB, we proposed that the Longgang and Nangrim Blocks were initially broken apart through an intracontinental rifting that was triggered by delamination of an old thickened lithosphere at ca. 2.2–2.1 Ga, before they were finally collided with each other at ca. 1.90 Ga.