Zircon U–Pb and Lu–Hf isotopic and geochemical constraints on the origin of the paragneisses from the Jiaobei terrane, North China Craton

Abstract

Clastic sedimentary rocks are important tracers to understand the evolution of the continental crust. Whole-rock major and trace element data, zircon U–Pb dating and Hf isotopic data for the paragneisses from the Jiaobei terrane are presented in this study in order to constrain their protoliths, provenance and tectonic setting. The paragneisses are characterized by enrichment in Al2O3 and TiO2, negative DF (DF=10.44−0.21SiO2−0.32Fe2O3T−0.98MgO+0.55CaO+1.46Na2O+0.54K2O) values and the presence of aluminum-rich metamorphic minerals (e.g., garnet and sillimanite). Together with the mineral assemblages and zircon features, it can be inferred that the protoliths of these rocks are of sedimentary origin. The K–A (A=Al2O3/(Al2O3+CaO+Na2O+K2O), K=K2O/(Na2O+K2O)) and log(Fe2O3/K2O)–log(SiO2/Al2O3) diagrams indicate that they belong principally to clay–silty rocks with some contributions from graywacke. A series of geochemical indexes, such as the widely employed CIA (CIA=[Al2O3/(Al2O3+CaO∗+Na2O+K2O)]×100; molar proportions) and ICV (ICV=(Fe2O3+MnO+MgO+CaO+Na2O+K2O+TiO2)/Al2O3) values, and the A–CN–K diagram for the paragneisses indicate relatively weak weathering in the source rocks and negligible post-depositional K-metasomatism. In addition, their REE patterns, low Cr/Zr (0.61–1.99), high Zr/Y (4.81–23.59) and Th/U (3.21–40.67) ratios, the low to moderate contents of Cr (197–362ppm) and Ni (6.68–233ppm), and source rock discrimination diagrams collectively suggest that the sediments of the protoliths of the paragneisses in the Jiaobei terrane were derived from the source with intermediate–acidic composition, probably granitic-to-tonalitic rocks. In combination with geochronological and isotopic studies on the paragneisses and the basement rocks in the Jiaobei terrane, it is suggested that the Archean–early Paleoproterozoic granitic rocks in the Jiaobei terrane possibly provided the most important source materials. In addition, the protoliths of the paragneisses might have been deposited during 2.47–2.42Ga. High and heterogeneous ICV values of the paragneisses could imply a chemically immature source. A number of geochemical indicators and tectonic discrimination diagrams together indicate an island arc or active continent margin setting for the deposition of the protoliths of the paragneisses in the Jiaobei terrane. Together with the nearly contemporaneous igneous rocks, it can be inferred that the convergent margin setting was possibly operative during the Late Neoarchean–Early Paleoproterozoic transition in the Jiaobei terrane.