Field occurrences and Nd isotopic characteristics of the meta-mafic-ultramafic rocks from the Caozhuang Complex, eastern Hebei: Implications for early Archean crustal evolution of the North China Craton

Abstract

Crust-mantle differentiation and growth of early crust are critical issues in the field of solid earth science and the proper understanding of how Earth evolved during Eoarchean-Paleoarchean greatly relies on these issues. The Caozhuang Complex, one of the oldest rock units in the North China Craton (NCC), is located in eastern Hebei and composed of the Caozhuang supracrustal rocks and the Huangbaiyu orthogneisses. The Caozhuang supracrustal rocks are considered to be the oldest sedimentary sequences in the NCC, into which the Huangbaiyu grey gneisses with ∼3.3 Ga zircon ages intruded. In previous studies, the Caozhuang supracrustal rocks are represented by fuchsite-bearing quartzites with ∼3.8 Ga detrital zircons and metamorphosed mafic rocks (amphibolites) with ∼3.5 Ga single-stage Nd model age relative to depleted-mantle (TDM(Nd)).Here we present our new study on meta-mafic–ultramafic rocks of the Caozhuang supracrustal sequence in accordance with detailed large-scale field mapping, petrology, elemental and Sm-Nd isotopic analyses, and then discuss the possible crust-mantle differentiation events that could take place during Eoarchean and Paleoarchean in the Caozhuang area.The meta-mafic-ultramafic rocks can be subdivided into two groups based on field occurrence. Group One rocks are amphibolites and hornblendites that occur as layers or slabs only in sedimentary sequences, which are typically accompanied by banded iron formations (BIFs). By contrast, Group Two never coexists with BIFs, and they mainly occur as amphibolites and hornblendites lenses or deformed dykes and sills in supracrustal sequences and orthogneisses, with blastophitic textures preserved. Sm-Nd isotopic analyses show that rocks from Group One have a Sm-Nd isochron age of 3580 ± 98 Ma and εNd(t) of +0.5 to +1.3, corresponding to TDM(Nd) of ∼3.7–3.5 Ga; while Group Two rocks yield a Sm-Nd isochron age of 3179 ± 90 Ma and εNd(t) between +4.6 and +6.6, corresponding to TDM(Nd) of ∼3.3–3.1 Ga. Geochemical characteristics of the two groups, such as moderate Mg#, negative correlation between Fe2O3T and MgO, and positive correlation between Ni and MgO, suggest they both experienced fractional crystallization of ferromagnesian minerals (dominated by olivine and clinopyroxene) during emplacement of magma(s). And combined with the flat HREEs patterns, and N-MORB-like trace element spider diagrams and positive εNd(t) values, it could be indicated that they could be partial melting products of ancient depleted mantle in the spinel stability fields. Based on the consistent whole rock Sm-Nd isochron ages with TDM(Nd), we suggest that two episodes of crustal growth occurred during ∼3.7–3.5 Ga in late Eoarchean and ∼3.3–3.1 Ga in late Paleoarchean in the Caozhuang area. The depleted characteristics of Group One indicate lithospheric mantle in the Caozhuang area might have been depleted before ∼3.7–3.5 Ga. And the Sm-Nd isochron ages of Group One basic volcanic rocks constrained that Caozhuang supracrustal sequence may have deposited during ∼ 3.7–3.5 Ga. Basic dykes or sills with TDM(Nd) of ∼3.3–3.1 Ga and time-equivalent TTG gneisses represent a significant thermal event in late Paleoarchean. The meta-mafic-ultramafic rocks in the Caozhuang Complex are an important window to reveal and understand the early crustal evolution of the NCC, which probably has global significance.