Evidence of the Neoarchean in the North China Craton

Zircon U–Pb–Hf isotopes and geochemistry of Neoarchean dioritic-trondhjemitic gneisses, Eastern Hebei, North China Craton: Constraints on petrogenesis and tectonic implications

Authors:

Bai et al

Abstract:

Dioritic-trondhjemitic gneisses of Neoarchean age are widely distributed throughout the Gualanyu–Shangying area in Eastern Hebei, North China Craton. These orthogneisses consist of dioritic, trondhjemitic, and quartz-rich trondhjemitic gneisses, which were subjected to amphibolite- to granulite-facies metamorphism. LA-ICP-MS zircon U–Pb isotopic dating reveals that the magmatic precursors of these rocks were contemporaneously emplaced from 2535 ± 23 to 2513 ± 8 Ma, reflecting an important Neoarchean dioritic-trondhjemitic magmatic event followed by sequential Proterozoic tectonothermal events between 2472 ± 60 and 2307 ± 50 Ma.

Dioritic and trondhjemitic gneisses exhibit similar zircon ɛHf (t) values of +1.5 to +5.2 and +2.9 to +5.3, respectively, indicating their depleted mantle affinity. Geochemical analysis shows that the dioritic gneisses exhibit low SiO2 contents (55.4%–63.0%), various MgO contents (2.07%–4.78%) and Mg# (36.07–63.56), and high (La/Yb)N ratios (5.45–15.19). In contrast, the trondhjemitic gneisses exhibit higher SiO2 contents (67.4%–73.8%) and (La/Yb)N ratios (12.86–76.73), and lower MgO contents (0.66%–2.67%), although their Mg# (36.29–61.41) are similar to those of dioritic gneisses. These geochemical and zircon Lu–Hf isotopic features, together with geochemical modeling, suggest that the dioritic magmas were derived mainly from the partial melting of a depleted mantle wedge that was metasomatized by slab-derived melts or fluids, and that the trondhjemitic magmas were formed by fractional crystallization of the dioritic magmas. Quartz-rich trondhjemitic gneisses exhibit extremely high SiO2 contents (77.4%–84.6%) with zircon ɛHf (t) values ranging from −3.4 to +4.1, suggesting that their precursors formed after the partial melting of a mixed magmatic source. It is suggested that the precursors comprised a mafic member derived from a mantle wedge altered by oceanic slab melts or fluids, and a felsic member derived from the partial melting of ancient crustal material. These new data, combined with previous studies of contemporaneous metavolcanic rocks in Gualanyu-Shangying area and surrounding areas, suggest that the magmatic precursors of these Neoarchean dioritic-trondhjemitic gneisses formed in a subduction-related tectonic environment at an E-W strike convergent plate margin. Furthermore, the Eastern Block of the North China Craton had a major lateral accretion of crustal growth in the north margin in the Neoarchean, whereas vertical crustal growth and recycling under plume tectonic backgrounds occurred in the interior of the Eastern Block.