Origin of the ca. 3.1 Ga Luanjiajie rock assemblage in the northeastern margin of the North China Craton: New constraints on the Mesoarchean geodynamic regime

Abstract

The North China Craton (NCC) is one of the oldest cratons on Earth and preserves abundant Mesoarchean igneous rocks. However, the petrogenesis of the Mesoarchean magmatism and the associated geodynamic regime in the NCC remain controversial. In this paper, we conducted a systematic study to constrain the petrogenesis of the recently discovered Mesoarchean Luanjiajie rock assemblage (LRA) and infer the regional Mesoarchean tectonic setting. The LRA is composed mainly of tonalite–trondhjemite–granodiorite (TTG) gneiss with minor dioritic gneiss and amphibolite. The amphibolite shows intrusive contact with the surrounding TTG gneiss, whereas the dioritic gneiss displays transitional boundary with the TTG gneiss. Zircon U-Pb dating revealed that these three lithologies were roughly contemporaneous, with crystallization and metamorphic ages of ca. 3.1 and ca. 2.5 Ga, respectively. Geochemical characteristics (e.g., high Sr/Y and LaN/YbN ratios and weak positive Eu anomalies), zircon Hf isotopic features (TDM2 ages of 3822 to 3531 Ma with εHf(t) values of −2.44 to +0.97), and slightly elevated δ18O values (+5.48‰ to +7.24‰) indicate that the ca. 3.1 Ga TTG was derived from the partial melting of ancient hydrated low-K mafic rocks. The Luanjiajie TTG gneiss and other reported Mesoarchean TTGs in the NCC are classified mainly as medium- to low-pressure types. The linear trends between SiO2 content and Fe2O3T, MgO, Y, Yb, Sm, and Gd contents of the ca. 3.1 Ga dioritic gneiss reflect that the dioritic gneiss might be a product related to amphibole fractionation of the coeval TTG magma. The ca. 3.1 Ga amphibolite displays OIB-affinity geochemistry and positive εHf(t) (up to +4.65) and εNd(t) (+0.2 to +7.7) values. The amphibolite was most likely formed through the partial melting of a depleted garnet-peridotite mantle and underwent minor crustal contamination during magma ascent. Hence, the LRA was most likely formed under a plume-related tectonic setting involving basaltic magma underplating and recycling of ancient crust. Combining our new results with existing zircon Hf data, we suggest that the NCC experienced initial continental nuclei formation during the Eo- to Paleoarchean, and followed by episodic crustal growth and recycling during the Meso- to Neoarchean.