Late Neoarchean crust–mantle interaction and tectonic implications in western Shandong Province, North China Craton: Evidence from a granodiorite pluton and associated magmatic enclaves

Abstract

This study presents original data on the microstructure, mineralogy, whole–rock geochemistry, and isotopes of a Late Neoarchean granodiorite pluton and associated mafic microgranular enclaves (MMEs) in western Shandong Province of the eastern part of the North China Craton (NCC). The MMEs hosted in the granitic pluton are more mafic, and amphibole and biotite are the predominant mafic minerals. The sharp contacts and fine–grained textures of the MMEs indicated a rapid quenching process during a magma mingling event. Zircon and titanite UPb dating indicated that the host granodiorite pluton formed at circa 2546 Ma, and the MMEs formed at approximately 2538 Ma. The dating results indicated a crust–mantle interaction. Geochemically, the host pluton has high silica contents (SiO2 = 67.68–68.51 wt%), Mg# values (51–53) and low Aluminum Saturation Index ratios (A/CNK = 0.92–0.95), which are indicative of I–type granitic affinity. Moreover, they show arc–type trace element features characterized by enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) (e.g., Cs, Rb, K, and Pb), but depletion in high field strength elements (HFSEs) (e.g., Nb, Ta, P, and Ti). In contrast, the MMEs have lower silica contents (SiO2 = 52.07–56.03 wt%), but higher Mg# values (56–68) and Cr, Ni and Cu contents of 454–633, 159–225 and 98.3–207 ppm, respectively, corresponding to a mantle component. Similar to the host pluton, the MMEs show enrichment of LREEs and LILEs, but depletion of HFSEs, reflecting an arc–type magma origin. Isotopically, the host granodiorite and MMEs both have relatively depleted NdHf isotopic compositions, with εNd(t) values of −0.67–1.22 and − 0.31–1.89, and εHf(t) values of 5.63–2.03 and − 0.18–4.48, respectively. Zircon oxygen isotopes reveal that the host granodiorite and MMEs have slightly higher oxygen isotopes than the normal mantle zircon (5.3‰), with mean δ18O values of 6.35‰ and 6.81‰, respectively, indicating assimilation of minor supracrustal materials in the magma source. However, a combination of Nd–Hf–O isotopes suggests that the MMEs haveslightly enriched isotopic compositions than the host granodiorite pluton, suggesting a “reversed isotope” feature. This important finding also enlightens that “reversed isotope” phenomenon represented magma mingling between isotopically relatively enriched mafic and relatively depleted felsic magmas. Furthermore, the Ce4+/Ce3+ ratios in zircon from the host granodiorite and MMEs indicated a low oxygen fugacity, with mean values of 24.25 and 39.69, respectively. Although the host granodiorite and MMEs have the similar oxygen fugacity ranges, the MMEs have high Cu concentrations of 98.3–207 ppm (mean value = 151 ppm), indicating a Cu–rich magma source. Integrating the petrogenesis of the Late Neoarchean granodiorite pluton and associated MMEs, it is inferred that the host granodiorite pluton was derived from partial melting of mafic lower crust with minor insignificant ancient materials, and the MMEs were derived from partial melting of depleted mantle metasomatized by sulfur–rich sediment melts/fluids (chalcophile element). Tectonically, our samples reveal an affinity with an arc in a syn–collisional setting. Together with regional rock assemblages, we suggest that the onset of plate tectonics (subduction) was to occur by the end of Neoarchean in the NCC.