Geochemistry and geochronology of the TTG-sanukitoid suite in the Zhulagou area: Constraints on the Neoarchean crustal evolution of the western North China Craton

Abstract

The Archean basement in the Yinshan Block of the western North China Craton consists of a variety rock of assemblage, including TTG (tonalite-trondhjemite-granodiorite) rocks and some dioritic gneiss (sanukitoid). Understanding the petrogenesis of these rocks is crucial for comprehending the late Neoarchean crustal evolution of the western North China Craton. This study provides a new geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic data for the TTG-sanukitoid suit that was newly identified in the Zhulagou area. The late Neoarchean TTG gneisses were emplaced around 2.54–2.50 Ga, while dioritic gneiss was emplaced between 2.53 and 2.50 Ga. The TTG gneisses exhibit high SiO2 (64.46–72.24 wt%) and Al2O3 (13.37–16.48 wt%) contents, relatively elevated (La/Yb)N and Sr/Y ratios, as well as low Y and Yb contents. They display variable εHf(t) (+0.94 to +3.57) and εNd(t) (−1.02 to +0.82) values, with relatively low Pb isotopic compositions, suggesting that they were derived from the partial melting of a thickened mafic lower crust. On the other hand, dioritic gneiss shows moderate SiO2 (58.05–61.87 wt%) content, high MgO (2.62–4.06 wt%), Al2O3 (14.42–18.44 wt%) and Cr contents, but relatively low (La/Yb)N values, showing geochemical affinity to the Archean sanukitoids. The dioritic gneiss exhibits variable εHf(t) values (−2.29 to +5.56) and εNd(t) (−2.05 to +2.02), with relatively low Pb isotopic compositions. Furthermore, the presence of amphibolite xenoliths, along with the high Mg# contents (36.84–53.49), indicating the addition of mantle-derived components to its source region. Therefore, the formation of the dioritic gneiss is primarily attributed to partial melting of the thickened (mafic) lower crust and the addition of mantle material. By combining previous studies with the new data presented in this study, we propose that a late Neoarchean tectonic regime involving a mantle plume is the most favorable explanation for the two major magmatic events in the Yinshan Block. Firstly, the earlier pulse/stage of mantle plume resulted in the ∼2.7 Ga thickened (mafic) lower crust and a limited amount of TTG rocks. Subsequent pulse/stage of mantle plume triggered partial melting of pre-existing thickened (mafic) lower crust and generated 2.54–2.50 Ga TTG rocks and dioritic gneiss.