Petrogenesis and tectonic implications of the ca. 2.5 Ga granitoid assemblage in the eastern Zhongtiao region, southern North China Craton

Abstract

The key to unraveling the crustal evolution and extensive magmatism of the North China Craton (NCC) at the Archean–Proterozoic boundary (ca. 2.5 Ga) is to investigate the petrogenesis of the volumetrically dominant granitoids. In this contribution, we report new petrological, mineralogical, and geochemical data for the Huping and Luojiahe trondhjemites, which are ca. 2.5 Ga granitoid assemblages from the eastern Zhongtiao region of the southern NCC. The Huping trondhjemites have high SiO2 and Na2O contents and high (La/Yb)N and Sr/Y values, which are consistent with most Tonalite–trondhjemite–granodiorite (TTG) rocks worldwide. Evolved whole-rock Nd and Hf isotopic compositions indicate that the Huping trondhjemites were derived from partial melting of preexisting ca. 2.7 Ga mafic crust, and high MgO, Cr, and Ni contents suggest the possible involvement of depleted mantle in their source region. Integrated petrological, mineralogical, and geochemical data indicate that the Huping trondhjemites were likely formed by partial melting of delaminated lower crust. In contrast, the Luojiahe trondhjemites are Na- and Si-rich with negative Eu and Sr anomalies. Their geochemical characteristics suggest that they were likely formed by partial melting of low-K mafic crust that was shallower than the source of the Huping trondhjemites. The higher LREE content of allanite from the Luojiahe trondhjemites relative to that from the Huping trondhjemites indicates the Luojiahe trondhjemites formed at a higher melting temperature. The occurrence of the two types of coeval granitoids indicates that ca. 2.5 Ga crustal anatexis occurred at different depths. Furthermore, a strong heat flux at the base of the southern NCC at ca. 2.5 Ga is evident, which might have been introduced by mafic magma underplating. Based on our new data and the records of coeval magmatism in the region, this study suggests that lithospheric delamination and large-scale mantle-derived magma underplating beneath the thickened crust could have played important roles in the extensive magmatism of the southern NCC at the Archean–Proterozoic boundary.