Zircon U–Pb ages and geochemistry of the late Archaean granitoids in the Zanhuang Complex: Records of an arc–continent collision event at the end of Archaean

Abstract

The late Archaean witnessed a critical transition of the composition of the continental crust from predominantly sodic tonalite‐trondhjemite‐granodiorites (TTGs) to a combination of TTGs and increasing proportion of potassic calc‐alkaline granitoids. This shift is also well recorded in the Zanhuang Complex located in the central part of the North China Craton (NCC). In this work, we present a detailed geochemical study of these granitoids and subdivide them into four groups, that is, TTGs, sanukitoids, crustal‐sourced granitoids, and hybrid granitoids. The TTGs in the Zanhuang Complex may have formed at a longer and earlier period during ~2.7 to ~2.5 Ga as the first stage. They show typical TTG characteristics, such as high SiO2, Na2O, and Sr/Y and depletion of Nb, Ta, Ti, and HREE. The TTGs are interpreted to have formed in a subduction‐related environment. The other three groups of granitoids formed at a shorter and later stage. The sanukitoid sample shows high MgO and Mg# and other geochemical signatures of typical Archaean sanukitoid affinity, yielding a weighted mean zircon 207Pb/206Pb age of 2,517 ± 5 Ma as the crystallization age. The sanukitoid magma derived from partial melting of a hydrous mantle peridotite source metasomatized by slab‐derived melts or fluids. The crustal‐sourced granitoids, including a new sample (~2,510 Ma) in this work, the Jiandeng granite (~2,490 Ma), and the Huangcha/Wangjiazhuang granite (2,488 to 2,517 Ma) show signatures of a mainly crustal source such as high SiO2 and K2O, low MgO and Mg# values, and weakly peraluminous. The hybrid granitoids represented by the Haozhuang granitoid (2,511 to 2,528 Ma) are characterized by enrichment in both incompatible (LILE and LREE) and compatible (Mg, Ni, and Cr) elements and may have formed by mixing/contamination of multiple magmas or sources. The temporal and spatial distributions of these groups of granitoids indicate that the Zanhuang Complex experienced a subduction–collision event between the Fuping arc and the Eastern Block of the NCC at the end of Archaean.