Age and depositional setting of the Paleoproterozoic Gantaohe Group in Zanhuang Complex: Constraints from zircon U–Pb ages and Hf isotopes of sandstones and dacite

Abstract

The Gantaohe Group, located in the Zanhuang area, contains typical North China Craton Paleoproterozoic strata, and is one of the best-preserved examples of these rocks. It is therefore a significant area for deciphering evolutionary process in the trans-North China Orogen. In this paper, we present petrologic, detrital and volcanic zircon U–Pb ages and Lu–Hf isotopes from three formations of the Gantaohe Group. The zircons from a conglomerate at the base of the Gantaohe Group yielded a young population of ca. 2.1Ga. Dacite from the upper section of the Nansizhang Formation yielded zircon 207Pb/206Pb ages of 2090±14Ma. Thus, the initial deposition of the Gantaohe Group occurred at ∼2.1Ga. The Group underwent greenschist to low amphibolite facies metamorphism, identical to regional 1.87–1.85Ga metamorphism in Zanhuang Complex. Therefore, we propose that the Gantaohe Group was deposited in the period between 2.1Ga and 1.9Ga. Sandstones in the Gantaohe Group are dominated by Q (monocrystalline and polycrystalline quartz) and F (K-feldspar and plagioclase) with only minor lithic fragments, suggesting that detrital components were mainly derived from a cratonic source. Detrital zircons from sandstones exhibit a dominant population of 2.55–2.5Ga grains with minor contributions of ∼2.7Ga and ∼2.1Ga grains, which further indicate they were dominantly sourced from ∼2.5Ga TTG gneisses, late Archean Zanhuang and late Archean to Paleoproterozoic Guandu Groups, with subordinately input from ∼2.7Ga TTG gneisses and the Paleoproterozoic Xuting granite. Given that the sandstone clastic components chiefly comprise quartz and feldspar, there is a lack of volcanic lithics, and the major detrital zircons are 400Ma older than the initial deposition age, we propose that the Gantaohe Group was deposited in a rift-related setting far from any back-arc basin at ∼2.1Ga or 1880–1810Ma syn-collisional processes. A few 2.5–2.7Ga zircons have positive εHf(t) values identical with the contemporaneous depleted mantle and the 207Pb/206Pb ages are similar to their single stage model ages (TDM1), which suggests some degree of 2.5–2.7Ga crustal growth. Most ∼2.5 zircons, however, show 207Pb/206Pb ages that are much younger than the Hf single stage model ages, which indicates intensive crustal reworking in the late Neoarchean. The zircons collectively present two stage model ages ranging from 2.6 to 2.9Ga. Therefore, we suggest that the 2.6–2.9Ga periods represents a time of intensive crustal growth in the Zanhuang Complex.