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
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. 2.1 Ga. Dacite from the upper section of the Nansizhang Formation yielded zircon 207Pb/206Pb ages of 2090 ± 14 Ma. Thus, the initial deposition of the Gantaohe Group occurred at ∼2.1 Ga. The Group underwent greenschist to low amphibolite facies metamorphism, identical to regional 1.87-1.85 Ga metamorphism in Zanhuang Complex. Therefore, we propose that the Gantaohe Group was deposited in the period between 2.1 Ga and 1.9 Ga. 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 2.55-2.5 Ga grains with minor contributions of ∼2.7 Ga and ∼2.1 Ga grains, which further indicate they were dominantly sourced from ∼2.5 Ga TTG gneisses, late Archean Zanhuang and late Archean to Paleoproterozoic Guandu Groups, with subordinately input from ∼2.7 Ga 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 400 Ma 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 ∼2.1 Ga or 1880-1810 Ma syn-collisional processes. A few 2.5-2.7 Ga 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 2.5-2.7 Ga 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 2.9 Ga. Therefore, we suggest that the 2.6-2.9 Ga periods represents a time of intensive crustal growth in the Zanhuang Complex.