Zircon U-Pb ages and Lu-Hf isotope compositions from clastic rocks in the Hutuo Group: Further constraints on Paleoproterozoic tectonic evolution of the Trans-North China Orogen
The Hutuo Group, as one of the classic examples of Paleoproterozoic strata, plays an important role in establishing evolutionary processes in the Trans-North China Craton. In this contribution, we present petrologic, detrital zircon U-Pb ages and Lu-Hf isotopes from three subgroups of the Hutuo Group. Sandstones in the Doucun and Dongye Subgroups are dominated by Q (monocrystalline and polycrystalline quartz) and F (K-feldspar and plagioclase) with minor lithic fragments, suggesting that the detrital components were mainly derived from both the continental block and recycled orogen. In contrast, clastic components in the Guojiazhai Subgroup are mainly Q and lithic fragments, indicating they were derived predominantly from the recycled orogen. The ages of detrital zircons from sandstones in the Doucun and Dongye Subgroups are mainly concentrated at ca. 2.5 Ga and 2.2-2.1 Ga with minor 2.7 Ga zircons also present which indicates they were dominantly sourced from 2.5 Ga Wutai, Fuping and Zanhuang Complexes, and Paleoproterozoic intrusives. The Guojiazhai Subgroup displays a different zircon age population of ca.2.4 Ga, 2.2-2.1 Ga and 2.0-1.9 Ga, which indicates likely derivation from Paleoproterozoic intrusives in the Wutai, Lüliang and Hengshan areas. Based on the observation that sandstone clastic components in the Doucun and Dongye Subgroups are dominantly quartz, feldspar and sedimentary lithic fragments, but not volcanic lithics, we propose that they were deposited in a rift-related setting. Zircons from the lower sequence of the Hutuo Group yielded a young population of 2140 Ma. Considering the volcanic rocks of 2140 ± 14 Ma at the base of the group and 2.2-2.0 Ga magmatism along the TNCO, we propose that Doucun and Dongye Subgroups formed at 2.2-2.0 Ga. The youngest, ca. 1.9 Ga, zircons in the Guojiazhai Subgroup indicate this Group was deposited during closure of the rift at 1.9-1.8 Ga. The two stage model ages of the detrital zircons mainly range from 2.6 to 2.9 Ga with a minor ∼2.5 Ga contribution. Therefore, we infer that 2.6-2.9 Ga represents a period of intensive crustal growth in the Trans-North China Orogen, but that some degree of crustal growth continued to ∼2.5 Ga.