Depositional age and provenance of the Wutai Group: Evidence from zircon U-Pb and Lu-Hf isotopes and whole-rock geochemistry

The Hengshan-Wutai-Fuping area is situated in the middle segment of the Trans-North China Orogen and consists of the upper amphibolite- to granulite-facies Hengshan and Fuping Complexes and the intervening greenschist- to lower amphibolite-facies Wutai Complex. The Wutai Complex is composed of Neoarchean to Paleoproterozoic granitoids and low-grade metamorphosed supracrustal rocks, of which the latter are further divided into the Wutai and Hutuo Groups. Geochemistry of the meta-sedimentary samples from the Wutai Group indicates weak to moderate source weathering, dominantly compositional variation trend and major felsic with few mafic source rocks. On the tectonic discrimination plots, most of the samples display continental island arc and active continental margin features. U-Pb ages of detrital and xenocrystic zircons from the Wutai Group yield two major age populations of 2565-2465 Ma and 2729-2676 Ma and a subordinate age population of 2372-2331 Ma. The major ∼2522 Ma and ∼2699 Ma zircons were most likely derived from the Wutai meta-volcanic rocks. The minor ∼2348 Ma zircons are only found from the upper Gaofan Subgroup and most likely sourced from the 2360-2330 Ma orthogneisses in the Hengshan Complex. The maximum depositional ages of 2535-2516 Ma from the meta-sedimentary rocks are consistent with the eruption ages of 2548-2513 Ma from meta-volcanic rocks in the lower Shizui and middle Taihuai Subgroups, indicating coeval sedimentation and volcanism in a fore-arc or intra-arc basin. This favors a tectonically juxtaposed contact, rather than a layer-cake model. However, the maximum depositional age of 2348 Ma and the 2176-2161 Ma felsic intrusions suggest a much younger formation age for the Gaofan Subgroup.