Sedimentology, stratigraphy, and detrital zircon geochronology of Mesoproterozoic strata in the northern Helan Mountains, western margin of the North China Block

Abstract

We reconstruct the early tectonic and depositional history of the western part of the North China Block (NCB) through a detailed sedimentological, chemostratigraphic, geochronological, and paleoenvironmental analysis of Mesoproterozoic strata of the northern Helan Mountains, northwest China. Siliciclastic strata of the Huangqikou Formation represent terrestrial and shallow marine deposits. Shallow marine dolostone of the overlying Wangquankou Formation comprise a transgressive systems tract with a spectacular variety of stromatolite morphologies, including unique sharp-peaked forms, common in Mesoproterozoic strata. The significant spatial distribution of approximately age-equivalent strata of the Wangquankou carbonate in the NCB argues against the interpretation that they represent aulacogen deposits. Mesoproterozoic strata in the field area are capped by the Cambrian Maozhuang Formation. We report a new trilobite fauna from this formation, including Solenoparia (Plesisolenoparia) ruichengensis, a diagnostic fossil of the Oryctocephalus salteri zone, Wuliuan Stage, Miaolingian Series. This constrains the upper limit of the hiatus associated with the Mesoproterozoic–Cambrian unconformity and the timing of the early Paleozoic marine transgression in this area. Carbonate δ13C data for the Wangquankou Formation vary from −1.4 to 0.9‰. Covariance of carbon and oxygen isotope data from 30 to 49 m suggests variable diagenetic alteration during recrystallization, likely influenced by meteoric fluids. Isotope data for the rest of the section show low-amplitude variations, consistent with low variability in Mesoproterozoic strata worldwide. Detrital zircon UPb age spectra for the Mesoproterozoic and Cambrian strata show 1760–1820 Ma and 2400–2510 Ma peaks, which indicate local derivation from crystalline basement rocks in this region of the NCB. There are no reliable ages for Mesoproterozoic strata of the western margin of NCB, but our zircon data provide a maximum depositional age of ~1610 Ma for the Wangquankou Formation. Geochronologic data also show a consistent provenance throughout the Mesoproterozoic section, and for both Middle Cambrian and Middle Ordovician strata, which could suggest a remarkable paucity of major igneous events in this western part of the NCB over a considerable time span (approximately 1 billion years).