SEDIMENTOLOGY, STRATIGRAPHY AND DETRITAL ZIRCON GEOCHRONOLOGY OF MESOPROTEROZOIC STRATA IN NORTHERN HELAN MOUNTAINS, CHINA

Mesoproterozoic rocks of the western margin of the North China Block have received little attention despite the presence of exceptional exposures of carbonate strata that record a surprisingly low level of deformation relative to most rocks of this age worldwide. We reconstruct the early tectonic and depositional history of the western part of the North China Block through a detailed sedimentological, geochronological, chemostratigraphic, and paleoenvironment analysis for the Huangqikou and Wangquankou formations in the northern Helan Mountains. Red beds and overlying white quartz arenite of the Huangqikou Formation represents terrestrial and shallow marine deposits, respectively, with the former representing alluvial fan to fluvial environments, possibly in a rift setting. Overlying stromatolite-bearing dolostone of the Wangquankou Formation records shallow marine deposits of a transgressive systems tract with numerous m-scale parasequences. Stromatolitic beds record various morphologies, including common planar, domal, columnar forms, as well as unique sharp-peaked forms. δ¹³C data from the Wangquankou Formation average ~0‰ and show approximately ±1‰ variations with δ¹⁸O data ranging from -6 to -10‰. Potential chemostratigraphic correlations with other Mesoproterozoic sections are difficult to establish due to a lack of precise age constraints, but show significant short-term variability that hold great potential for regional correlation to other sections in the North China Block. Four detrital zircon U–Pb age spectra from the Mesoproterozoic strata and one from overlying Cambrian rocks all contain 1760–1820 Ma and 2400–2510 Ma peaks. Along with published data, these suggest no significant changes in provenance in this region from the Mesoproterozoic to Middle Ordovician.