CRYOGENIAN TO EARLY CAMBRIAN EVOLUTION OF THE PHOSPHORITE-BEARING KHOVSGOL BASIN, MONGOLIA

Strata of the Neoproterozoic-early Cambrian Khovsgol Group (Khovsgol Aimag, Mongolia) record evidence of major climatic, geochemical, and paleobiological changes near the end of the Proterozoic. Along with diamictites associated with the Sturtian and Marinoan snowball earth glaciations, the Khovsgol Group contains the Khesen phosphorite, which, in addition to being one of the largest ore-grade phosphorite deposits in the world, contains a Doushantuo-Pertatanka-type microfossil assemblage. Previous workers have assigned Ediacaran ages to the Khesen phosphorite, with some authors suggesting a genetic link between the Marinoan glaciation and phosphogenesis.

We incorporate constraints from new geologic mapping, chemostratigraphy, and geochronology into both a new age model for the emplacement of Khovsgol Group strata as well a general tectonic model for the creation and evolution of the Khovsgol Basin in the late Neoproterozoic and Early Cambrian. Importantly, our age model suggests that the fossiliferous Khesen phosphorite is of basal Cambrian (Nemakit-Daldynian) age, separated from Cryogenian and Ediacaran strata by a major depositional hiatus. This result confirms the global diachroneity of major Ediacaran-Cambrian phosphorite deposition, and represents the youngest known occurrence of Doushantuo-Pertatanka-type microfossils in the stratigraphic record. While the Tonian and Cryogenian strata of the Khovsgol Group were likely deposited along a rifted passive margin, we suggest that the upper Khovsgol Group strata were deposited into a forearc environment associated with the incipient collision of the Agardag Arc along the western margin of the Tuva-Mongolia microcontinent. Furthermore, emplacement of the Khesen phosphorite was closely associated with an episode of forebulge migration, during which primary authigenic phosphatic material emplaced on local paleotopographic highs was redistributed as granular allochems in regionally extensive allodapic carbonates. Finally, our basinal model suggests strong similarities in the tectonic evolution of the Khovsgol and Zavkhan paleobasins, lending insights into the paleogeography and accretionary history of the Mongolian core of the Central Asian Orogenic Belt.