Early Palaeozoic sedimentary record and provenance of flysch sequences in the Hovd Zone (western Mongolia): Implications for the geodynamic evolution of the Altai accretionary wedge system

摘要

Sedimentological and detrital zircon provenance study of the Lower Palaeozoic Zuunnuruu, Tsetseg and Sagsai sedimentary formations was carried out in the eastern part of the Floyd Zone in western Mongolia. Sedimentological analysis has revealed two distinct and consecutive types of sedimentary environments. The Lower Ordovician-earliest Silurian sediments had dominantly volcano-sedimentary character, interpreted as reflecting deposition in a proximal part of a Pacific-type accretionary wedge. The upper, mainly Devonian part of the profile has generally siliciclastic flysch-like nature and indicates platform-type depositional setting related to the synextensional thinning of the accretionary wedge-system. Detrital zircon age populations of all the three studied formations uniformly show a dominant Neoproterozoic-Ordovician age group at ca. 560-460 Ma, a broad Neo- to Mesoproterozoic peak at ca. 1050-720 Ma, several minor Meso- to Palaeoproterozoic age clusters at ca. 1.4, 1.9 and 2.4 Ga and ca. 400-360 Ma peaks in the youngest Devonian formation. The early Palaeozoic part of the age spectra is interpreted as detritus mainly derived from the magmatic rocks of the Cambrian-Ordovician Ikh-Mongol Arc System within the nearby Lake Zone and the youngest ages from the neighbouring Devonian granites. The other sources were identified as more distal Tonian magmatic-arc complexes, Rodinia break-up-related volcanic rocks and basement of the Precambrian Zavkhan and Baidrag continental blocks even further east. The maximum sedimentary ages, determined by the youngest detrital zircons, shift the end of deposition of the Sagsai Formation at least to the latest Devonian. Nearly identical detrital zircon age spectra from Lower Palaeozoic sequences of the Hovd Zone and other parts of the Altai belt support an existence of a single giant accretionary complex developed along the entire outer margin of the Ikh-Mongol Arc System. The change in the sedimentary style suggests the latest Ord