Ages and tectonic attributes of Late Palaeozoic stratigraphic units in the Tarbagatay, West Junggar (NW China): revisited

Abstract

ABSTRACT As the youngest single age of detrital zircons is used as the maximum depositional age, the ages and tectonic attributes of the Late Palaeozoic stratigraphic units in the Tarbagatay area, West Junggar, NW China, become very confusing. An improved statistical approach indicates that 5, 6 and 7 concordant zircons are required for the maximum depositional age at 95% confidence level when the quantity of total concordant zircon in a sample does not exceed 50, 140 and 432, respectively, but the defined maximum depositional age is valid only when it is compatible with geological facts. Otherwise, it is still uncertain if the age is valid. This approach is applied to resolve a chronological chaos in the Late Palaeozoic stratigraphic units in the Tarbagatay area. As a result, the maximum depositional ages obtained from the detrital zircon samples in literature and this study are at 95% confidence level, most of which are well compatible with the fossils in the units, intrusive relationships of plutons to country rocks and ages of interbedded volcanic rocks. The valid maximum depositional ages and other geological facts definitely show the presence of the Devonian, Early Carboniferous and Early Permian stratigraphic units in the Tarbagatay area. The Late Devonian-Early Carboniferous marine-dominated units occurred in the Zharma-Saur arc and its transition to a back-arc basin to the south during the southward subduction of the Irtysh-Zaisan Ocean, whereas the Permian terrestrial unit was formed in a post-collisional setting after the closure of the Irtysh-Zaisan Ocean in the Late Carboniferous.