Composition, Provenance, and Tectonic Setting of the Southern Kangurtag Accretionary Complex in the Eastern Tianshan, NW China: Implications for the Late Paleozoic Evolution of the North Tianshan Ocean

Abstract

The Kangurtag belt in the Eastern Tianshan, connecting the Dananhu Arc with the Central Tianshan Arc, contains diagnostic rocks of accretionary origin, and thus provides key information about the evolution of the North Tianshan Ocean. The Southern Kangurtag belt is composed of two types of mélange. Type I mélange consists of Enriched Mid-Ocean Ridge Basalt-type pillow basalts, draped by biohermal limestones and carbonate-siliceous sediments of a slope facies, and siliceous argillites from a hemipelagic-pelagic environment that together make up a seamount assemblage. In Type II mélange, Normal Mid-Ocean Ridge Basalt and ribbon cherts were dismembered and entrained in a clastic matrix, showing a “block-in-matrix” structure. Detrital zircons of four sandstones from Devonian and Carboniferous strata within the mélanges have a predominant age population of 410–430 Ma and a distinct Proterozoic cluster around 1.4–1.6 Ga. The εHf(t) values of Phanerozoic zircons range from −25.1 to +8.6. Such age patterns, typical of the Central Tianshan Arc, and the Hf isotopic data indicate that these sedimentary successions were deposited on the northern margin of the Central Tianshan Arc. The youngest detrital zircon age of 317 Ma provides an upper limit for the time of formation of the Southern Kangurtag accretionary complex. Therefore, we suggest that the Southern Kangurtag belt comprises an accretionary complex that developed during southward subduction of the North Tianshan Ocean beneath the Central Tianshan Arc. This subduction began in the Early Ordovician and may have lasted until the Late Carboniferous–Permian.