Carboniferous tectonic incorporation of a Devonian seamount and oceanic crust into the South Tianshan accretionary orogen in the southern Altaids

Abstract

The southern Altaids is of critical significance for a better understanding of the orogenic architecture and continental growth in Central Asia. The tectonic setting of the South Tianshan, a typical Paleozoic accretionary orogen within the southern Altaids, is controversial. This study reports new structural relations, geochemistry, and geochronology of previously assigned Late Silurian–Early Devonian strata in the Karaghol area of the South Tianshan. The constituent rocks are pillow basalts, ribbon cherts, limestones, siliceous mudstones, and turbiditic sandstones. Field relations show that these rocks are characterized by fault-bound tectonic slices and mélanges with typical block-in-matrix structures. The pillow basalts show N-MORB, E-MORB, and OIB geochemical fingerprints, and our new LA-ICP-MS zircon U–Pb dates indicate that the maximum eruption age of the OIB-type basalts was ca. 386 Ma, whereas the Maximum Depositional Ages (MDAs) of the turbiditic sandstones were ca. 369 Ma and 355 Ma. The age spectra of the detrital zircons, which define prominent peaks at ca. 370 Ma, ca. 420 Ma, ca. 770 Ma, ca. 960 Ma with subordinate Proterozoic ages, are a good match with the igneous ages of intermediate to felsic magmatism in the Yili–Central Tianshan arc. We conclude that the previously assigned Late Silurian–Early Devonian strata in the Karaghol area are mostly dismembered structural components and mélanges that belonged to an accretionary complex of Late Devonian to Early Carboniferous age, into which fragments of a Late Devonian seamount along with South Tianshan oceanic crustal rocks were likely emplaced during north-dipping subduction in the Early Carboniferous. This well-documented accretionary orogen in the Chinese South Tianshan is a key constraint on the accretionary orogenesis of the southern Altaids.