Detrital and igneous zircon ages for supracrustal rocks of the Kyrgyz Tianshan and palaeogeographic implications

Abstract

We report detrital zircon ages for Precambrian and early Palaeozoic metasediments from the Tianshan orogen in Kyrgyzstan, an important component of the Central Asian Orogenic Belt and compare these with published ages from the Chinese Tianshan and the Tarim craton. These data provide information on possible source terrainsand suggestthatPrecambrianbasementiswidespreadintheTianshanandmay,atleastinpart,representcontinental fragments rifted off the Tarim craton during the early history of the Central Asian Orogenic Belt. Distinct differ- ences in the Precambrian zircon age distribution between the North and Middle Tianshan of Kyrgyzstan support earlier ideas that these two terranes had different crustal histories prior to their amalgamation in the early Palaeozoic. We envisage an archipelago-type scenario for the Palaeo-Asian Ocean south of the Siberian craton in the late Neoproterozoic to early Palaeozoic in which numerous island arcs and Precambrian crustal fragments derivedfromTarimdriftednorthwards (inpresent coordinates) and wereamalgamatedand tectonicallystacked togetherduringseveraloceanclosureandaccretion–collisionevents.Thisissurprisinglysimilartowhathasbeen envisaged for the evolution of Indonesia where Mesozoic rifting of fragments from the Australian margin was followed by Cretaceous collisions and Cenozoic collision of Australia with the SE Asian margin. In both Central Asia and Indonesia continental crust has arrived in the region in multiple episodes and has been fragmented and juxtaposed by subduction-related processes. Continental growth during this process was minimal.