Continuous denudation and pediplanation of the Chinese Western Tianshan orogen during Triassic to Middle Jurassic: Integrated evidence from detrital zircon age and heavy mineral chemical data

Abstract

The Mesozoic tectonic framework of the Chinese Tianshan orogen is ambiguous. However, thick Mesozoic sediments both inside and in front of the orogen can provide clues to solve this problem. LA-ICPMS U–Pb dating on detrital zircons and electron microprobe analysis on detrital heavy minerals are carried out on the Triassic to Middle Jurassic sediments along the Kuqa River section in the northern Tarim basin and in two intermountain basins (Bayanbulak basin and Gongnaisi valley), to reveal the change in provenance through time and space. Zircons from the Kuqa River section are mainly of magmatic origin and their ages have a wide range from 2800 to 234Ma, 70.5% of which are Palaeozoic. This age signature indicates a main provenance from the Tianshan orogen. Detrital heavy minerals (garnet and tourmaline) from the Kuqa River section suggest a complex provenance from granitic and metamorphic rocks of the South Tianshan belt, the South Tianshan suture and the Yili-Central Tianshan block. Detrital zircon U–Pb ages of the Early-Middle Jurassic sandstones from the Gongnaisi valley and Bayanbulak basin are predominantly Palaeozoic, without Precambrian, indicating a simple source from the Yili-Central Tianshan block. The different features of detrital zircon ages and heavy mineral chemical compositions in individual sample reflect provenance changes due to continuous denudation and pediplanation of the Chinese West Tianshan orogen in an extensional setting during Triassic to Middle Jurassic: (1) during Early to Middle Triassic, the South Tianshan belt towered to the north of Tarim basin, acting as a watershed and being the main source area for northern Tarim basin; (2) since Late Triassic to Early Jurassic, material from southern Yili-Central Tianshan block was largely transported to Tarim basin because of the continuous denudation of the South Tianshan belt; (3) in Middle Jurassic, the Chinese Western Tianshan orogen reached a peneplain state, resulting in the direct deposition in intermountain basins and the decrease of sediment supplying from Yili-Central Tianshan block to Tarim.