Significant Carboniferous magmatism and continental growth in the northern West Tianshan orogen, NW China: Revealed by detrital zircon U-Pb and Lu-Hf analyses for turbidites from the North Tianshan Accretionary Complex

Abstract

We conducted an integrated study including U-Pb dating and Lu-Hf isotope analysis for detrital zircons from the turbidites of the North Tianshan Accretionary Complex, northern Chinese West Tianshan, to constrain their depositional age, source characteristics and tectonic setting. Almost all the detrital zircons display well developed oscillatory zoning textures and have high Th/U ratios, reflecting an igneous origin. Based on the analytical results, the depositional ages of the four samples were 303 Ma–312 Ma, 310 Ma and 321 Ma. The studied samples all display unimodal detrital zircon spectra, with few Precambrian zircons, coinciding with the age spectra of sediments deposited in convergent basins, most likely a forearc basin. Most of the zircons and other mineral grains have euhedral to subhedral shapes, reflecting a relatively short transportation distance. Considering the occurrence of volcanic lithic fragments in thin-sections of the sandstones, we propose that the northern edge of Yili Block, which is widely accepted as a Paleozoic continental arc formed by subduction of the North Tianshan Ocean, was the most likely source area for the North Tianshan turbidites. There are 137 Lu-Hf isotope analyses on zircons with concordant U-Pb ages, all showing positive εHf(t) values (εHf(t) = +2.9–+15.8), and young TCDM ages (328–1152 Ma, peak at 400–800 Ma). The Hf isotope characteristics indicate important growth of juvenile crust and minor crustal contamination during the formation of the zircons’ host rocks. When we examine the Hf isotope of reported magmatic rocks from the northern Yili Block, we found an increasing trend of εHf(t) values from the Early Paleozoic to Late Paleozoic. This indicates that there was a large amount of juvenile material extracted from the depleted mantle in the Carboniferous. Taking into account of previous research, we conclude that the northern West Tianshan was a subduction-arc system during the Carboniferous, and the studied turbidites from the North Tianshan Accretionary Complex were sourced from the magmatic arc of the northern Yili Block and most likely deposited in a fore-arc basin in the Late Carboniferous.