Provenance of detrital zircons in the Late Triassic Sichuan foreland basin: constraints on the evolution of the Qinling Orogen and Longmen Shan thrust-fold belt in central China

In this article, we present <i>in situ</i> U–Pb and Lu–Hf isotope data for Upper Triassic detritus in the Sichuan region of northwestern South China, which was a foreland basin during the Late Triassic. The aim is to determine the provenance of sediments in the foreland basin and to constrain the evolution of the surrounding mountain belts. U–Pb age data for the Late Triassic detrital zircons generally show populations at 2.4–2.6 Ga, 1.7–1.9 Ga, 710–860 Ma, 410–460 Ma, and 210–300 Ma. By fitting the zircon data into the tectonic, sedimentologic, and palaeographic framework, we propose that the north Yangtze Block and South Qinling–Dabie Orogen were the important source areas of sediments in the northern part of the foreland basin, whereas the Longmen Shan thrust-fold belt was the main source region for detritus in other parts of the foreland basin. The South Qinling–Dabie Orogen could also have served as a physical barrier to block most detritus shed from the southern North China Block into the foreland basin during the sedimentation of the Xujiahe Formation. Our results also reveal that part of the flysch from the eastern margin of the Songpan–Ganzi region had been displaced into the Longmen Shan thrust-fold belt before the deposition of the foreland basin sediments. In addition, the Lu-Hf data indicate that Phanerozoic igneous rocks in central China show insignificant formation of the juvenile crust.

本文报道了华南西北部四川地区晚三叠世碎屑沉积物的原位（in situ）U-Pb与Lu-Hf同位素数据，该区域在晚三叠世时期为前陆盆地（foreland basin）。本研究旨在明确该前陆盆地沉积物的物源，并约束周边造山带的演化过程。晚三叠世碎屑锆石的U-Pb年龄数据普遍呈现2.4~2.6 Ga、1.7~1.9 Ga、710~860 Ma、410~460 Ma以及210~300 Ma的年龄组分。通过将锆石数据与构造、沉积学及古地理框架相结合，本文提出扬子北缘与南秦岭-大别造山带（South Qinling–Dabie Orogen）是该前陆盆地北部沉积物的重要物源区，而龙门山冲断褶皱带（Longmen Shan thrust-fold belt）则是前陆盆地其余区域碎屑物的主要物源区。南秦岭-大别造山带还可作为物理屏障，阻止绝大多数源自华北地块（North China Block）南部的碎屑物进入须家河组（Xujiahe Formation）沉积时期的前陆盆地。本研究结果还显示，松潘-甘孜地块（Songpan–Ganzi region）东缘的部分复理石沉积物，在该前陆盆地沉积前就已经位移至龙门山冲断褶皱带内。此外，Lu-Hf同位素数据表明，中国中部的显生宙火成岩仅形成了极少量的新生地壳（juvenile crust）。