Provenance of Middle to Late Triassic sedimentary rocks in the Zoige Depression in the NE part of the Songpan–Ganzi Flysch Basin: Petrography, heavy minerals, and zircon U–Pb geochronology

Abstract

The Zoige Depression is an important depocenter zone within the northeast of the Songpan–Ganzi Flysch Basin, which is bounded by the South China, North China, and Qiangtang blocks, and also forms the northeastern margin of Tibetan Plateau. This paper discusses the provenance of Middle to Late Triassic sedimentary rocks in the Zoige Depression using petrography, heavy mineral assemblages, and zircon U–Pb geochronology. The results demonstrate that the detritus is derived from multiple source regions. Four distinct parent rocks can be distinguished based on the heavy mineral assemblages and lithic fragments: Provenance 1 predominantly comprises intermediate–acidic volcanic rocks; Provenance 2 includes high‐grade metamorphic rocks; Provenance 3 contains a mixture of various detrital components; and Provenance 4 primarily consists of mafic volcanic rocks. The different U–Pb ages of the zircons from the Middle to Late Triassic ranging from 260 to 280, 429–480, 792–974, and 1,800–2,500 Ma represent distinct source regions, which are comparable to the 4 provenances mentioned above: Provenance 1 (260–280 Ma), Eastern Kunlun Orogen; Provenance 2 (429–480 and/or 1,800–2,500 Ma), Qinling orogeny (mainly in North Qinling); Provenance 3 (1,800–2,500 Ma), the North China Block; and Provenance 4 (792–974 Ma), the Yangtze Block. Overall, the detritus in the Middle Triassic (Ladinian, T2zg) primarily originates from the Eastern Kunlun Orogen and North Qinling. During the Late Triassic (Early Carnian, T3z), the southern margin of the North China Block was likely transported westward to the basin by a river network between the North China and South China blocks flowing through the Qinling region, because of the predominance of the detrital zircon age ranging from 1,800 to 2,500 Ma and the occurrence of quartz sandstone with visible enlargement texture. Since the Late Triassic (Middle Carnian, T3zh), great changes have occurred in the source terrains, such as the absence of sources of the Eastern Kunlun Orogen and North China Block and the predominance of Yangtze Block. This drastic change can be explained by the Triassic collision between the South China and North China blocks, and the clockwise rotation of the South China Block progressively closed the basin and uplifted the Qinling orogeny.