How was the Triassic Songpan‐Ganzi basin filled? A provenance study

Abstract

The Triassic Songpan‐Ganzi complex comprises >200,000 km2of 5–15 km thick turbiditic sediments. Although surrounded by several magmatic and orogenic belts, the Triassic high‐ and ultrahigh‐pressure Qinling‐Tongbai‐Hong'an‐Dabie (QTHD) orogen, located several hundred kilometers to the east, was proposed as its major source. Middle to Late Triassic samples from the northern and southern Songpan‐Ganzi complex, studied using detrital white mica40Ar/39Ar ages, Si‐in‐white mica content, and detrital zircon U/Pb ages, suggest that the northern Songpan‐Ganzi deposystem obtained detritus from the north: the north China block, east Kunlun, northern Qaidam, Qilian, and western Qinling; the southern Songpan‐Ganzi deposystem was supplied from the northeasterly located Paleozoic QTHD area throughout the Ladinian and received detritus from the Triassic Hong'an‐Dabie orogen during the Carnian, indicative of exhumation of the orogen at that time. The QTHD orogen fed the Norian samples in the southeastern southern Songpan‐Ganzi deposystem, signifying long drainage channels along the western margin of the south China block. An additional supply from the Emeishan magmatic province and/or the Yidun arc is suggested by the paucity of white mica in the southern Songpan‐Ganzi deposystem. Mica ages of Rhaetian sediments from the northwestern Sichuan basin best correlate with those of the Triassic QTHD orogen. Our Si‐in‐white mica data demonstrate that the high‐ and ultrahigh‐pressure rocks of the Hong'an–Dabie Shan were not exposed in the Middle to Late Triassic.