Zircon U–Pb age constraints on the provenance of Upper Oligocene to Upper Miocene sandstones in the western Qiongdongnan Basin, South China Sea

Abstract The provenance of Upper Oligocene to Upper Miocene sandstones in the western Qiongdongnan Basin was studied by zircon U–Pb ages. More than 200 zircon grains were separated from the sandstone of the Upper Oligocene Lingshui, Lower Miocene Sanya, and Upper Miocene Huangliu formations, and spot analysed for U–Pb dating. Most zircon grains from the Upper Oligocene Lingshui and Upper Miocene Huangliu formations had rounded corners, suggesting that the grains were transported a relatively long distance before deposition, while grains from the Lower Miocene Sanya Formation were usually angular in shape, implying a shorter transport distance from their provenance. The Lingshui Formation contained exclusively Cenozoic, Mesozoic, Palaeozoic, and Proterozoic zircon ages, and the age spectrum showed two major peaks at ca. 247 and 441 Ma, along with nine subordinate age peaks at ca. 35, 305, 517, 623, 811, 852, 911, 1158, and 1905 Ma. This indicates that Upper Oligocene sediments in the northwestern area of the Qiongdongnan Basin may have originated from the Red River system, which had four major peaks at 254, 418, 751, and 1848 Ma, and five subordinate peaks at ca. 32, 81, 514, 606, and 967 Ma. Results suggest that sediments from the Red River system entered the Qiongdongnan Basin as early as the Late Oligocene. The zircon age spectrum of the Sanya Formation had two major peaks at 99 and 237 Ma, indicating that Lower Miocene sandstones in the northwestern area of the Qiongdongnan Basin were mainly from Hainan Island, with ages ranging from 110 to 90 to 280–220 Ma. This study infers that the sandstones may be the paleo-Red River capture by the Yangze River, which resulted in a large decrease of sediment supply from the paleo-Red River system, such that the sediments of the paleo-Red River did not develop in the western area of the Qiongdongnan Basin during the Early Miocene. Results indicate that if the Red River capture existed, it would have predated the Early Miocene. The zircon age spectrum from Upper Miocene sediments of the western Qiongdongnan Basin showed five major age peaks at ca. 27, 88, 256, 415, and 785 Ma, indicating that its provenance was the Red River system. The Late Miocene provenance change from Hainan Island to the Red River may have been in response to a major phase of the Tibetan Plateau uplift.