Detrital zircon U-Pb ages and Hf isotopes of Lower-Middle Devonian to Middle Jurassic sandstones in the Qinfang basin, southern South China block: Constraints on provenance and tectonic setting

Abstract

• The Qinfang basin is a remnant of the Paleo-Tethyan (Ailaoshan) Ocean. • The basin is not a part of the Indochina block. • The basin and its adjacent east and west sides may share a common East Gondwana source in the period of Silurian . • The adjacent west side of the Qinfang basin may be still tectonically active at least until the Middle Jurassic . The affinity and tectonic setting of the Qinfang basin have long been an enigma. Here we conduct detrital zircon U-Pb geochronology and Hf isotope analysis of Lower-Middle Devonian to Middle Jurassic samples from the basin. Along with published age data and paleontology from the basin and its east and west sides, this study constrains the sediment provenance and tectonic setting of the basin. Sandstones of Devonian-Jurassic exhibit an age distribution with major age groups at 2700–2300 Ma (absence for Jurassic), 1200–800 Ma, 600–400 Ma, and 334–246 Ma (for Jurassic alone). Zircons (peak ~2500 Ma) were sourced from the Yangtze block, while ~1200–800 Ma grains may originate from the Yunkai massif and Jiangnan orogen. ~600–400 Ma zircons came from Yunkai massif and/or Cambrian to Silurian sediments in the basin. Carboniferous-Early Triassic (334–246 Ma) zircons were probably supplied by an older sequence in the Great Youjiang basin and the Darongshan-Shiwandashan igneous rocks. The Qinfang basin and its two sides are considered to share a common East Gondwana source in Silurian. The similarity in age spectra between Silurian-Devonian samples in the basin and Ailaoshan belt suggests that the basin is a remnant of the Paleo-Tethyan (Ailaoshan) Ocean. However, the distinct age distribution between the basin and Indochina block shows that the basin is not a part of the Indochina block. By Middle Jurassic following the final closure of the Ailaoshan Ocean in the Late Triassic, the adjacent western area may be still tectonically active, thereby feeding the basin with detritus of Early Permian.