The Source And Tectonic Setting Of The Changhsingian K-Bentonites In The Huaying Mountain Region, South China

Numerous volcanic ash (or K-bentonite) beds are ubiquitous in the Permian-Triassic boundary (PTB) successions in South China and are conjectured to have been a critical factor in global events, such as the Permian-Triassic mass extinction. However, the precise location and tectonic setting of related volcanism remain debated. In this study, we present whole-rock geochemistry, zircon U-Pb ages, trace element compositions and in situ Hf isotope analyses of K-bentonites found in the Late Permian Changxing Formation in the Huaying Mountain region, eastern Sichuan Province, northern South China. The two samples yield U-Pb zircon ages of 253.7 +/- 1.4 Ma (QJP-38-R-1 below) and 253.0 +/- 2.0 Ma (QJP-38-R-2 above). Zircons with ages of 240-280 Ma from sample QJP-38-R-1 have negative epsilon(Hf)(t) values from -16.65 to -1.65 and two-stage model (T-DM(c)) ages from 1.38 to 2.34 Ga, whereas those from QJP-38-R-2 have mainly negative eHf(t) values from -7.28 to 0.13 and T-DM(c) ages from 1.28 to 1.74 Ga. The low epsilon(Hf)(t) values and T-DM(c) ages suggest that the original magmas mainly involved Paleoproterozoic-Mesoproterozoic crustal components and a few mafic mantle materials. Moreover, zircon trace element characteristics show that the two samples were sourced from a subduction-driven magmatic arc. The zircons from QJP-38-R-1 are primarily derived from S-type granites that formed in continental granitic rocks related to a continental arc tectonic setting, whereas the zircons from QJP-38-R-2 are mainly derived from S-type granites in oceanic granitic rocks related to an island arc tectonic setting. The K-bentonites near the PTB in South China may have originated from felsic volcanism as a result of the convergence of the Paleo-Tethys Ocean. The integration of zircon Hf isotope and trace element compositions indicates that the PTB K-bentonites in South China may have been derived from a series of volcanic eruptions occurring within a short interval at the continental margins around South China. Volcanic eruptions took place at the Ailaoshan-Sanjiang continental margins in or near southwestern South China and may be the major source of K-bentonites. Volcanic eruptions took place at the East Kunlun or West Qinling continental margins and may have provided a small number of original materials. This result indicates that these K-bentonites may have multiple sources.