Zircon U-Pb ages, trace elements, and Hf isotopes establish a genetic link between volcanic ash beds and porphyritic intrusions during early Silurian in the South China Block

Abstract

The provenance of the Late Ordovician to Early Silurian volcanic ash beds (VABs) in the South China Block (SCB) remains debated. In this study, we provide new zircon U-Pb age, trace element, and Hf isotope data of bentonites, crystal tuffs, and porphyritic intrusions from the Longmaxi Formation of the Dabashan area in the northern margin of the SCB with a view to investigate the composition, tectonic setting, and source of their parent magmas. The zircon U-Pb ages show that the volcanic eruption and magma emplacement occurred in the Early Silurian (ca. 440–434 Ma). The zircon trace element compositions are consistent with A-type parent magmas formed in intraplate extensional settings. The positive ε_Hf(t) values (+1.7 to +6.9) of zircon indicate an enriched mantle source for the magma, with possible crustal input. The similar U-Pb ages, trace element compositions, and ε_Hf(t) values of zircon grains from VABs and porphyritic intrusion in the Dabashan area suggest that their parent magmas shared similar sources. These A-type parent magmas could have originated in intraplate extension settings on the passive margin of the northern SCB during the subduction of the Proto-Tethys Ocean towards the North Qinling Terrane. Our results suggest that the Paleo-Tethys oceanic basin did not open before the Early Silurian (ca. 434 Ma) in the Dabashan area.