Late Permian High-Ti and Low-Ti Basalts in the Songpan–Ganzi Terrane: Continental Breakup of the Western Margin of the South China Block

Abstract

Although the Emeishan Large Igneous Province (ELIP) has been thoroughly researched, the role of the ELIP in the tectonics of the Songpan–Ganzi extensional basin in the eastern Tibetan Plateau has long been argued without any corroborated and robust evidence. We have investigated the basalt succession of the Dashibao Formation along Xindianzi and Xuecheng sections in the southeast margin of the Songpan–Ganzi Terrane (SGT). New SIMS zircon U-Pb ages and geochemical features of the Dashibao Formation are reported in this paper. Zircons of basalt sample XDZ02-1 yielded a weighted mean age of 259.1 ± 1.66 Ma, which is in alignment with the period when the main eruption of the ELIP occurred. Zircons from two tuff samples XDZ05-1 and XC05-2, overlying the basalt succession, were dated at 251.8 ± 1.57 Ma and 251.5 ± 0.27 Ma, respectively. These new dating results revealed a ~10 Ma eruption for the Dashibao basalts. The Dashibao basalts are geochemically classified into alkaline basalts (Group 1) and tholeiitic basalts (Group 2), which are a part of the Emeishan basalt. We thus propose that the Dashibao basalts erupted in a continental rift setting, located at the margin of the ELIP. The temporal and spatial coincidence of the Dashibao basalt, ELIP, and continental rifting in the western margin of the South China Block suggest that the continental breakup is a response to the Permian mantle plume that triggered the separation of the SGT in the eastern Tibetan Plateau.