Why are Low-Ti Basalts of the Siberian Traps Large Igneous Province Similar to Island Arc Basalts?

Tholeiitic and alkaline basalts are predominant rock types in the Late Permian - Early Triassic Siberian Traps Large Igneous Province (STLIP). These basalts belong to high-Ti and low-Ti series of rocks. A peculiarity of the low-Ti basalts is the virtual similarity with typical island arc basalts. On a primitive mantle normalized diagram, both the low-Ti basalts of the STLIP and island arc basalts exhibit prominent depletion of Th, Ta-Nb, Pr and Sr relative neighboring elements. As an example, the low-Ti basalts are characterized by almost the same trace element abundances and compositional trends as basalts from Klyuchevskoi volcano, which belongs to the modern volcanic arc of Kamchatka. Using ratios of element pairs with similar rock-melt distribution coefficients such as Sr-Pr, Nb-U, K-Nb and Ce-Pb we infer three principal components: 1) oceanic sediments (or upper crust), 2) melts of oceanic basalt type (either middle oceanic ridge or oceanic island basalt types), and 3) melts of island arc basalt type (or lower crust). Basalts of the high-Ti and low-Ti series of the STLIP form two nearly perpendicular trends between the first and second and between the first and third components. Such trends, and especially the approaching of the trends towards the same component, which is similar to oceanic sediments cannot be explained by a model of plume-lithosphere interaction. An impact-induced melting model could explain the trace-element data satisfactorily, but it can also be rejected because volcanism of the STLIP initiated in the Late Permian, a few Ma before the Permo-Triassic stratigraphic boundary. Prolonged subduction beneath the Siberian part of Pangea occurred in the Permian and Triassic. Remnants of the Mongolo-Okhotsk slab beneath Siberia are still visible in seismic tomography images. Therefore, we suggest that the origin of the STLIP is related to subduction processes. Melts from subducted sediment-bearing oceanic crust played a major role in triggering magmatic processes in the sublithospheric and lithospheric upper mantle. The low-Ti basalts were produced within the sublithospheric upper mantle, which was highly metasomatized by subduction-derived fluids. This explains both the extremely large volume of melts and trace element similarity with island arc basalts.