Evaluation of Different Models for the Origin of the Siberian Traps

Various evidence, including size and volume of the Siberian Traps, timing and duration of eruptions, paleotectonic and paleogeographic reconstructions, lithospheric structure, heat flow, trace element and radiogenic isotope compositions of lava are reviewed in this chapter. The major evidence may be summarized as follows. The Siberian Traps erupted in a number of brief volcanic events from the Late Permian until the end of the Middle Triassic. They occupied a vast region (about 7×10 km) in a back-arc tectonic setting. The overall volume of erupted rocks was as much as ~ 4×10 km with most of the volume erupted within the Tunguska syncline. This syncline experienced long-term subsidence before initiation of the volcanism and the region is now underlain by thinnest lithosphere, which is about 180 km thick. Two types of trace element patterns are observed in the Siberian Traps; subordinate high-Ti ocean-island-basalt-like patterns, and dominant low-Ti island-arc-basalt-like patterns. In radiogenic isotope and trace element coordinates, mixing trends between these two types of magma are absent or, at least, not evident. Some volcanic rocks contain primary magmatic mica. These are considered in light of different models. Each model can explain, or was thought to explain, particular observations. However, some evidence can be fatal for some models. For example, the enormous size and volume cannot be explained in the framework of impact and edge-driven convection models, and is problematic for lithospheric delamination models. Plume models face problems explaining the uplift/subsidence pattern and the absence of mixing curves between expected high-Ti primary plume melts and contaminated low-Ti melts. Therefore a model which relates Siberian Trap magmatism and subduction is suggested. In this model, subducting slabs brought significant amounts of water into the mantle transition zone. Consequent release of water from the transition zone lowered the solidus of the upper mantle, leading to voluminous melting. Major supporting observations for this model include: (1) the tectonic position of the Siberian Traps in a back-arc setting of Permian subduction systems, (2) island-arc-basalt-like trace element patterns for the majority of the erupted basalts, (3) primary mica found in volcanic rocks and (4) experimental data on