Evaluation of different models for the origin of the Siberian traps

Various types of evidence, including the size and volume of the Siberian Traps, the timing and duration of eruptions, paleotectonic and paleogeographic reconstructions, lithospheric structure, heatflow, and the 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 ( approximately 7X10 (super 6) km (super 2) ) in a back-arc tectonic setting. The overall volume of erupted rocks was as much as approximately 4X10 (super 6) km (super 3) , 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 a relatively thin lithosphere, which is approximately 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 of the Siberian Traps cannot be explained in the framework of impact and edge-driven convection models and are problematic for lithospheric delamination models. Plume models face problems in explaining the uplift and subsidence pattern and the absence of mixing curves between expected high-Ti primary plume melts and contaminated low-Ti melts. Therefore, a model that 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 the high water capacity of the mantle transition zone and its recharging via the subduction process.