The Collisional System in the Northeastern Siberian Craton and a Problem of Diamond-Bearing Lithospheric Keel

摘要

The diamond-bearing kimberlite diatremes in the northeastern Siberian Craton allow the tectonic models of the formation of Earth's crust to be compared with the specific features of the underlying upper mantle. The Mesoarchean (3.2-2.8 Ga) stage in the evolution of the craton was characterized by the creation of sialic continental masses that amalgamated into the Pangea-0 supercontinent 2.8-2.6 Ga ago. Pangea-0 subsequently broke up into new microcontinents that amalgamated again 1.9-1.8 Ga ago into the Siberian Craton in its present-day configuration as a part of the Pangea-1 supercontinent. This sequence of events is supported by the isotopic ages of ancient terranes, their subsequent thermal transformation, and the generation of collision-related granitoids. Interpretation of seismic profiles indicates that the attributes inherent to the Phanerozoic collision prisms have been retained in the crust of the Paleoproterozoic fold system. These attributes include the Earth's crust as thick as 58 km, fragments of the lower, high-velocity layer (presumably, underplating basaltic rocks), and deep-seated blocks of high-velocity upper mantle raised to the level of the lower crust. These crustal units are underlain by the high-velocity diamond-bearing lithospheric mantle that has thickened to 260-300 km. The thickness of this mantle layer decreases to approximately 200 km toward the margins of the region, and a local swell has been identified as a lithospheric keel (root). The spatial relationships between the crustal and mantle structures are evident. It would be the simplest to suggest that this keel was formed as a result of accretion of the lithospheric mantle fragments together with the crustal terranes attached to their roofs. Such a suggestion comes into conflict with the commonly accepted views on the exclusively crustal nature of the continental collision and may become a subject of further investigations.