Structure of the Mantle-Crust Transitional Zone in Modern and Ancient Spreading Centers, the Central Atlantic and Polar Urals

摘要

Geological structures, igneous and metamorphic events occurring in various geodynamic environments are compared in the context of boundary formation between the residual mantle and crustal plutonic complexes. The evolution of geological structures and the composition of a stable association of mafic and ultrama-fic rocks in the mantle-crust transitional zones are deduced from a comparison of the complexes exposed in particular segments of the well studied modern spreading centers in the Central Atlantic and in ophiolitic massifs of the Urals formed as a result of oceanic crust spreading in the suprasubduction environment. It is concluded that the structure and composition of oceanic and ophiolitic complexes providing insights into the origin of the Moho discontinuity are principally similar. In both cases, the spatiotemporal variations of the mantle material ascent give rise to the segmentation of embryonal oceanic crust along the spreading axis and to the wavelike undulations of the Moho topography (change of the Moho depth), which are correlated with variable thickness and structure of the entire mantle-crust transitional zone. It has been suggested that a difference in the structure of the mantle-crust transitional zone and hence in the dynamics of its evolution in the Mid-Atlantic-Ridge and in the suprasubduction paleospreading centers is caused by the greater thickness of oceanic lithos-phere in the suprasubduction domains. Metamorphism of ultramafics and gabbro, previously referred to the col-lisional or orogenic events, is also known from the spreading centers of oceanic floor. This implies that ophio-lites bear information not only on the mantle, but also on the crustal history of oceanic complexes.