Microtectonics of Gabbroids in the Mid-Atlantic Ridge off Sierra Leone from 5° to 7° N

摘要

The structure of gabbroic rocks from the Mid-Atlantic Ridge from 5° to 7° 10′ N (Sierra Leone Non-transform Offset Zone) is considered. This paper is based on the materials obtained from dredging during several cruises of the Russian research vessels in 2000-2003. The structural assemblages characterizing evolution of gabbroids at the magmatic stage and at stages of ductile deformation and brittle failure are recognized. Unde-formed, slightly deformed, partially and completely recrystallized, and mylonitized gabbroids are distinguished by their structural features. Large and small neoblasts corresponding to various conditions of rock transformation occur in recrystallized gabbroic rocks. Gabbroids of the banded complex that reveal magmatic lineation were formed under conditions of viscous laminar flow of an incompletely solidified magmatic melt along the walls of magma chambers. Massive gabbroic rocks crystallized under static conditions. The large neoblasts grew in the process of recrystallization owing to the diffusion and dislocation creep in the crystal lattice of minerals. The small neoblasts largely arose as a result of dislocation gliding and dislocation creep. The structure evolution of gabbroids bears indications of retrograde metamorphism under gradually falling temperature, increasing stress, and the effect of hydrothermal fluid during the uplift of basic rocks. The large neoblasts in gabbroids originate at a great depth and the small neoblasts appear nearer to the surface, while brittle failure occurred in the uppermost crust. The gabbroic rocks rose up owing to the ductile and cataclastic flow along the faults that developed mainly under conditions of simple shear.