A conceptual model for the formation of a km-scale gabbroic body in a fossil slow spreading center (Corsica ophiolites, France)

The Pineto gabbroic sequence from Jurassic ophiolites of Corsica (France) represents a lower crustal section of an embryonic slow spreading center. The sequence has a thickness of ~2 km and mostly consists of troctolites and minor olivine-gabbros in its lower portion, and clinopyroxene-rich gabbros at higher stratigraphic levels. The gabbroic sequence also includes sparse olivine-rich troctolites, olivine-gabbronorites and oxide-gabbros and locally encloses up to 100 m thick bodies of mantle peridotites. Primitive troctolites occur at different stratigraphic heights, in both lower and upper portions of the sequence, thereby indicating that the building of the gabbroic section involved different primitive melt injections. Petrological variations and cooling rate estimates (from -2.2 to -1.7 °c/yr log units) as a function of the stratigraphic eight allowed us to define a conceptual model for the growth of the km-scale gabbroic sequence. The proposed conceptual model shows that the primitive melts are intruded within a lithospheric mantle, cooled by active hydrothermal circulation. These primitive melts crystallize in situ, thereby forming sills consisting of primitive troctolites. The melts residual after the formation of the troctolites migrate through focused flow and develop sills that are progressively more evolved upward. When new primitive melt injections involve the lithospheric section, these primitive melts may intrude the lithospheric mantle at different depths forming primitive troctolites at different stratigraphic levels. The melts residual after the formation of the second stage primitive troctolites migrate upward and yield a series of variably evolved sills intruding the first stage sill series. This model shows that evolved gabbroic sills derived from different primitive melt injections may be physically associated explaning the complex architecture of the gabbroic complex.