Building of the deepest crust at a fossil slow-spreading centre (Pineto gabbroic sequence, Alpine Jurassic ophiolites)

This work presents new field and petrological data on a poorly known lower crustal section from the Alpine Jurassic ophiolites, the Pineto gabbroic sequence from Corsica (France). The Pineto gabbroic sequence is estimated to be ~1.5 km thick and mainly consists of clinopyroxene-rich gabbros to gabbronorites near its stratigraphic top and of troctolites and minor olivine gabbros in its deeper sector. The sequence also encloses olivine-rich troctolite and mantle peridotite bodies at different stratigraphic heights. The composition and the lithological variability of the Pineto gabbroic sequence recall those of the lower crustal sections at slow- and ultra-slow spreading ridges. The gabbroic sequence considered in this study is distinct in the high proportion of troctolites and olivine gabbros, which approximately constitute 2/3 of the section. In particular, the lower sector of the Pineto gabbroic sequence shows the existence of large-scale fragments of the deepest oceanic crust displaying a highly primitive bulk composition. The mineral chemical variations document that the origin and the evolution of the Pineto gabbroic rocks were mostly constrained by a process of fractional crystallisation. The clinopyroxenes from the olivine gabbros and the olivine-rich troctolites also record the infiltration of olivine-dissolving, Cr2O3-rich melts that presumably formed within the mantle, into replacive dunite bodies. Cooling rates of the troctolites and the olivine gabbros were evaluated using the Ca in olivine geospeedometer. We obtained high and nearly constant values of -2.2 to -1.7 °C/year log units, which were correlated with the building of the Pineto gabbroic sequence through multiple gabbroic intrusions intruded into a cold lithospheric mantle.