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

Abstract

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 gab- bros in its deeper sector. The sequence also encloses olivine-rich troctolite and mantle peridotite bodies at dif- ferent stratigraphic heights. The composition and the lith- ological 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 gab- broic sequence shows the existence of large-scale frag- ments of the deepest oceanic crust displaying a highly primitive bulk composition. The mineral chemical varia- tions 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 geospee- dometer. 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 litho- spheric mantle.