Melt transport and deformation history in a nonvolcanic ophiolitic section, northern Apennines, Italy: Implications for crustal accretion at slow spreading settings

Field observations and petrological and geochemical data are used to constrain a conceptual model for the formation of a gabbro-peridotite section from Ligurian ophiolites (Italy). The studied section is attributed to an intraoceanic domain of the Jurassic Ligurian-Piedmontese basin and is characterized by the lack of a basalt layer, similar to nonvolcanic segments from (ultra)slow spreading ridges. The proposed model shows a “hot” lithospheric evolution in which melt transport in the mantle under spinel to plagioclase facies conditions occurred mostly in the form of grain-scale porous flow. We recognize a series of melt/peridotite interaction events, either diffuse or channeled, which modified the composition of the moderately depleted precursor mantle. In particular, localized infiltrations of MORB-type melts gave rise to formation of spinel websterite layers close to the lithosphere-asthenosphere boundary. The peridotite-websterite association was involved in a spinel facies deformation attributed to emplacement of asthenospheric material at the base of the lithosphere. The “hot” lithospheric evolution is followed by an evolution characterized by melt transport through fractures, which started with crystallization of melt into troctolite to olivine gabbro dikes. Both mantle structures and gabbroic dikes are locally crosscut by gabbroic sills. As the mantle section cooled significantly, the dip of the melt migration structures evolved from subvertical to subhorizontal. The growth of a gabbroic pluton (up to ~400 m thick) that is intruded into the mantle sequence is attributed to accretion of gabbroic sills. The tectonomagmatic history recorded by the gabbroic pluton after its solidification is characterized by ductile shearing developed from near-solidus to amphibolite facies conditions.