Igneous and metamorphic evolution of the Ligurian Tethys crust
Dipartimento di Scienze della Terra, Università di Pavia, Italy
The gabbros from the Northern Apennine ophiolites formed by the intrusion of N-MORB type melts in a heterogeneous mantle section. These melts underwent an igneous differentiation process controlled by fractional crystallisation, most likely associated with late percolation in the gabbroic cumulate pile of a volatile-bearing igneous agent, such as a trondhjemite-type melt. Similar to the gabbros, associated basalts were derived from N-MORB type melts, variably evolved through fractional crystallisation. The gabbros record a polyphase, pre-orogenic tectono-metamorphic evolution, characterised by progressive temperature decrease. The early high-temperature (T ~900 °C) recrystallisation is localised along ductile shear zones and occurred in the absence of seawater-derived fluids. The subsequent evolution, from amphibolite-facies to low-temperature conditions, was assisted by seawater-derived fluids and commonly associated with brittle deformations. As a whole, the gabbros and basalts from the Northern Apennine ophiolites bear striking petrographic and geochemical similarities to those recovered from modern magma-poor slow-spreading centres. A remarkable exception is represented by the External Liguride gabbros of M. Roccabruna. These gabbros are slightly older (179 ± 9 Ma) than the oldest pelagic sediments of the Ligurian Tethys and lack any evidence for interaction with seawater-derived fluids. The M. Roccabruna gabbros therefore testify to mantle melting prior to continental break-up, similar to what is inferred for the “syn-rift” gabbros from the ocean-continent transition of the Western Iberian margin.