Seminari di Scienze della Terra
Universita’ di Genova
11 Settembre 2006

 Multidisciplinar Approach to Natural Triggers of Lithosphere Evolution

Igneous and metamorphic evolution of the Ligurian Tethys crust

Riccardo Tribuzio

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.


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