The Jurassic Ligurian Tethys, a fossil ultraslow-spreading ocean: The mantle perspective
G. B. Piccardo, 2008. "The Jurassic Ligurian Tethys, a fossil ultraslow-spreading ocean: The mantle perspective", Metasomatism in Oceanic and Continental Lithospheric Mantle, M. Coltorti, M. Grégoire
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Alpine–Apennine ophiolites derive from the Jurassic Ligurian Tethys oceanic basin formed by lithosphere extension and failure in the pre-Triassic Europe–Adria system. The basin was floored by mantle peridotites and was characterized by along-axis alternation of avolcanic and volcanic segments. Lithosphere extension and thinning caused asthenosphere adiabatic upwelling and decompressional melting. Mid-ocean ridge basalt (MORB)-type melts diffusely percolated through and reacted with the overlying lithospheric peridotites, which were strongly modified, both depleted (harzburgites and dunites) and enriched (plagioclase peridotites), by melt–peridotite interaction and melt refertilization. The stratigraphic–structural features (mantle at the sea floor and alternation of avolcanic and volcanic segments) coupled with petrological features (presence of alkaline melts and strongly heterogeneous, melt-modified peridotites) allow us to interpret the Ligurian Tethys as a Jurassic analogue of modern ultraslow-spreading oceans. The Liguria Mode for the inception of an oceanic basin consists of: (1) the rifting (continental) stage, dominated by extension of continental lithosphere and tectonic exhumation of lithospheric mantle; (2) the drifting (transition) stage, characterized by melt-related processes (i.e. inception of asthenosphere partial melting and MORB melt percolation through the overlying mantle lithosphere); (3) the spreading (oceanic) stage, characterized by failure of the continental crust, sea-floor exposure of mantle peridotites and discontinuous MORB extrusion.
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Twenty years have passed since Menzies & Hawkesworth extended the concept of metasomatism to mantle processes. The aim of this book is to gather together progress made on this topic since then. Most of the 14 papers reported in the volume rely on in situ major and trace element analyses of minerals and glasses in mantle xenoliths, and deal with different kinds of metasomatic agents at variable fluid/rock ratios in tectonic settings as different as intra-plate, mid-ocean ridge (ophiolites) and supra-subduction. The book contributes to the wide debate on the nature of the fluids migrating into the mantle wedge, as well as on the different residential times of the subduction signature. In addition papers on intra-plate settings deal with the problem of relating various metasomatic signatures to one single metasomatic event through an infiltration-reaction process.