HR: 0830h
AN: V11E-0536    [PDF]
TI: Melt Percolation and Melt Rock Interaction in the Ophiolitic Peridotites of the Alpine Apennine System (Italy): a Main Step in the Rift Evolution of the Jurassic Ligurian Tethys
AU: * Piccardo, G B
EM: piccardo@dipteris.unige.it
AF: DIPTERIS, University of Genova, C.so Europa 26, Genova, 16132 Italy
AU: M\"{u}ntener, O
EM: Othmar.Muentener@unine.ch
AF: Geological Institute, University of Neuchatel, Rue E. Argand 11, Neuchatel, 2007 Switzerland
AU: Pettke, T
EM: thomas.pettke@erdw.ethz.ch
AF: Institut of Isotope Geochemistry and Mineral Resources, ETH, Sonneggstr. 5, Zurich, 8092 Switzerland
AU: Zanetti, A
EM: zanetti@crystal.unipv.it
AF: Istituto di Geoscienze e Georisorse - CNR, Sezione di Pavia, Via Ferrata 1, Pavia, 27100 Italy
AB: Ophiolites exposed along the Alpine - Apennine chain represent the oceanic lithosphere of the Jurassic Ligurian Tethys basin which separated the Europe and Adria plates. Pre-oceanic rifting of the Europe-Adria lithosphere caused: i) the tectonic exhumation of the sub-continental lithospheric mantle, and ii) the adiabatic upwelling and decompressional partial melting of the asthenosphere. Mantle peridotites of the Alpine-Apennine ophiolites (Lanzo (Western Alps) Erro-Tobbio (Liguria), Ligurides (Northern Apennines), Monte Maggiore (Corsica)) are variably depleted spinel lherzolites. They show a complete equilibrium recrystallisation under spinel-facies conditions at T = 900-1100$\deg$C, attained during their accretion to the thermal lithosphere.Prior to their exposure at the sea-floor in mid-Jurassic time, the peridotites were affected two main mantle processes which are related to the rifting stage of the basin: 1) a subsolidus decompressional evolution, as evidenced by the development of km-scale extensional shear zones and by incipient recrystallisation under decompression to plagioclase-facies conditions; 2) the porous flow percolation and impregnation, as indicated by textural, chemical and thermal modifications of the pristine spinel peridotites, and the subsequent focused melt migration in replacive dunite channels. Recent field, petrologic and geochemical investigations indicate that, during the pre-oceanic rifting stage of the basin, pyroxene-undersaturated melts migrated through the extending mantle lithosphere via diffuse and reactive porous flow, and became progressively pyroxene-(silica)-saturated. Reactive melt percolation produced depletion of the lower lithospheric mantle levels (i.e. pyroxenes dissolution) and impregnation and chemical refertilization of the shallower mantle levels (i.e. addition of basaltic components, in form of interstitial gabbroic microgranular aggregates, and significant trace element enrichment of the mantle minerals, when they attained the geochemical equilibrium with the impregnating melts). Thermometric estimates reveal that magmatic aggregates and mantle porphyroclasts in the impregnated peridotites record the same equilibrium temperatures (T$>$1250$\deg$C) indicating a significant heating of the lithospheric mantle during melt percolation. Accordingly, the lithospheric mantle represented by the Alpine-Apennine ophiolitic peridotites was subjected to significant thermochemical erosion (chemical refertilization plus heating) during the asthenosphere/lithosphere interaction which accompanied the rifting stage of the basin. The widespread occurrence of impregnated plagioclase peridotites within the Alpine - Apennine ophiolites indicates that the asthenosphere/lithosphere interaction via melt percolation played a fundamental role in the opening of the Jurassic Ligurian Tethys.
DE: 1025 Composition of the mantle
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8434 Magma migration
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting