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Geological Society, London, Special Publications; 2003; v. 218; p. 69-89;
DOI: 10.1144/GSL.SP.2003.218.01.05
© 2003 Geological Society of London

Tethyan Ophiolites in the Alpine-Himalayan Orogenic System

Melt migration in ophiolitic peridotites: the message from Alpine-Apennine peridotites and implications for embryonic ocean basins

Othmar Müntener1 & Giovanni B. Piccardo2

1 Institute of Geology, University of Neuchâtel, 11 rue Emile Argand, CH-2007 Neuchâtel, Switzerland othmar.muentener{at}unine.ch
2 Dipteris, Università di Genova, Corso Europa 26, I-16132 Genoa, Italy

Results of a field study as well as petrological and geochemical data demonstrate that substantial portions of the lithospheric mantle, exhumed during opening of the Jurassic Piedmont Ligurian ocean, were infiltrated by and reacted with migrating melts. Intergranular flow of ascending liquids produced by the underlying hot asthenosphere dissolved clinopyroxene ± spinel and precipitated orthopyroxene + plagioclase ± olivine, forming orthopyroxene + plagioclase-rich perioditite. Migrating liquids became progressively saturated in clinopyroxene, and then precipitated microgranular aggregates of clinopyroxene-bearing gabbronorite. Later, diffuse porous melt flow was replaced by focused porous flow, producing a system of discordant dunite bodies. Upon cooling, liquids migrating in dunite channels became progressively saturated in clinopyroxene and plagioclase, forming interstitial clinopyroxene at olivine triple points followed by clinopyroxene ± plagioclase megacrysts and gabbro veinlets within the dunite, and gabbro dykelets within plagioclase peridotites. Subsequent cooling during continued exhumation was accompanied by intrusion of kilometre-scale gabbroic dykes evolving from troctolite to Mg-Al and Fe-Ti gabbros. Migrating liquids, which infiltrated peridotite and formed gabbroic rocks, span a wide range of compositions from silica-rich single melt fractions to T- and N-MORB (mid-ocean ridge basalt), characteristic of the melting column beneath midocean ridges. Explanations for the progressive evolution of an igneous system from diffuse to focused porous flow and finally dyking include the competing effects of heating of the lithospheric mantle by ascending magmas from the underlying hot asthenosphere and conductive cooling by exhumation. Whether or not rift-related melt infiltration and heating is recorded by exhumed subcontinental lithospheric mantle along ocean-continent transitions and/or oceanic lithospheric mantle along slow-spreading ridges depends on the relative position to the underlying upwelling asthenosphere.





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