Giovanni B. Piccardo, Alberto Zanetti, Gianluca Spagnolo, Eugenio Poggi


New field, petrographic-structural and petrologic-geochemical investigations have been dedicated to the spinel and plagioclase peridotites cropping out at Mt. Arpone and Mt. Musinè, within the Southern Body of the Lanzo Ophiolitic Massif (Western Alps). Field mutual relationships indicate that coarse granular, “reactive” spinel peridotites formed at the expense of pristine lithospheric mantle protoliths. Most of the spinel peridotites were successively transformed in “impregnated”, plagioclase-rich peridotites. Metre-wide bands of granular “replacive” spinel harzburgites and dunites and decametre- to hectometre-long bodies of granular “replacive” spinel dunites cut across the plagioclase peridotites, together with some gabbroic dikelets and dikes.
It is recognized that all these rock types were formed by melt-related processes: i) the reactive spinel peridotites originated by the diffuse percolation of silica-undersaturated melt increments which dissolved pyroxenes of the pristine lithospheric peridotites and crystallised new olivine; ii) the plagioclase peridotites originated by diffuse percolation of silica-saturated melt increments which crystallised interstitially within the peridotite, causing impregnation and refertilization of previous lithospheric as well as reactive spinel peridotites; iii) the replacive spinel harzburgite and dunite bands and bodies were formed by the focused percolation of silica-undersaturated MORB-type liquids, which dissolved plagioclase and pyroxenes, along preferential channels within the plagioclase peridotites. Once formed, these channels were utilized by aggregate MORB melts to migrate to shallow crustal levels.
This study sheds further light on the evolution of the melt dynamics (from single melt increments to aggregate MORB) and the migration mechanisms (from diffuse to focused porous flow to diking) during the progressive exhumation of this lithospheric mantle section, in response to the lithosphere extension leading to formation of the Jurassic Ligurian Tethys.



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