Petrology of ultramafic and mafic rocks from the Lanzo peridotite body (Western Alps)

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The Lanzo lherzolites (west of Torino, Italy) exhibit a history of decompression at relatively high temperature (1150-950°C) from the spinel-lherzolite field to the plagioclase-lherzolite field. These ultramafics have clinopyroxenes with Na contents intermediate between those of the sub-oceanic peridotites and those of the sub-continental peridotites; moreover they do not exhibit a marked depletion in easily fusible elements (e.g. Na, Ti and Al) with respect to primitive mantle abundances, and are less depleted than other ophiolitic ultramafics of the Western Alps. The associated dykes of gabbro and porphyritic basalt derive from tholeiitic melts which show many affinities with N-MORB and with the other Piedmont ophiolites; they were produced by partial melting and low-pressure fractionation of an already depleted asthenospheric source. Thermobarometric data and mantle source/extracted melts relationships suggest that the Lanzo lherzolites cannot represent residua after extraction of N-MORB-type melts. Consequently the Lanzo ultramafics might be a section of sub-continental lithosphere which suffered a polyphasic history of partial melting and a decompression during attenuation and rifting of the European-“African” continental crust. Later, during the Jurassic, they were intruded by N-MORB-type melts produced at depth and were involved in more “mature” oceanic-type stages which led to the opening of the small Piedmont-Ligurian basin.