The small ultramafic body at Mt. Moncuni consists of impregnated plagioclase peridotites, metre-scale masses of spinel dunites and harzburgites, and widespread gabbroic and porphyritic dykes. Metre- to decametre-scale shear zones cut the plagioclase peridotites and deform the pre-existing gabbroic dykes. Within the shear zones and in the host peridotite, millimetre- to decimetre-wide, decimetre- to metre-long, veins of pseudotachylytes are present, both concordant and discordant to the tectonite-mylonite foliation of shear zones. Late coarse-grained porphyritic dykes cut across both deformed plagioclase peridotites and shear zones. The formation of decametre-scale shear zones and the sequence of metamorphic assemblages developed in the shear zones, under increasing fluid activity, from amphibole-bearing plagioclase-peridotite- facies to amphibolite-facies, show that these rocks underwent progressive exhumation to shallower lithospheric levels, under progressively decreasing pressure and temperature conditions. Millimetre-wide pseudotachylyte veins are concordant with the fault planes of the shear zones (fault-vein type A) showing ultra-fine grained cryptocrystalline/microcrystalline and microlitic/ spherulitic textures and rather large (up to 10-15% by volume) amounts of clastic olivine grains or aggregates and lithic mylonitic clasts. Decimetrewide, metre-long, pseudotachylyte veins (injectionvein type B) cut discordantly the foliation of the host deformed peridotite and exhibit spinifex textures and limited presence (<1% by volume) or absence of exotic lithic aggregates. Pseudotachylytes type B show peridotitic major and trace element bulk rock compositions. The occurrence of ultramafic pseudotachylytes is consistent with the occurrence of upper mantle earthquakes. The formation of millimetre-wide ultrafine grained type A pseudotachylyte veins suggests that the fault planes acted as loci for the seismogenic release of the accumulated shear stress. The coseismic shear heating and the associated temperature increase produced the strongly localized, almost complete melting of the host deformed peridotite, forming melts with peridotitic composition. Strong localized heating of the host rock (up to 1450°C) was associated with intrusion of parental melts of type B pseudotachylytes, where rapid crystallization produced spinifex textures, typical of komatiitic (peridotitic) magmas. The mutual relations between the early (i.e., preshear zones) MORB-type gabbroic dyke intrusion, the formation of shear zones and pseudotachylytes, and the crosscutting of the late (i.e., post-shear zones) MORB-type porphyritic dyke intrusion give clear time constraints for the upper mantle seismicity. The investigated pseudotachylytes are records of Jurassic earthquakes related to extensional faults in the upper mantle, during its exhumation from subcontinental lithospheric levels towards the sea-floor of the Jurassic Ligurian Tethys.

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Keywords Jurassic Ligurian Tethys, Lanzo peridotite massif, Lithospheric mantle, Ultramafic pseudotachylytes, Western Alps ophiolites
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Journal Periodico di Mineralogia
Piccardo, G.B. (Giovanni B.), Ranalli, G, Marasco, M. (Michela), & Padovano, M. (Matteo). (2007). Ultramafic pseudotachylytes in the Mt. Moncuni peridotite (Lanzo Massif, western Alps): Tectonic evolution and upper mantle seismicity. Periodico di Mineralogia, 76(3), 181–197. doi:10.2451/2007PM0024