Subsolidus Emplacement of Mantle Peridotites during Incipient Oceanic Rifting and Opening of the Mesozoic Tethys (Voltri Massif, NW Italy)

The Erro–Tobbio (ET) peridotite (Voltri Massif, NW Italy)represents a fragment of subcontinental mantle, emplaced athigh crustal levels during rifting and opening of the Piemonte–Ligurianocean, the Alps–Apennine part of the Mesozoic Tethys.The ET peridotite is dominated by spinel-bearing lherzolites,with minor dunites, spinel websterites and plagioclase-bearinglherzolites. Granular spinel lherzolites in the ET peridotiteare transected by five generations of shear zone structures:porphyroclastic spinel-bearing tectonites, plagioclase-, hornblende-and chlorite-bearing peridotite mylonites, and serpentinitemylonites. There is a systematic correlation between the microstructuresand the composition of the constituent mineral phases. Thesecompositional trends are related to changing conditions of Pand T during the pertinent stages of syntectonic recrystallizationin the shear zones. Geothermobarometry shows that the shearzone structures developed at progressively lower P and T conditions.The P–T path obtained for the ET peridotite indicatessubsolidus uplift, from deep levels in the subcontinental mantletowards the ocean floor. Uplift is associated with limited ‘wet’partial melting. This subsolidus trajectory is particularlyconsistent with the thermal history expected for the footwallof a lithosphere-scale, dipping extensional shear zone, anddiffers from those of more oceanic peridotites showing an adiabaticuplift history at much higher temperatures presumably relatedto convective upwelling. The shear zone structures in the ETperidotites are therefore interpreted as fragments of an extensionaldetachment system. This interpretation is consistent with theoverall asymmetric architecture of the coeval passive marginsbordering the Piemonte–Ligurian ocean. It is suggestedthat the uplift of the ET peridotites occurred by tectonic denudation,in a slightly to strongly asymmetric oceanic rift. ^* Present address: Shell Int. Petroleum Maatschappij, P.O. Box 162, 2501 AN The Hague, Netherlands     

Alpine olivine- and titanian clinohumite-bearing assemblages in the Erro-Tobbio peridotite (Voltri Massif, NW Italy)

The microstructures in the Erro-Tobbio peridotite indicate several stages of recrystallization of olivine + titanian clinohumite-bearing assemblages. The development of these assemblages is closely associated with serpentinite mylonites, in which they occur in shear bands and foliations and are inferred to have grown synkinematically, in veins, and as post-kinematic radial aggregates. In the peridotite wall-rock adjacent to these mylonites, the same assemblages have recrystallized statically at the expense of original olivine and pyroxenes, mesh-textured chrysolite and antigorite veins. In addition, the olivine-bearing assemblage occurs in widespread vein systems. The brittle deformation of the peridotite resulting in the development of these vein systems is closely related to ductile deformation of metagabbroic dykes in the peridotite. Although early metasomatism resulted in extensive rodingitization of the gabbros, some dykes show an eclogitic assemblage of Na-clinopyroxene + garnet + chloritoid + chlorite ± talc. These observations, the microstructures and the mineral chemistry all suggest that the assemblages in the ultramafic rocks and metagabbros developed during a prograde evolution towards high pressures (>13–16 kbar, 450–550° C), and during subsequent decompression. This metamorphic evolution is considered to be related to Late Cretaceous intraoceanic subduction in the Alps-Apennine system and closure of the Piedmont-Ligurian basin.