Emplacement at granulite facies conditions of the Sesia-Lanzo metagabbros:an early record of Permian rifting?

The evolution of the pre-Alpine Corio and Monastero metagabbros points to s trong chemical and mineralogical similarities with that of other Permian ga bbro bodies of the Alps, which are concentrated in the Southalpine and Aust roalpine domains. The structural and metamorphic pre-Alpine evolution of th ese gabbros records a re-equilibration following the emplacement in the dee p crust (P = 0.6-0.9 GPa and T = 850 +/- 70 degreesC), exhumation through a mphibolite facies conditions (P = 0.5-0.35 GPa and T = 570-670 degreesC), f ollowed by a greenschist facies imprint (0.25 less than or equal to P less than or equal to 0.35 GPa and T < 550 degreesC). This retrograde P-T evolut ion suggests that the exhumation occurred in a high thermal gradient regime , as that induced by upwelling of an asthenospheric plume during continenta l rifting. This would be consistent with the crustal thinning known to have occurred in both the Southalpine and Austroalpine domains during Permian t imes. The gabbros and their country acid granulites are spatially associate d with the serpentinised subcontinental mantle of the Lanzo Massif. This li thologic association and the metamorphic evolution is similar to that of th e Fedoz gabbro (Austroalpine Domain of the Central Alps) and completely dif ferent from that observed in passive margins, where no remnants of the lowe r crust occur and the upper granitic crust directly overlies the serpentini zed lherzolites. The location of Permian gabbro bodies in the Austroalpine and Southalpine domains and their absence in the Helvetic domain is evidenc e for asymmetric rifting. (C) 2001 Elsevier Science B.V. All rights reserve d.