Lawsonite, blueschist and eclogite formation in the southern Sesia zone (western Alps, Italy)
published: Mar 21, 1989
ArtNo. ESP147050101012, Price: 29.00 €
Abstract The southern Sesia zone in the western Italian Alps consists of four tectonic sub-units separated by major thrusts. These sub-units include : "gneiss minuti" (GM), blueschist-facies "polymetamorphic paraschists" (PP) similar to the "II zona diorito-kinzigitica", "eclogitic micaschists" (EM) and blueschist-facies "Rocca Canavese thrust sheets" (RCT). During Alpine metamorphism these sub-units underwent a polyphase early- Alpine history at high-P, followed by a late-Alpine low-P event. Evidence for an early-Alpine prograde path has been only found in the EM and GM. The early-Alpine metamorphic peaks occurred at T = 500-550 °C and P = 13-20 kbar in the EM, T = 400-450 °C and P ≥11-12 kbar in the GM and at T = 300-400 °C and P ≥ 8-10 kbar in the PP and RCT. Decompression during the early-Alpine event occurred at decreasing T in the EM (from T = 500-550 °C to T < 450-500 °C) and at more or less constant T in the other sub-units. In the southern EM, the early-Alpine post-eclogitic recrystallization produced lawsonite porphyroblasts which grew in equilibrium with quartz ± jadeite. Formation of lawsonite occurred at low XCO2 ; it was favoured by a temperature decrease after the HP climax and probably the syn-metamorphic infiltration of H2O-rich fluids along preferential channels. Lawsonite also occurs in the other Sesia sub-units, where it crystallized during the metamorphic climax. The post-eclogitic T decrease inferred here for the southern EM is less evident in more northerly parts of this sub-unit ; it may be related to a process of conductive-cooling during early-Alpine exhumation and the stacking at deep crustal levels with the relatively cooler blueschist-facies sub-units. After stacking and recrystallization under fairly comparable blueschist-facies conditions, the different Sesia sub-units underwent a low-P event of probable late-Alpine age. Such a low-P event caused a metamorphic zonation, with lower T in the structurally higher and eastern RCT.