Incomplete blueschist re-crystallization in high grade metamorphics from the Sesia-Lanzo unit (Vasario-Sparone subunit, Western alps): a case history of metastability

The Vasario-Sparone (VS) subunit is part of the Sesia-Lanzo unit (Western Alps) and represents a thin and discontinuous slice of pre-Alpine deep continental crust. It is tectonically interposed between the "eclogitic mica schists" and the "gneiss minuti" subunits. Metastable relics and a sequence of incomplete transformations controlled by a local equilibrium reaction mechanism are preserved in the VS subunit. The VS rocks record a pre-Alpine high grade of metamorphism (T=780±50°C, P≈6 kbar) which is comparable to the metamorphism dated around 500 Ma in the Ivrea Zone. During the Alpine evolution the VS rocks sustained an early blueschist stage (T=300-400°C, P≥8-10 kbar) of probable Cretaceous age and a later low-T, low-P stage of Tertiary age. The blueschist transformations are governed by slow reaction kinetics, by the heterogeneous distribution of pervasive deformations and by syn-metamorphic fluid phase. In particular, transient kyanite formed in the early stages of the blueschist re-crystallization in response to a process controlled by fluid-deficient conditions and by internal buffering of aH2O at low values