Ordovician barrow-type metamorphism in the Strona-Ceneri Zone (northern Italy) dated by U-Pb on staurolite

The crustal segment of the Strona-Ceneri Zone (SCZ) in northern Italy shows a complex polymetamorphic evolution that is bracketed by the emplacement age of 450-460 Ma old granitoids and the Middle Carboniferous age of mica and amphibole presumably related with the D3-phase. The age of the strongest tectonometamorphic event (D2) in the SCZ, which took place under amphibolite facies conditions at 600 +/- 30 degrees C and 8 +/- 1 kbar, resulted in the growth of abundant syntectonic staurolite in a garnet mica schist formed during the D2 clockwise prograde P-T evolution that is typical for Barrow-type metamorphism. The U-Pb and Pb-Pb systematics of staurolite demonstrate that staurolite formed during an Ordovician event and had lost lead during a Variscan overprint. Lead loss occurred prefer ably from microfractures and crystal defects, whereas lead from structural sites seems not to have been affected. The staurolite gives an Ordovician age for the D2 event, which implies that the metamorphic and structural memory of the SCZ was acquired during at least two distinct tectonometamorphic events.