Rl. Romer et L. Franz, Ordovician barrow-type metamorphism in the Strona-Ceneri Zone (northern Italy) dated by U-Pb on staurolite, SCHWEIZ MIN, 78(3), 1998, pp. 383-395
Citations number
57
Categorie Soggetti
Earth Sciences
Journal title
SCHWEIZERISCHE MINERALOGISCHE UND PETROGRAPHISCHE MITTEILUNGEN
The crustal segment of the Strona-Ceneri Zone (SCZ) in northern Italy shows
a complex polymetamorphic evolution that is bracketed by the emplacement a
ge of 450-460 Ma old granitoids and the Middle Carboniferous age of mica an
d amphibole presumably related with the D3-phase. The age of the strongest
tectonometamorphic event (D2) in the SCZ, which took place under amphibolit
e facies conditions at 600 +/- 30 degrees C and 8 +/- 1 kbar, resulted in t
he 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 a
nd crystal defects, whereas lead from structural sites seems not to have be
en 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.