Pre-Variscan deformation, metamorphism and magmatism in the Strona-Ceneri zone (southern Alps of northern Italy and southern Switzerland)

The Strona-Ceneri Zone comprises the deformed and metamorphosed remains of a Late Proterozoic or Paleozoic subduction-accretion complex that was intruded by Ordovician granitoids. Xenoliths within these granitoids include relies of MOR-basalt that experienced eclogite facies metamorphism (690-750 degrees C, > 15-16 kbar) and paragneiss that underwent D1 deformation at amphibolite facies conditions. Pre-intrusive exhumation of the eclogites involved nearly isothermal decompression to 9-11 kbar sometime before their equilibration under prograde D2 amphibolite facies conditions (570-630 degrees C, 7-9 kbar). D2 mylonitization at these conditions coincided with and outlasted the intrusion of large volumes of Ordovician granitoids. Most of these granitoids stem from mixing of mantle-derived melts with aluminous metasediments in the Strona-Ceneri Zone. Pure S-type granitoids, the so-called Ceneri granitoids, derived from partial, biotite-dehydration melting of such metasediments under fluid-depleted, granulite facies conditions. These granitoid magmas then intruded and were emplaced syntectonically at the current erosional surface in the Strona-Ceneri Zone. The metasediments of the Strona-Ceneri Zone were probably deposited during Cadomian orogenesis and/or Cambro-Ordovician rifting. Eclogitization of oceanic crust and subsequent uplift is either Cadomian or Ordovician age. Ordovician magmatism and related D2 deformation and metamorphism are ostensibly related to subduction and/or collision along the northern margin of Gondwanaland.