ZIRCON TYPOLOGY IN METASEDIMENTS FROM THE STRONA-CENERI ZONE (SERIE DEI LAGHI, WESTERN SOUTHERN ALPS) - INDICATIONS ON THEIR PROTOLITHS ANDEVOLUTION

The typologic study of zircon populations from Gneiss Minuti and Cener igneiss (Strona-Ceneri Zone, Serie dei Laghi, Western Southern Alps) g ave interesting indications on their protoliths and evolution. Both ro ck types contain detrital zircon populations, mainly deriving from the erosion of intrusive calcalkaline rocks. Despite some differences in the degree of mechanical abrasion, the homogeneity of zircon character istics within each sample suggests that each population mainly derives from a single source rock; similarity of the typologic characteristic s and progressive variations of the typologic distributions among the different samples suggest that the igneous source rocks of the sedimen ts were genetically linked. The Cenerigneisses petrographically and ch emically show mixed sedimentary and igneous character; beside the typi cal zircons of paragneisses, they also contain euhedral, non-abraded z ircons of granitic origin. These crystals are similar to the populatio ns of some two-mica orthogneisses, representing the Hercynian metamorp hic product of granitoid rocks which intruded the Serie dei Laghi in t he Ordovician. The study of Cenerigneiss samples from the Vira-Magadin o area (Switzerland), where the Serie dei Laghi is affected by a subse quent very limited partial melting (attributed to a Permian thermal di sturbance induced by the uprise of mafic magmas), shows that ''recryst allized'' and non-recrystallized zircons coexist in the same sample. T herefore, the recrystallization process at high grade metamorphic cond itions does not affect the whole zircon population at the same time.