GEOLOGICAL AND PETROLOGICAL STUDY OF THE CRYSTALLINE UNITS BETWEEN POPOLASCA AND PONTE DI CASTIRLA (NORTH-CENTRAL CORSICA, FRANCE)

Abstract

The Popolasca area (north of Corte) shows a complex geological framework. In particular, it is characterized in its western part, by Permian-?Triassic granitoids (Popolasca Plutonic Complex) which intruded Paleozoic sedimentary and volcanic successions, whereas towards east, an west–verging complex pile of tectonic “slices” (Popolasca Slices) is present and it is made up of Upper Paleozoic- ?Triassic metagranitoids, Permian metavolcanites, and Carboniferous, Triassic-Jurassic and Eocene metasedimentary successions. The latter (Castiglione metasandstone Fm.) are represented by meta-breccias, meta-conglomerates with nummulite-bearing limestone and metapelites. The boundary beetwen the Popolasca Plutonic Complex and the Popolasca Slices is represented by a NStrending high-angle fault. The Popolasca Slices are affected by a polyphased metamorphic and deformative evolution consisting in at least three events (the former two are syn-metamorphic in the Green-Schists facies): D1 is represented by the main continuous penetrative schistosity (S1) sub-parallel to the bedding and rarely associated to isoclinal folds; D2 is characterized by isoclinal to close mesofolds with associated spaced S2 axial plane foliation; D3 consists of metric to hectometre scale F3 folds with subhorizontal axial planes. Blue amphibole were found only in the Permian metagranitoids of the Popolasca Slices within some NS- trending shear zones. Microprobe analyses performed on these minerals reveal riebeckite to crossite compositions. Moreover, the AlVI/Si IV values point to pressures >5 Kb (13-15 km). These blue amphiboles form static fan-like aggregates in correspondence of fractures or grew on some K-Feldspars phenocrysts, but it still difficult to define the relationships with the Green-Schists blastesis. Two hypotheses for the genesis of these HP-LT minerals can be suggested: 1) a blastesis due to lithostatic pressure (10-15 Km depth); 2) a blastesis due to over-pressured (idrostatic pressure) fluids confined within the shear-zones. In the last hypothesis, the amphiboles could represent the metamorphic peak of such rocks which was reached only within shear zones in pre-D2 times or, perhaps, also during the D2 event.