The Larderello Plutono-Metamorphic Core Complex: Petrografic Data

Abstract

Terranes which have suffered LP-HT metamorphism (De Yoreo al., 1991 for explication about Pressure Metamorphism) are of great interest because they be related to extensional lithospheric processes (Thompson and Ridley, 1987; Wickham and Oxburgh, 1985,1987) accompanied to magmatic heat advection (De Yoreo et 1989; Barton and Hanson, 1989; Jones and Brown, 1990; Haugered and Zen, 1991; et al., 1992). The extensional processes may, for example, be expressed by low-angle normal shear zones (detachments). This type of structures has been widely described, particularly in continental domains which have undergone an earlier orogeny (Wernicke, 1981, 1982; Davis, 1983). The magmatic product often associated to extensional tectonic is due to the partial melting of crustal rocks; the felsic magma rises to the crust and in high crustal level (Soula, 1982; Castro, 1987). Some field examples of this association have been described from the continental collision in the Variscan belt, especially in the Massif Central area and in the axial zone of the Pyrenees Saint Blanquat et al., from high temperature part of paired metamorphic belts related to subduction in the Aegean Sea (Jansen al., 1976; Lister et al., 1984; Avigad et al., 1991) and Western United States (Cordillera-type LPM; Barton et al, 1988). In the Larderello-Travale area, previous studies have show (a) the presence of a LP-HT metamorphism associated with migmatization and young granitoid rocks the frequent superposition of mylonitic textures over magmatic and metamorphic assemblages. The core of metamorphic and magmatic rocks have been named Larderello Plutono-metamorphic Core Complex by Franceschini ( 1994a). In this paper I present the result of a petrographic study which complete and data already obtained by Franceschini 1994 c). Arguments based on petrology and bibliography for the high temperature metamorphism associated with crustal extension are presented. The age and significance of the crustal extension and magmatic advection are discussed taking into consideration the regional geological framework.