3 - SOUTHERN TUSCANY - A - ARGENTARIO PROMONTORY

Abstract

The Argentario Promontory consists of a complex stack of Tuscan, Ligurian and Ligurian-Piedmontese Units (Lazzarotto et al., 1964; Decandia and Lazzarotto, 1980). From bottom to top they are (Fig. 1): 1. Monticiano-Roccastrada Unit. It comprises, from bottom to top: a- Mt. Argentario Formation. Graphitic phyllites with metasiltstone interbeds grading upwards into grey-whitish quartzites and metaconglomerates, with local graphite-rich phyllitic intercalations. This formation has been correlated to the Upper Carboniferous-Lower Permian successions cropping out along the Mid Tuscan Ridge (e.g. the St. Lorenzo Schists of the Pisani Mts. and Iano Schists and Sandstones) and in eastern Elba (Rio Marina Fm.). b- “Verrucano”. Alternating violet and greenish phyllites with local carbonate nodules and levels (caliche), quartzites and quartzitic white-pink metaconglomerates of Late Ladinian?- Carnian age. This succession, which shows an overall fining- and thinning-upward trend (including many metric/decametric positive cycles), is related to continental braided rivers passing upward to a coastal plain environment (Uncini, 1985). In the syntectonic quartz-rich segregations of the “Verrucano” rocks, Theye et al. (1997) found Mg-carpholite which testifies a HP-LT event (P> 8kb T = 300-400°C). c- Tocchi Formation. It consists of vacuolar, polymictic breccia made up of yellowish carbonatic and grey to green phyllitic clasts. These diagenetic and/or tectonic breccias derive from Carnian siliciclastic-carbonatic lithotypes which constitute the transition from the continental “Verrucano” and the Norian carbonatic shelf deposits. 2. Tuscan Nappe. This non-metamorphic unit is constituted by the “Calcare Cavernoso”: grey and dark grey, vacuolar calcareous and calcareous-dolomitic breccias with local karst structures. At places, these breccias include boulders and slices of stratified calcareous-dolomitic successions. Cataclastic textures are frequently recognisable. 3. Cala Piatti Unit. It is made up of Triassic recrystallised carbonate succession: a. Lower?-Middle Triassic stratified grey-pinkish limestones, b. Middle-Upper Triassic stratified black limestones, c. Upper Triassic? massive or poorly bedded grey dolostones. This succession represents one of the rare evidence of Early?-Middle Triassic marine sedimentation in the Northern Apennines (see also the Punta Bianca-Massa succession: Martini et al., 1986). 4. Cala Grande Unit. It is made up of HP-LT metamorphic rocks. In particular, two lithologic associations are recognisable: a. Varicoloured (reddish and grey-green) phyllites with grey and dark grey calcschists and recrystallised limestones; b. Metaophiolites (gabbros, basalts, serpentinites and ophiolitic breccias). Particularly, in b. HP-LP minerals are present: glaucophanitic amphibole (Mgriebeckite to crossite)+lawsonite+phengitic mica±Na-pyroxene (Gottardi, 1957; Ricci, 1972; Azzaro et al., 1977; Theye et al., 1997). The presence of Fe-carpholite relics within syn-tectonic quartz veins of the metasedimentary lithotypes (Theye et al., 1997) suggests T= 300-350°C and P>7 kb peak metamorphic conditions in agreement with the Blueschists facies minerals of the Metaophiolites. This unit was correlated to the “Schistes Lustrés” of the Alpine Corsica (Ricci, 1972; Ricci and Serri, 1975). Polymictic tectonic breccias, mainly made up of elements and slices of “Calcare Cavernoso” and/or “Verrucano” (but locally also of the two uppermost units), separate the Tuscan Nappe from the overlying Cala Piatti Unit and this latter from the Cala Grande Unit. The complex tectonic stack of Mt. Argentario is related to the Oligo-Miocene compressional stage (which produced the piling up of the “Schistes Lustrés” onto the Tuscan Units and the metamorphism of the Monticiano-Roccastrada Unit), but also to the post-compressional events (Burdigalian- early Tortonian) linked to the extensional tectonics of the Apennines and the Tyrrhenian Sea area (Bartole, 1995; Carmignani et al., 1995, cum bibl.). During the extensional events, the tectonic lamination of the Tuscan Nappe through low-angle normal faults, and the formation of the Mt. Argentario polymictic breccias occurred.