THE HP-LP META-OPHIOLITIC UNIT AND VERRUCANO OF THE CALA GRANDE AREA IN THE ARGENTARIO PROMONTORY (SOUTHERN TUSCANY, ITALY): STRUCTURAL-METAMORPHIC EVOLUTION AND REGIONAL CONSIDERATIONS

Abstract

Ophiolite-bearing HP-LT metamorphic rocks (Schistes Lustres or Calcschists with ophiolithes Auctt.) are present in southern Tuscany and Tuscan Archipelago (Mt. Argentario Promontory, Giglio Island, Elba Island and Gorgona Island). They are interpreted as fragments of oceanic crust which experienced an initial HP-LT metamorphic evolution during the closure of the Ligurian ocean and successively, mooving eastwards, were enclosed together the Ligurian and Tuscan Units in the Apennine chain. The HP-LT ophiolitic succession of the Mt. Argentario is that of the Cala Grande Unit which is constituted by grey-green to red-violet metapelites, marble, calc- schists, metacherts and associated metamorphic ophiolitic rocks (metaserpentinites, metagabbros and metabasalts). The petrographical-structural studies performed by the authors allow to reconstruct the complex metamorphic and tectonic evolution of such successions. In the metabasites, the primary femic magmatic minerals and those of the oceanic metamorphism are overprinted by: 1) a polystadial HP-LT event in the blueschist facies (Dp, peak conditions P = 7-8kb for T = 340-350°C); 2) a greenschist facies event(s) characterized by the chloritization of the Na-amphiboles, development of a main penetrative schistosity and of top-to-NE and top-to-SW shear bands. In the associated metasediments, the HP-LT event is testified by the schistosity of the NE-vergent Dp event which is overprinted by two greenschists tectono-metamorphic events (NE-vergent Dgs1 and SW-vergent Dgs2). The tectonic slices of Verrucano (Triassic quartz-rich metasediments of the Tuscan Monticiano-Roccastrada Unit) intercalated in the Cala Grande Unit show a similar structural-metamorphic evolution, but with different orientation of the mineralogic lineations within the D1 foliations. This data point to a post-D1 emplacement of the Verrucano slices onto the Cala Grande Unit, probably during the D2 event of the Northern Apennines tectogenesis. The structural-metamorphic evolution of the studied area is also compared to that of other metamorphic successions of the Tuscan Archipelago and of the Southern Tuscany.