3 - SOUTHERN TUSCANY - B - COLLINE METALLIFERE

Abstract

In the last decades the geology of Southern Tuscany (Giannini et al., 1971; Boccaletti et al., 1982, cum bibl.) has been greatly improved by lithostratigraphic, biostratigraphic and geophysic studies, and by deep geothermal drillings (with a depth down to 3000 m b.g.l.) (Bertini et al., 1991; 1994; Elter and Pandeli, 1996, cum bibl.). The geological setting of this area is composed, from top to bottom, of the following tectono-stratigraphic units (Figs. 1 and 2): a. Neo-autochthon (post-orogenic). It is formed by the sediments filling the extensional basins, generally oriented along to the Apenninic chain. They unconformably lie on the stack of Ligurian and Tuscan Units, and are represented by lacustrine-marine (late Tortonian-Messinian), marine (Pliocene) and lacustrine-fluvial (early Villafranchian-Quaternary) sediments. b. Ligurian, sub-Ligurian and epi-Ligurian Units. They are composed of five tectonic units: 1- Upper Ophiolitic Unit (Late Jurassic-Early Cretaceous: Vara Supergroup), largely represented by Palombini Shales with some ophiolitic outcrops. 2- Lower Ophiolitic Unit, (Jurassic-Early Cretaceous: Vara Supergroup), including thick ophiolitic successions. This unit is stratigraphically capped by: 3- The syn-orogenic Upper Paleocene- Middle Eocene epi-Ligurian Lanciaia Fm. (ophiolitic breccias and sandstones, grading upward to a mainly carbonate-siliciclastic flysch). 4- Monteverdi Marittimo Unit (Southern Tuscany Elminthoid Flysch Group): consisting of a calcareousmarly- arenaceous flysch of Late Cretaceous- Early Paleocene age. 5- Canétolo Unit (Paleocene- Middle/Late Eocene), made up of shales with limestone, calcarenite and graywacke intercalations. This unit belongs to the sub-Ligurian paleogeographic Domain (close to the Adria margin). c. Tuscan Nappe. It is composed, from top to bottom, of the following non-metamorphic formations: Macigno (Late Oligocene-Early Miocene), “Scisti Policromi” (Early Cretaceous- Oligocene), Tuscan Cherts (Malm), Posidonia Marlstones (Dogger), “Rosso Ammonitico” and Limano Cherty Limestones (Middle-Late Liassic), “Calcare Massiccio” (Late Triassic?-Early Liassic), Spezia Fm. (Rhaetian), Burano Anhydrites (Norian-Rhaetian). In this succession lateralvertical facies changes and sedimentary unconformities are present, mainly in Middle-Late Jurassic times. Furthermore, the Tuscan Nappe is often tectonically laminated or locally lacks (“Serie Toscana Ridotta” = Reduced Tuscan Series Auctt.) by low-angle extensional faulting with flat-ramp-flat geometry (Fig. 3). The Tuscan Nappe lies on the Tuscan Metamorphic succession (Monticiano-Roccastrada Unit) which is quoted below. d. Monticiano-Roccastrada Unit. The upper part of this unit is a complex pile of tectonic slices made up of epimetamorphic Lower to Upper Paleozoic formations, Triassic “Verrucano”, Tocchi Fm. and non-metamorphic Upper Triassic anhydrite-carbonate successions. The lower part is made up of ?Upper Cambrian-?Lower Ordovician siliciclastics and metabasites (Phyllitic-quartzitic Complex), which include relics of Hercynian schistosity. In the subsurface of Larderello geothermal area, the Phyllitic-quartzitic Complex overlies ?Lower Paleozoic/?pre-Paleozoic Micaschist Complex (see Stop 5). The Colline Metallifere area is characterised by high heat flow (>650 mWm-2 in the Larderello geothermal field), by lithosphere thinning (Moho at a depth of about 22 km in the Grosseto area), by the uplift of the metasomatised and partially melted mantle, and by anatectic crustal magmatism (Boccaletti et al., 1985; Mongelli et al., 1991).