Geologia dell'area di Boccheggiano e Serrabottini (Colline Metallifere, Toscana meridionale)

Geological survey (see attached map, scale 1:10.000), and petrographic-structural studies in the Massa Marittima mining region (fig. 1) have provided new elements for determining the stratigraphic place of the so-called "Formazione Filladica di Boccheggiano" (Signorini, 1964; Costantini et alii, 1994). This formation is one of the most problematic and debated of the Palaeozoic Tuscan crystalline basement because of lack of fossils and its uncertain stratigraphic position in the context of the metamorphic Tuscany units (Bertini et alii, 1991; Costantini et alii, 1994). To reduce misunderstanding, we have not used the name "Formazione Filladica di Boccheggiano" because in Tuscany it indicates a group of rocks recognised both in outcrops and subsurface and considered in stratigraphic relationships. In this paper, we propose that these rocks are a tectonic wedge complex piled up during the Apennines orogeny. The wedge is composed of slices represented by portions of Triassic (Anidriti di Burano and Tocchi Formations) and by Palaeozoic pre-Sudetic (quartzitic-phyllites Group) and post-Sudetic (Filladi e quarziti del T. Mersino Formation) successions. The geological survey conducted in both the Serrabottini and Boccheggiano areas allowed recognition of several formations belonging to the following units or complexes (fig. 2). These are, from top to bottom: - Recent continental deposits; - Neoautochthonous Complex; - Ligurian units; - Tuscan Nappe Unit; - Monticiano-Roccastrada Unit. The Monticiano Roccastrada Unit is represented in outcrop by the Tocchi Formation (Costantini et alii, 1981) and Filladi del Torrente Mersino Formation (Bertini et alii, 1991). The latter has been studied in detail because of the main target of this study. A brief description of the formation is presented below. In the "Filladi e quarziti del Torrente Mersino" three main lithological associations are distinguished (fig. 3): A. Grey to black phyllites and quartzites (fM), consisting of phyllites (f) and quartzitic phyllites (fq) with thin beds of pale grey fine to coarse-grained quartzites (fgq) (figs. 4-6); B. Green phyllites and quartzites (fM1) made up of chlorite rich, often quartzitic phyllites and metapsammites (fig. 7); C. Quartzitic metaconglomerates (fM2) (fig. 8) consisting of lenticular, often erosive, decimetric to metric beds of pale grey poorly sorted metapsephites. Clasts are mainly composed of white quartz and minor, often graphitic, phyllites, quartzitic phyllites and microquartzites (lidites). The quartzitic-phyllitic- matrix, containing variable amounts of graphite, is abundant. Relationships between the above lithologies observed in both areas indicate that the grey to black phyllites and quartzites (fM) are the lowest part of the succession. Lenses of green phyllites and quartzites fM1 are found in fM, but normally fM1 rests on it. Quartzitic metaconglomerates (fM2) normally are at the top of the succession. The total thickness of the outcropping successions is more than 300 m. Their lithological and microstructural characteristics suggest a possible correlation with the Tuscan formations of Late Carboniferous-Early Permian age (i.e. "Scisti di S. Lorenzo", "Arenarie e Scisti di lano", "Formazione di Rio Marina", Bagnoli et alii, 1979; Puxeddu et alii, 1984). In addition, microstructural data reveal that these rocks are devoid of relicts of pre-Alpine schistosity .