Abstract

In the Mt. Amiata volcano-geothermal area the Palaeozoic and Early-middle Triassic successions have been drilled by geothermal boreholes. The stratigraphic and structural features of these successions have been described in previous papers for the eastern part of the geothermal area (Piancastagnaio Geothermal field). In this paper, the geological features of the Palaeozoic and Triassic successions drilled in the Bagnore geothermal field and Poggio Nibbio area (W and S of the Mt. Amiata geothermal area, respectively) are presented and discussed in the framework of the inner Northern Apennines geological setting. Cuttings and cores samples of five geothermal boreholes drilling the upper crust down to 4500 m below the surface have been analysed. These boreholes drilled the stacked sedimentary tectonic units (Ligurian and Subligurian Units and Tuscan Nappe) overlying the Triassic and Paleozoic metamorphic successions. Below the Late Triassic evaporites, representing the lowermost stratigraphic unit of the Tuscan Nappe, the Verrucano Group (Early-Middle Triassic) and the phyllite succession (Palaeozoic) have been perforated. The Verrucano Group is represented by a stratigraphic succession (from 15 to 160 m in thick composed of quartzose metasandstones and metapelites. The lithological information deriving from cuttings and, rarely, from cores samples are insufficient to distinguish the formations of the Verucano Group recognised in the outcropping successions of southern Tuscany. The Palaeozoic phyllite succession consists of at least 2535 m of a monotonous succession, probably doubled by contractional structures, composed of micaceous-quartzose metasandstones and graphite-rich metapelites, showing comparable characters with: i) the Farma Fm. exposed in the Middle Tuscan Ridge; ii) the Formation A drilled in the Piancastagno geothermal field; iii) the Palaeozoic succession exposed in the Mt. Romani. A peculiar stratigraphic level for the Bagnore geothermal field has been recognised in the BG25 borehole within the core sampled at 2204-2206 m below the surface, where feldspar grains appear together with quartz and phyllosilicate grains. The rocks belonging to the Verrucano and Palaeozoic phyllite succession were affected by polyphase deformation during the Alpine-Apennines tectonic evolution. In particular, three superposed tectonic foliations characterise the rocks fabric: a) the first tectonic foliation developed under metamorphic conditions with a syn-kinematic paragenesis consisting of fine grained white mica, chlorite, quartz, carbonates, and oxides; b) the second tectonic foliation was accompanied by synkinematic metamorphism characterised by fine grained white mica, quartz and oxides, only recognised in the BG20, BG3bis, BG13 and BG25 boreholes; c) the third tectonic foliation was never accompanied by blastesis. The stratigraphic succession drilled by the PN8 borehole was also characterised by chloritoid, and calcite-quartz veins which superimposed on the second tectonic foliation (S2). The stratigraphic and structural features of the successions drilled in the Bagnore and Poggio Nibbio area agree with the geological setting described for the Palaeozoic and Early-Middle Triassic successions exposed in the southern Tuscany or drilled in the Piancastagnato field.

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