The Triassic and Palaeozoic successions drilled in

BROGI (Boll. Vol. 127 Fasc.3-2008)

Boll.Soc.Geol.It. (Ital.J.Geosci.), Vol. 127, No. 3 (2008), pp. 599-613, 8 figs., 2 tabs.

Queste bozze, corrette e accompagnate dall’allegato preventivo firmato e dal buono d’ordine, debbono essere restituite immediatamente alla Segreteria della Società Geologica Italiana c/o Dipartimento di Scienze della Terra Piazzale Aldo Moro, 5 – 00185 ROMA

The Triassic and Palaeozoic successions drilled in the Bagnore geothermal field and Poggio Nibbio area (Monte Amiata, Northern Apennines, Italy) ANDREA BROGI (*)

ABSTRACT

RIASSUNTO

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 Palaeozoic 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 thickness) mainly 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 Verrucano 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 graphiterich metapelites, showing comparable characters with: i) the Farma Fm. exposed in the Middle Tuscan Ridge; ii) the Formation A drilled in the Piancastagnaio 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 AlpineApennines 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 syn-kinematic 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 Piancastagnaio field.

Le successioni triassiche e paleozoiche perforate nelle aree geotermiche di Bagnore e Poggio Nibbio (Monte Amiata, Appennino Settentrionale).

KEY WORDS: Triassic succession, Palaeozoic succession, Tuscan Metamorphic Units, Mt. Amiata geothermal area, Northern Apennines.

(*) Dipartimento di Scienze della Terra, Università di Siena, Via Laterino, 8 - 53100 Siena. E-mail: [email protected]

Nell’area vulcanica del Monte Amiata le successioni paleozoiche e del Triassico inferiore-medio sono state incontrate soltanto in sondaggio durante l’esplorazione geotermica. Le caratteristiche stratigrafiche e strutturali di tali successioni sono state descritte in dettaglio nell’area più orientale del Monte Amiata, nel campo geotermico di Piancastagnaio (ELTER & PANDELI, 1991). In questo lavoro vengono descritte le caratteristiche geologiche delle successioni metamorfiche incontrate in sondaggio nel campo geotermico di Bagnore e nell’area mineraria di Poggio Nibbio, collocati rispettivamente a sudovest ed a sud dell’edificio vulcanico del Monte Amiata. Sono state analizzate le successioni stratigrafiche attraversate da cinque sondaggi profondi, uno dei quali (BAG25) ha raggiunto la quota di 4500 m al di sotto della superficie. Tutti i sondaggi hanno attraversato le unità sedimentarie di copertura dell’Appennino Settentrionale (Unità Liguri, Subligure e Falda Toscana) che si trovano al di sopra delle successioni metamorfiche triassiche e paleozoiche. Al di sotto delle evaporiti del Triassico superiore, che costituiscono la base della Falda Toscana, sono state incontrate le successioni filladico-quarzitiche del Gruppo del Verrucano (Triassico inferiore-medio) e della successione filladica paleozoica. Il Gruppo del Verrucano è costituito da una successione spessa da 15 a 160 m, principalmente composta da metarenarie e metapeliti quarzose. Le informazioni litologiche ricavate dall’analisi dei cuttings e, più raramente, delle carote sono scarse e quindi insufficienti per distinguere le formazioni riconosciute e descritte nelle successioni di Verrucano affioranti in Toscana meridionale (COSTANTINI et alii, 1988; ALDINUCCI et alii, 2003). Per quanto riguarda la successione filladica paleozoica, essa è rappresentata da una successione monotona, spessa almeno 2535 m, probabilmente interessata da raddoppi tettonici, costituita da metarenarie quarzoso-micacee e da metapeliti e filladi carboniose che, per le loro caratteristiche litologico-petrografiche, possono essere confrontabili con alcune successioni paleozoiche descritte in affioramento o attraversate da sondaggi: Formazione del Farma descritta nella Dorsale Monticiano-Roccastrada (COSTANTINI et alii, 1988), Formazione A descritta nel sottosuolo del campo geotermico di Piancastagnaio (PANDELI & PASINI, 1990; ELTER & PANDELI, 1991), Successione paleozoica descritta nei Monti Romani (BAGNOLI et alii, 1980; MORETTI et alii, 1991). Un livello stratigrafico peculiare è stato riconosciuto nel sondaggio BG25 in corrispondenza della carota prelevata alla profondità di 2204-2206 m, dove sono stati osservati livelli di metarenarie quarzoso-micacee contenenti diffusi clasti alterati di feldspato. Le rocce appartenenti al Gruppo del Verrucano ed alla successione filladica paleozoica sono state interessate da una evoluzione tettonica polifasata, sviluppata durante l’evoluzione tettonica alpinoappenninica. In particolare, sono state riconosciute tre foliazioni tettoniche sovrimposte. La prima si è sviluppata in condizioni di metamorfismo, associato al quale si sono formati: mica bianca, clorite, quarzo, carbonati e ossidi. La seconda foliazione tettonica, anch’essa sviluppata in condizioni metamorfiche, è stata accompagnata da una paragenesi costituita da mica bianca, quarzo e ossidi, riconosciuta nella maggior parte dei sondaggi (BG20, BG3bis, BG13, BG25); la terza foliazione, sviluppata soprattutto nei livelli fillosilicatici, non è stata accompagnata da metamorfismo. Nell’area di Poggio Nibbio, nel sondaggio PN8 sono state riconosciute associazioni mineralogiche post-cinematiche, principalmente rappresentate da cloritoide,

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The Palaeozoic and Early-Middle Triassic rocks of the Northern Apennines collisional belt belong to the deepest tectonic units piled upon the African margin during the collisional stage (CARMIGNANI et alii, 2001). In spite of this, the Palaeozoic and Triassic successions are discontinuously exposed in the hinterland of the Northern Apennines (i.e. southern Tuscany and Northern Tyrrhenian Sea). They are mainly exposed in structural highs cropping out in the southern Tuscany and Tuscan Archipelago (CARMIGNANI & LAZZAROTTO, 2004) (fig. 1) and have been also drilled in the Larderello-Travale geothermal area and the Colline Metallifere region during geothermal and mining exploitations (CASTELLUCCI et alii, 1983; GIANELLI et alii, 1988; PANDELI et alii, 1991; BERTINI et alii, 1991, 1994; ELTER & PANDELI, 1990, 1994; BATINI et alii, 2003; PANDELI et alii, 2005a). Their occurrence at shallow depth is due to the exhumation of the deeper crustal levels as

consequence of the extensional tectonics (BROGI, 2008) started from the Early-Middle Miocene (CARMIGNANI et alii, 1994). The Palaeozoic and Early-Middle Triassic successions described in southern Tuscany recorded polyphase tectonics during the Northern Apennines tectonic evolution (COSTANTINI et alii, 1988; ELTER & PANDELI, 1990, 1994; CONTI et alii, 1991; LAZZAROTTO et alii, 2003). Nevertheless, pre-Visean successions also recorded oldest tectono-metamorphic events. The most representative event is indicated by the occurrence of a relic tectonic foliation with associated a low-grade metamorphism, developed during the evolution of the Variscan oregenic belt (CONTI et alii, 1991; COSTANTINI et alii, 2002; PANDELI et alii, 1994, 2005). Additionally, the tectono-metamorphic events related to the Northern Apennines tectonic evolution were associated to syn-kinematic HP-LT and greenschist grade metamorphisms (THEYE et alii, 1997; GIORGETTI et alii, 1998; BRUNET et alii, 2000). HT-LP metamorphism, due to the emplacement of granitoids at shallow levels coevally with the extensional regime, locally superimposed on the previous syn-kinematic metamorphic events (DEL MORO et alii, 1982; ROSSETTI et alii, 2008). During the Northern Apennines collisional stage, the Palaeozoic and Triassic successions were imbricated in duplex structures which gave rise to a very thick tectonic wedge known as the «complesso a scaglie» (PANDELI et alii, 1991), hereafter the CS (fig. 2). The CS was overlie by a very thick sedimentary tectonic pile composed of the Tuscan Nappe (the deepest sedimentary tectonic unit) and the Subligurian and Ligurian Units (CARMIGNANI et

Fig. 1 - Location of the study area. The dark grey areas indicate the outcrops of the metamorphic successions, Early-Middle Triassic and Palaeozoic in age. Light grey areas indicate the outcrops of the sedimentary tectonic units and Neogene-Quaternary deposits. – Ubicazione dell’area di studio. Le aree rappresentate dal colore grigio scuro indicano gli affioramenti di successioni metamorfiche del Triassico inferiore-medio e del Paleozoico. Le aree di colore grigio chiaro indicano gli affioramenti delle unità sedimentarie di copertura ed i depositi neogenico-quaternari.

Fig. 2 - Tectonostratigraphic column reconstructed in the LarderelloTravale geothermal area. CS: «complesso a scaglie» (see the text for more information). – Rapporti tettono-stratigrafici tra le unità riconosciute nell’area geotermica di Larderello-Travale. CS: «Complesso a scaglie» (si veda il testo per informazioni più dettagliate).

calcite e quarzo, che si sono sviluppate successivamente alla seconda foliazione tettonica (S2). La calcite ed il quarzo, inoltre, sono presenti in vene millimetriche e centimetriche che si sono sviluppate successivamente alla foliazione tettonica S3. Le caratteristiche stratigrafiche e strutturali delle successioni metamorfiche non presentano particolari differenze da quanto documentato nelle successioni affioranti della Toscana meridionale e, nel loro insieme, sono confrontabili con quelle descritte nel vicino campo geotermico di Piancastagnaio.

TERMINI CHIAVE: Successione triassica, successione paleozoica, Unità Metamorfiche toscane, area geotermica del Monte Amiata, Appennino Settentrionale. INTRODUCTION

TRIASSIC AND PALAEOZOIC SUCCESSIONS DRILLED IN THE MT. AMIATA AREA

alii, 2001). The Late Triassic evaporites (Burano Fm.) correspond to the stratigraphic horizon separating the sedimentary tectonic pile from the CS. This formation was also tectonically interbedded within the CS (PANDELI et alii, 1991; BERTINI et alii, 1994b; BROGI & LIOTTA, 2006) (fig. 2) and also separates the sedimentary tectonic pile from the metamorphic successions. The Mt. Amiata volcano-geothermal area (figs. 1 and 3) is characterised by broad expositions of Subligurian and Ligurian Units and Tuscan Nappe, unconformably overlie by Miocene and Pliocene continental and marine sediments (CALAMAI et alii, 1970) (fig. 3). Deep geothermal boreholes represent the only way to investigate the metamorphic successions in the Mt. Amiata region. The stratigraphic and structural features of these successions have been already described for the eastern part of the geothermal area (Piancastagnaio geothermal field, PANDELI et alii, 1988; PANDELI & PASINI, 1990; ELTER & PANDELI, 1991). However, in this paper we describe the coeval successions drilled in the Bagnore geothermal field and Poggio Nibbio area (SW and S of the Mt. Amiata volcano, respectively, see fig. 3), where boreholes reaching down to 4,5 km below the topographic surface intercepted a very thick succession of these metamorphic rocks. In particular, the stratigraphic logs and the microstructural setting of the Palaeozoic and Triassic successions drilled by 5 boreholes (fig. 3), perforated during the 1980’s and 1990’s, are illustrated and discussed in the framework of the Mt. Amiata and inner Northern Apennines geological settings.

GEOLOGICAL BACKGROUND

The geological setting of the Mt. Amiata area derives from collisional and post-collisional tectonics which affected the Northern Apennines from the Early Cretaceous (BOCCALETTI et alii, 1981). Most authors suppose that convergent and collisional stages (Cretaceous-Early Miocene) determined the east-verging structural emplacement of the tectonic units, whereas a subsequent extensional stage (Early Miocene-Quaternary) produced the thinning of the overthickened crust and lithosphere (JOLIVET et alii, 1990; BERTINI et alii, 1991; CARMIGNANI et alii, 1994; BALDI et alii, 1995; MANTOVANI et alii, 1995, 1997; BARCHI et alii, 1998; LIOTTA et alii, 1998; BROGI et alii, 2003; LAVECCHIA et alii, 2004), resulting in about 20-22 and 30-50 km respectively (PANZA et alii, 1980; PONZIANI et alii, 1994; DECANDIA et alii, 1998; LAVECCHIA et alii, 2004). Extension produced also a widespread Neogene magmatism (PUXEDDU, 1984; SERRI et alii, 1993; ACOCELLA & ROSSETTI, 2002; ROSSETTI et alii, 1998; PECCERILLO et alii, 2001; DINI et alii, 2005) and the rapid surface uplift (DALLMEYER et alii, 1995; DALLMEYER & LIOTTA, 1998). This has been evaluated in the Mt. Amiata region as being at least 2000 m (PASQUARÉ et alii, 1983; GIANELLI et alii, 1988; DISPERATI & LIOTTA, 1998; ACOCELLA, 2000). Several regional tectono-stratigraphic units characterise the upper crust of the Mt. Amiata volcano-geothermal area; they are, from top to bottom (fig. 4a). The magmatic complex. This is mainly represented by the Mt. Amiata volcano, composed of acid and intermediate products (FERRARI et alii, 1996, cum bibl.). This complex consists of dacitic, rhyodacitic and olivine-latitic rocks containing mafic enclaves, erupted in a period ranging from 300 to 190 ka (FERRARI et alii, 1996).

601

Neogene and Quaternary deposits (M-P-Q in fig. 4a). They consist of Middle Miocene-Quaternary continental and marine sediments filling the tectonic depressions, and Quaternary travertines. These deposits unconformably overlie the pre-Neogene substratum. The Ligurian and Subligurian Units. This includes the Ligurian Units (S. Fiora Unit: Early Cretaceous-Eocene; and Ophiolitic Unit: Middle Jurassic-Early Cretaceous) and the Subligurian Unit (Eocene-Oligocene) (fig. 4b). The Ligurian Units are composed of remnants of the Jurassic oceanic basement and its pelagic sedimentary cover. The Subligurian Unit is composed of a pelagic sequence belonging to a paleogeographical domain interposed between the Ligurian Domain and the Tuscan Domain. The Ligurian and Subligurian Units were thrust eastwards over the Tuscan Domain during the Latest Oligocene-Early Miocene. The Tuscan Nappe (TN in figs. 4a and 4c). This is related to part of the Late Triassic-Early Miocene sedimentary cover of the Adria continental palaeomargin. Its stratigraphic succession is made up of (from bottom to top): evaporite (Late Triassic), carbonate (Late TriassicEarly Cretaceous) and pelagic-turbiditic (CretaceousEarly Miocene) formations. The Tuscan Nappe was detached from its substratum along the Triassic evaporite horizon and was thrust over the outer palaeogeographical domains (Umbria-Marchigian Domain) during the Late Oligocene-Early Miocene contraction. Below the Tuscan Nappe, the Tuscan Metamorphic Complex has been encountered by deep boreholes (ELTER & PANDELI, 1991). This complex is composed only of the Monticiano-Roccastrada Unit (fig. 4a), which has been drilled down to 4500 m. The complex is composed of metamorphic sequences belonging to two groups: (a) the Triassic Verrucano Group (MRU3 in fig. 4a), composed of continental metapelites, metasandstones and metaconglomerates; (b) the Palaeozoic phyllite succession (MRU2 in fig. 4a) described for the Piancastagnaio area (fig. 4d), composed of graphitic phyllites and metasandstones of probable Carboniferous age (Formation A), ?Devonian hematite-rich and chlorite phyllites (Formation B), metasandstones with dolostone layers, and Late Permian fusulinid-bearing crystalline limestones and dolostones with intercalations of graphitic phyllites (Formation C) (PANDELI et alii, 1988; PANDELI & PASINI, 1990; ELTER & PANDELI, 1991). The occurrence of the Micaschist Group (MRU1 in fig. 4a) and possibly the Gneiss Complex (GC in fig. 4a) at depth has been documented by xenoliths in the Quaternary lavas (VAN BERGEN, 1983). Evidence for Middle-Late Miocene and Early-Middle Pliocene marine to continental sediments have been found in outcrops and deep boreholes in the surroundings of the Mt. Amiata volcano-geothermal area (fig. 3). These deposits unconformably overlie the previously described stratigraphic units, and infill the surrounding structural depressions (Cinigiano-Baccinello Basin: BOSSIO et alii, 1993, 1994; LANDI et alii, 1995; BENVENUTI et alii, 2001; the Velona and Val d’Orcia basins: BOSSIO et alii, 1993; BONINI et alii, 1999; GHETTI et alii, 2002; the Radicofani Basin: BOSSIO et alii, 1993; LIOTTA & SALVATORINI, 1994; LIOTTA, 1994, 1996; ACOCELLA et alii, 2002; BONINI & SANI, 2002) (fig. 3). The bedding orientation of the Pliocene sediments suggests the occurrence of a broad dome centred on the Mt. Amiata volcano which

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Fig. 3 - Geological sketch-map of the Mt. Amiata volcano-geothermal region and geological section across the Bagnore geothermal field. The location of the analysed boreholes are also indicated in the map. – Carta geologica schematica dell’area vulcanica e geotermica del Monte Amiata e sezione geologica attraverso il campo geotermico di Bagnore. Sono inoltre indicati i pozzi geotermici presi in considerazione in questo lavoro.


603

Fig. 4 - (a) Tectonostratigraphic units in the Mt. Amiata area; M-P-Q: Miocene, Pliocene and Quaternary sediments; MR – magmatic rocks; Tuscan Nappe (TN): TN2 – Early Miocene-Rhaetian sequence, TN1 – Late Triassic evaporites; Monticiano-Roccastrada Unit (MRU): MRU3 – Triassic Verrucano Group; MRU2 - Palaeozoic Phyllite-Quartzite Group; MRU1 – Palaeozoic Micaschist Group; GC – Gneiss Complex (from BATINI et alii, 2003). (b) Structural and stratigraphic relationships between the Ligurian and Subligurian Units. Ophiolitic Unit - S, G, D: serpentinites, gabbros, basalts, respectively Middle-Late Jurassic, gc1: Mt. Alpe cherts (Late Jurassic), c1: Calpionella Limestone (Early Cretaceous), c2: Palombini Shales (Early Cretaceous); S. Fiora Unit: SFF: Santa Fiora Fm. (Late Cretaceous), Pf: Pietraforte Fm. (Late Cretaceous), MMF: Monte Morello Fm. (Paleocene-Eocene); Subligurian Unit: CF: Canetolo Fm. (Paleocene-Eocene) (from BATINI et alii, 2003). (c) Stratigraphical succession of the Tuscan Nappe; Symbols: O - Macigno Fm. (Late Oligocene-Early Miocene), ce - Scaglia Toscana Fm. (Cretaceous-Early Oligocene), c1 - Maiolica Fm. (Early Cretaceous), g3 - Diaspri Fm. (Malm), g2m - Marne a Posidonomya Fm. (Dogger), g2s - Calcare Selcifero Fm. (Middle-Late Liassic), g2 - Calcare Rosso ammonitico Fm. (Early-Middle Liassic), g1 - Calcare Massiccio Fm. (Early Liassic), t2 - Calcari a Rhaetavicula contorta Fm. (Rhaetian), t1 - Burano Fm. and Calcare Cavernoso Fm. (Norian-Rhaetian) (from BATINI et alii, 2003). (d) Relationships between the Triassic and Palaeozoic formations belonging to the Monticiano-Roccastrada Unit drilled by geothermal wells in the subsurface of the Mt. Amiata area (from ELTER & PANDELI, 1991). – (a) Unità tettono-stratigrafiche nell’area del Monte Amiata; M-P-Q: Depositi miocenici, pliocenicie quaternari; MR – rocce magmatiche; Falda Toscana (TN): TN2 – successione compresa tra il Retico ed il Miocene inferiore, TN1 – evaporiti del Triassico superiore; Unità di Monticiano-Roccastrada (MRU): MRU3 – Gruppo del Verrucano (Triassico inf.-medio); MRU2 – Gruppo filladico-quarzitico (Paleozoico); MRU1 – Gruppo dei micascisti (Paleozoico); GC – Complesso degli Gneiss (da BATINI et alii, 2003); (b) Relazioni stratigrafiche e tettoniche tra le Unità Liguri e Subliguri. Unità Ofiolitifera - S, G, D: serpentiniti, gabbri, basalti, Giurassico medio-superiore; gc1: Radiolariti del Monte Alpe (Giurassico superiore), c1: Calcari a Calpionella (Cretaceo inf.), c2: Argille a Palombini (Cretaceo inf.); Unità di S. Fiora: SFF: Formazione di Santa Fiora (Cretaceo sup.), Pf: Pietraforte (Cretaceo sup.), MMF: Formazione di Monte Morello (Paleocene-Eocene); Unità Subligure: CF: Formazione di Canetolo (Paleocene-Eocene) (da BATINI et alii, 2003). (c) Successione stratigrafica della Falda Toscana; Simboli: O – Formazione del Macigno (Oligocene sup.-Miocene inf.), ce - Scaglia Toscana (Cretaceo-Oligocene inf.), c1 - Maiolica (Cretaceo inf.), g3 - Diaspri (Malm), g2m - Marne a Posidonomya (Dogger), g2s - Calcare Selcifero (Lias medio-sup.), g2 - Calcare Rosso ammonitico (Liassico inf.-medio), g1 - Calcare Massiccio (Lias inf.), t2 - Calcari a Rhaetavicula contorta (Retico), t1 – Anidridi di Burano e Calcare Cavernoso (Norico-Retico) (from BATINI et alii, 2003). (c) Relazioni tra le formazioni triassiche e paleozoiche appartenenti all’Unità di Monticiano Roccastrada perforate dai pozzi geotermici nel sottosuolo del campo geotermico di Piancastagnaio (from ELTER & PANDELI, 1991).

controls the geometric setting of the mainly Pliocene deposits (ACOCELLA, 2000). Small outcrops of Early Pliocene marine sediments, located close to the dome culmination, exceed the present-day altitude of about 900 m (CALAMAI et alii, 1970). They unconformably overlie the previously deformed Ligurian and Subligurian Units, which are laterally discontinuous and, in other cases, characterised by widespread reduced thickness, due to the lack of some stratigraphical units. Nevertheless, the Ophiolitic Unit, the uppermost Ligurian Unit in the tectonic pile, locally overlies the basal succession of the Tuscan Nappe (Triassic evaporites), suggesting the omission of both the carbonate and pelagic-turbiditic Tuscan Nappe successions, the Subligurian and part of the Ligurian Units. In this light, the contacts separating the Ligurian and Subligurian Units from the Tuscan Nappe correspond, at present, to low-angle normal faults (LANFs), as deeply discussed in BROGI (2004, 2006a). High-angle normal and transtensional faults, Pliocene and Quaternary in age, respectively, displaced the previously developed structures.

BOREHOLES

The analysed boreholes are: BG3bis, BG13, BG20, BG25 and PN8. They were conventional rotary-drilled large-diameter wells perforated by ENEL GreenPower for geothermal purposes. Their location is given in fig. 3. They encountered the sedimentary and metamorphic successions forming the inner Northern Apennines tectonic pile. The stratigraphic logs were sampled as cuttings (diameter up to 2 cm) or, rarely, as cores up to 3 m long. Thin sections of cuttings and cores of the Triassic and Palaeozoic rocks have been considered for this study (table 1). The sedimentary rocks, overlying the metamorphic ones, belong to the shallowest tectonic units, such as the Ligurian and Subligurian Units and the Tuscan Nappe (fig. 5). The drilled Ligurian Units are mainly composed of the Ophiolitic Unit consisting of the Argille a Palombini Fm., and the S. Fiora Unit, characterised by the calcareous-shaly succession with interbedded a very thick arenaceous turbiditic level (Pietraforte) reaching up to

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TABLE 1 Table indicating the analysed samples; the numbers coincide with the sampling depth. cu = cutting; co = core. – Tabella che indica i campioni analizzati. La numerazione di ciascun campione coincide con la sua profondità di campionamento. cu = cutting; co = carota.

Borehole BG3bis

Depth below the sea level and typology of sample (cu=cuttings; co=core) 1665cu

1670cu

1690cu

1700cu

1738cu

1790cu

1825cu

1850cu

1885cu

1920cu

1980cu

2020cu

2040cu

2080cu

2100cu

2108co

2111co

2160cu

2170cu

2230cu

2260cu

2280cu

2360cu

2370cu

2440cu

2460cu

2510cu

2550cu

2610cu

2661co

2664co

2680cu

2740cu

2800cu

2870cu

2970cu

3010cu

3111co

3113co

1481co

1484co

2020cu

2050cu

2100cu

2130cu

2150cu

2200cu

2250cu

2270cu

2300cu

2320cu

2340cu

2350cu

2400cu

2460cu

2500cu

2540cu

2620cu

2660cu

2700cu

2760cu

2800cu

2860cu

2880cu

2900cu

2960cu

3000cu

3050cu

3219co

3221co

BG20

1507co

1510co

2900co

2903co

3032co

3035co

BG25

1870cu

2005cu

2010cu

2020cu

2050cu

2100cu

2150cu

2200cu

2204co

2206co

2250cu

2270cu

2300cu

2380cu

2450cu

2500cu

2610cu

2640cu

2690cu

2730cu

2870cu

2910cu

3663co

3666co

BG13

PN8

900cu

910cu

920cu

930cu

940cu

960cu

970cu

980cu

990cu

1000cu

1010cu

1040cu

1050cu

1055cu

1060cu

1070cu

1080cu

1090cu

1100cu

1105cu

1100cu

1124co

1126co

1450cu

1850cu

1900cu

1908co

2450cu

2500cu

2550cu

2600cu

2650cu

2700cu

2750cu

2900cu

3000cu

3050cu

3080co

3261co

3262co

390 m, as documented in the borehole PN8. The Ligurian and Subligurian Units are separated from the Early-Middle Triassic and Palaeozoic metamorphic successions by means of the Late Triassic evaporites. Such a succession is characterised by the alternation of gypsum-anidrite and dark dolomite levels containing several brecciated levels (up to 100 m thick), drilled at different depths. The Tuscan Nappe Jurassic-Cretaceous carbonate succession has been not encountered by the analysed boreholes. Nevertheless, the Cretaceous-Oligocene pelagic-turbiditic Tuscan Nappe succession (Scaglia Toscana Fm.) was only drilled by the BG25 borehole. Such a borehole, encountered the Scaglia Toscana Fm. directly on the Late Triassic evaporites. Furthermore, the BG3b, BG13, BG20 and PN8 encountered the Ligurian Units (Ophiolitic Unit and/or S. Fiora Unit) on the Triassic evaporites. The metamorphic rocks of the Verrucano Group (Early-Middle Triassic) (MRU3 in fig. 4a) and the Palaeozoic phyllite succession (MRU2 in fig. 4a) were encountered below the Late Triassic evaporites. The Verrucano Group consists of a stratigraphic succession (from 15 to 160 m) mainly composed of quartzose metasandstones and metapelites. The lithological information deriving from cuttings and cores are insufficient to distinguish the stratigraphic units described in the outcropping successions of the southern Tuscany (COSTANTINI et alii, 1988; ALDINUCCI et alii, 2003). The Palaeozoic phyllite succes-

sion consists of at least 2535 m of a monotonous succession, probably doubled by collisional structures, composed of micaceous-quartzose metasandstones and graphiterich metapelites and phyllites. The stratigraphic features of the considered borehole logs have been resumed in the fig. 5. MICROSTRUCTURAL

SETTING

OF

THE

DRILLED

META-

MORPHIC ROCKS

Microstructural analysis has been carried out in thin sections of cores and cuttings collected from different depths, as indicated in the tables 1 and 2. In both the Early-Middle Triassic and Palaeozoic successions three widespread superimposed tectonic foliations have been recognised, suggesting a similar tectonic evolution that interested both the Triassic and Palaeozoic successions. The oldest tectonic foliations (S1 and S2) were associated with syn-kinematic metamorphic events; in contrast, the latest foliation (S3) was never accompanied by blastesis. The S1 is accompanied by the M1 metamorphic event, mainly consisting of fine-grained muscovite + quartz ± chlorite ± carbonates ± oxides (figs. 7A and 7B). The S1 foliation can be described as continuous schistosity (sensu PASSCHIER & TROUW, 1996) mainly defined by elongate quartz grains, and non-layered homogeneous distribution of platy mineral grains with a preferred orientation (fig. 7A). The planar fabric is mainly defined by


Topographic surface

BG 20

BG 25

BG 3bis

BG 13

PN 8

0m

250

500

750

1000

1250

1500

1750

2000

2250

2500

2750

3000

3250

3500 Volcanic Rocks 300/190ka

3750

4000

Ligurian and Subligurian Units Ophiolitic Unit, S. Fiora Unit, Canetolo Unit Jurassic-Eocene

TuscanNappe 4250

Pelagic-turbiditic succession Cretaceous-Early Miocene Evaporite succession Late Trias

4500

Monticiano Roccastrada Unit 4750

Verrucano Group Early-Middle Trias Phyllite-succession Late Palaeozoic

Fig. 5 - Simplified stratigraphic logs of the analysed boreholes. – Colonne stratigrafiche semplificate dei sondaggi analizzati.

605

606

A. BROGI

TABLE 2a

TABLE 2c

Lithological and structural features of the analysed samples collected by the BG20 borehole. – Caratteristiche litologiche e strutturali dei campioni derivanti dal pozzo BG20.

Lithological and structural features of the analysed samples collected in the PN8 borehole. – Caratteristiche litologiche e strutturali dei campioni derivanti dal pozzo PN8.

BG 20

PN 8

Sample

Lithology

Foliation

Sample

Lithology

Structures

1507

Phyllite

S1, S2, S3

900

Metasandstone

S1

1507

Metasiltite

S1

910

Metasandstone

S1

Metasandstone

S1, S2

1510

Calcschist

S1

920

2900

Graphitic metasandstone

S1, S2

930

Metasandstone

S1

940

Metasandstone

S1, S2

960

Metasandstone

S1, S2

970

Metasandstone

S1, S2

980

Metasandstone

S1

990

Metasandstone

S1

1000

Metasandstone, metapelite

S1, S2

1010

Metasandstone, graphitic metapelite

S1, S2

1040


S1, S2

1050


S1

1055


S1

1060


S1

1070


S1

1080

Metasandstone, graphitic phyllite

S1

1090


S1

1100


S1, S2

1105


S1

1110


S1

1124

Metapelite

S1, S2

1126

Metapelite

S1, S2

1450


S1, S2

2903

Graphitic metasandstone

S1, S2

3032

Graphitic metasiltite

S1, S2, S3

3035

Graphitic metasiltite

S1, S2, S3

TABLE 2b Lithological and structural features of the analysed samples collected in the BG13 borehole. – Caratteristiche litologiche e strutturali dei campioni derivanti dal pozzo BG13.

BG 13 Sample

Lithology

Structures

1481

Metasandstone

S1

1850


S1, S2

1484

Metasandstone

S1

1900


S1

2020


S1, S2, S3

1909


S1

2050


S1, S2, S3

2450


S1, S2

2100


S1, S2

2500


S1, S2


S1, S2

2130

Metasandstone

S1

2550

2150


S1, S2

2600


S1, S2

S1, S2

2650


S1, S2

2700


S1, S2

2750


S1, S2

2900


S1, S2

3000


S1, S2

3050


S1

3080


S1, S2

3261

Metasandstone

S1, S2

3262

Metasandstone

S1, S2

2200

Metasandstone

2250


S1, S2

2270


S1, S2, S3

2300


S1, S2

2320


S1, S2, S3

2340


S1, S2

2350


S1, S2

2400


S1, S2

2460


S1, S2

2500


S1, S2

2540


S1, S2

2620


S1, S2

2660


S1, S2, S3

2700


S1, S2, S3

2760


S1, S2, S3

2800


S1, S2

2860


S1, S2

2880


S1, S2, S3

2900


S1, S2

2960


S1, S2

3000


S1, S2, S3

3050

Metasandstone

S1, S2

3219


S1

3221


S1

flattened crystals, such as quartz, and, rarely, by rotation/reorientation of detrital phyllosilicates such as white mica and chlorite. Pressure solution and solution transfer (RUTTER, 1976; SPIERS et alii, 1990) were the main mechanisms that produced the flattening of the quartz grains, which enhanced preferential grain growth in the direction of the tectonic foliation (fig. 7B). S2 consists of a spaced schistosity (figs. 7C and 7D) (sensu PASSCHIER & TROUW, 1996) often associated with a syn-kinematic metamorphic event (M2) characterised by a mineralogical assemblage consisting of fine-grained muscovite + quartz ± carbonates ± oxides. Cleavage domains show a smooth shape delimiting lenticular


607

TABLE 2d

TABLE 2e

Lithological and structural features of the analysed samples collected in the BG25 borehole. – Caratteristiche litologiche e strutturali dei campioni derivanti dal pozzo BG25.

Lithological and structural features of the analysed samples collected in the BG3bis borehole. – Caratteristiche litologiche e strutturali dei campioni derivanti dal pozzo BG3bis.

BG 25

BG 3bis

Sample

Lithology

Structures

Sample

Lithology

Structures

1870

Metapelite, graphitic phyllite

S1, S2

1665


S1, S2

2005


S1, S2

1670


S1, S2

2010


S1, S2

1690


S1, S2

2020


S1, S2

1700

Metasandstone, metapelite, graphitic phyllite

S1, S2

2050


S1, S2

1738


S1

2100


S1, S2

1790


S1, S2

2150


S1, S2

1825


S1, S2

2200


S1, S2

1850


S1, S2

2204

Metasandstone

S1, S2

1885


S1, S2

2206

Metasandstone

S1, S2

1920


S1, S2

2250


S1, S2

1980


S1, S2

2270


S1, S2

2020


S1, S2

2300


S1, S2

2040


S1, S2

2380


S1, S2

2080


S1, S2

2100


S1, S2

2450


S1, S2

2108

Graphitic phyllite

S1

2500


S1, S2

2111

Graphitic phyllite

S1

2610


S1, S2

2160


S1, S2

2640


S1, S2

2170


S1, S2

2690


S1, S2

2230


S1, S2

2730


S1

2260


S1, S2

2870


S1

2280


S1, S2

2910


S1

2360


S1, S2

3663

Graphitic metapelite

S1

2370


S1, S2

3666

Graphitic metapelite

S1

2440


S1, S2

2460


S1, S2

2510


S1, S2

2550


S1, S2

2610


S1, S2

2661


S1, S2

quartzose microlithons containing the S1 oldest foliation oblique with respect to the S2 surface (fig. 7D). S3 foliation developed discontinuously only in the mica-rich layers (figs. 7E and 7F). This foliation, never accompanied by blastesis, is a smooth asymmetric crenulation cleavage, parallel and gradational, with a 10-30% volume percentage of cleavage domains (sensu PASSCHIER & TROUW, 1996). Post-kinematic minerals mainly consisting of chloritoid (figs. 8A and 8B), quartz and calcite (fig. 8C) formed after the development of the S2 and S3 foliations, respectively. Such a minerals have been observed only in the PN8 borehole. The chloritoid consists of isolated sub-millimetre crystals diffusely distributed within the rocks (fig. 8B). Contrarily, quartz and calcite are concentrated within millimetre and centimetre veins (fig. 8C) that crosscut the metamorphic rocks at high angle with respect to the main tectonic foliations (S1 and S2). DISCUSSION

The metamorphic successions drilled in the Bagnore Geothermal field and Poggio Nibbio area are not involved in widespread duplex structures as documented in the Larderello-Travale geothermal area (PANDELI et alii, 1991) and in the Colline Metallifere region (BERTINI et alii, 1994b). Nevertheless, the very thick (at least 2535 m) and lithologically monotonous Palaeozoic succession

2664


S1, S2

2680


S1, S2

2740


S1, S2

2800


S1, S2

2870


S1, S2

2970


S1, S2

3010


S1, S2

3111


S1, S2, S3

3113


S1, S2, S3

drilled below the Verrucano Group could suggest the occurrence of tectonic repetitions only doubling the Palaeozoic rocks, as documented in the Piancastagnaio Geothermal field (ELTER & PANDELI, 1991). On the other hands, the samples collected along the borehole logs (see tab. 1) are very discontinuous, and cannot permit to control the complete drilled succession. Therefore, tectonic repetitions are useful to justify the anomalous thickness of the Palaeozoic succession, but their occurrence cannot be demonstrated with certainty on the basis of the available data. The Palaeozoic succession shows very similar lithological and petrographical features with respect to: i) the Formation A drilled in the Piancastagnaio geothermal field (PANDELI et alii, 1988; ELTER & PANDELI, 1991); ii) the Farma Fm. exposed in the Middle Tuscan Ridge

608

A. BROGI

Fig. 6 - Photomicrographs of thin sections of the core sampled at 2204-2206 m within the BG25 borehole (plane polarised light, 25X). A) Feldspar grain, probably plagioclase; B) Altered feldspar grain. – Foto di sezione sottile della carota campionata alla profondità di 2204-2206 m nel pozzo BG25 (luce polarizzata, 25X). A) Granulo di feldspato probabilmente riferibile a plagioclasio; B) Granulo di K-feldspato completamente alterato.

(COSTANTINI et alii, 1988, cum bibl.) and iii) the Palaeozoic succession exposed in the Mt. Romani area (BAGNOLI et alii, 1980; MORETTI et alii, 1990). In the Piancastagnaio field the Formation A was drilled for about 800 m (ELTER & PANDELI, 1991), whereas the reconstructed stratigraphic thickness of the Farma Fm. cropping out in the Middle Tuscan Ridge did not exceed the 600 m (COSTANTINI et alii, 1988 cum bibl.; ALDINUCCI et alii, 2005). Small thickness has been reconstructed for the Mt. Romani Palaeozoic succession (MORETTI et alii, 1991). Differently from the Piancastagnaio area, the Palaeozoic succession drilled in the Bagnore and Poggio Nibbio areas are only characterised by graphitic quartzosemetasandstones. No carbonate levels, as documented in the Piancastagnaio Geothermal field (PANDELI & PASINI, 1990; ELTER & PANDELI, 1991), have been encountered. Moreover, it has been recognised a stratigraphic horizon, only drilled by the BG25 borehole at 2204-2206 m, characterised by altered feldspar grains contained within the quartzose metasandstones (fig. 6). This horizon is a peculiar stratigraphic feature for the analysed successions in the Bagnore and Poggio Nibbio areas. Feldspar grains

consists of clasts embedded within the quartzose and micaceous rocks. Their occurrence can be explained through two main hypotheses: (i) erosion of feldspar-rich crystalline rocks or, possibly, (ii) volcanic activity during the sedimentation. Optical analyses cannot be realised on all the feldspar grains due to their alteration (fig. 6B). Nevertheless, some of these are probably referable to plagioclase (fig. 6A). Independently from their composition, any of two hypotheses can be confirmed with certainty, but if the second one can be considered true, then this agrees with the possible Permian age of these sediments, according to the occurrence of magmatic products during the latest Palaeozoic (CASSINIS et alii, 1998; PANDELI, 2002). On the other hands, Permian age is referred to the Formation C (PANDELI & PASINI, 1990), to the Farma Fm. (ALDINUCCI et alii, this volume) and to the Arenarie di S. Pietro Fm. belonging to the Palaeozoic succession of Mt. Romani (NICOLARDI, 2002). The lithological affinities characterising the above mentioned successions exposed both in the Middle Tuscan ridge (Farma Fm.) and in the Mt. Romani (Arenarie S. Pietro Fm.) is also coupled with a similar tectonic setting with respect to that described in

Fig. 7 - Photomicrographs of thin sections of the metamorphic rocks sampled within boreholes drilled in the Bagnore and Poggio Nibbio areas. (A) Palaeozoic quartzose metasandstone sampled whitin the BG3bis borehole at 2108-2111m below the topographic surface. The metamorphic paragenesis, consisting of muscovite + quartz + calcite ± oxides, defines the S1 schistosity. (B) Triassic quartzsore metasandstone belonging to the Verrucano Group sampled at 940 m below topographic surface within the PN8 borehole. Quartz grains recrystallisation along the S1 schistosity are characterised by lobate boundaries and pressure shadows development where muscovite lamellae developed. (C) Palaeozoic quartzose metasandstone sampled at 2900-2903m below topographic surface within the BG20 borehole. S2 schistosity is characterised by a spaced tectonic foliation along which the white mica + quartz ± clorite ± oxides mineralogic paragenesis developed. (D) Detail of C: the S1 schistosity can be recognisable within microlithons defined by the S2 surfaces. (E) Palaeozoic quartzose metasandstone sampled at 3032-3035m below the topographic surface whitin the BG20 borehole. Three tectonic foliations can be observed. S1 and S2 are continuous and spaced schistosity, respectively; the S3 tectonic foliation can be described as spaced cleavage. (F) Detail of E. – Foto di sezioni sottili di rocce metamorfiche perforate nell’area di Bagnore e di Poggio Nibbio. A) Arenaria quarzosa paleozoica perforata a 21082111m al di sotto della superficie nel pozzo BG3bis. La paragenesi metamorfica sviluppata lunga la foliazione tettonica S1 è caratterizzata da muscovite + quarzo + calcite ± ossidi. B) Metarenaria quarzosa del Gruppo del Verrucano campionata a 940m al di sotto della superficie nel Pozzo PN8. La ricristallizzazione del quarzo lungo la superficie di foliazione S1 ha dato luogo a contorni lobati dei granuli e code di pressione in corrispondenza delle quali si sono sviluppate lamelle di mica bianca. C) Metarenaria quarzosa del paleozoico proveniente dalla carota prelevata alla profondità di 29002903m dalla superficie nel pozzo BG20. La superficie di foliazione tettonica S2 è caratterizzata da una scistosità spaziata lungo la quale si è sviluppata mica bianca+quarzo+clorite+ossidi. D) Dettaglio di C dove è visibile la foliazione tettonica S1 all’interno di un microlithon delimitato da piani di foliazione S2. E) Metarenaria quarzosa del Paleozoico appartenente alla carota prelevata alla profondità di 3032-3035m al di sotto della superficie, nel pozzo BG20. È riconoscibile la sovrapposizione di tre foliazioni tettoniche. La foliazione S1 e S2 corrispondono a scistosità definibili rispettivamente come continua e spaziata; la foliazione S3 corrisponde ad un clivaggio spaziato. F) dettaglio di E.


this paper for the drilled Palaeozoic succession (PIZZOLANTE, 2000; ALDINUCCI et alii, this volume). The Triassic succession of the Verrucano Group shows similar lithological features with respect to those described for the succession exposed in the Middle Tu-

609

scan Ridge (COSTANTINI et alii, 1988; ALDINUCCI et alii, 2003). Nevertheless, in the Bagnore and Poggio Nibbio areas the thickness of the Verrucano Group appears to be reduced with respect to that reconstructed in the Middle Tuscan Ridge. In addition, the analysed cuttings and

Fig. 7.

610

A. BROGI

Fig. 8 - Photomicrographs (plane polarised light, 25X) of thin sections of cores sampled at 3080m within the PN8 borehole. (A) Palaeozoic quartz-metasandstone; the S2 tectonic foliation is a crenulation cleavage without blastesis. B) Post-kinematic cloritoid crystallised after the S2 development. C) Post-kinematics quartz and calcite developed after the S2 development. – Foto di sezioni sottili (luce polarizzata, 25X) relative a carote prelevate nel sondaggio PN8 alla profondità di 3080m. (A) Metarenaria quarzosa paleozoica; la foliazione S2 coincide con un clivaggio di crenulazione al quale non è associata blastesi. B) Cristalli di cloritoide sviluppati successivamente alla foliazione tettonica S2. C) Cristalli di quarzo e calcite successivi allo sviluppo della foliazione tettonica S2.

cores samples only consist of metapelites and metasandstones, whereas metaconglomerates have been never recognised. Consequently, the occurrence of the lower member of the Civitella M.ma Fm. and Mt. Quoio Fm., mainly composed of quartzose metaconglomerates (COSTANTINI et alii, 1988; ALDINUCCI et alii, 2003) may be excluded. Considering the whole Mt. Amiata area, the Triassic succession of the Verrucano Group is discontinuous below the Late Triassic evaporites. In fact, the Seggiano 1 borehole (see location in fig. 3) encountered the Late Triassic evaporites directly overlying the Palaeozoic succession, the latter showing similar lithological characters with those described for the Bagnore and Poggio Nibbio areas. The absence of the Verrucano Group in the northern side of the Mt. Amiata can be explained through two hypotheses: a) tectonic elision related to low-angle normal faults which took place during the Middle-Late Miocene extensional tectonics, as described for the lateral segmentation («boudinage») of the Tuscan Nappe in the Mt. Amiata area (BROGI, 2004, 2006a); b) sedimentary causes due to the occurrence of topographic highs influencing the continental sedimentation. The microstructural setting of the analysed rocks highlight a common tectonic evolution for both the Early-Middle Triassic and Palaeozoic successions. Particularly, the microstructural setting is characterised by three superimposed tectonic foliations analogously with the corresponding Triassic and Palaeozoic metamorphic rocks cropping out in the exposed in the Monticiano Roccastrada Ridge (COSTANTINI et alii, 1988; CONTI et alii, 1991; LAZZAROTTO et alii, 2003; ALDINUCCI et alii, 2005; BROGI, 2006b, 2008) (fig. 1). The first tectonic foliation (S1) is always accompanied by blastesis; the second tectonic foliation (S2) is only locally pervasive, and is often accompanied by blastesis; the third tectonic foliation (S3), rarely developed, is never accompanied by blastesis. On the whole, these tectonic foliations are referred to Alpine deformational events (D1, D2 and D3) since they affected the Triassic rocks. The D1 deformational event is generally related to the Northern Apennines collisional event. The tectonic framework in which the D2 deformational event took place is under debate. This deformational event has been considered as related to ensialic shear zones activated during the Miocene extensional collapse of the chain (CARMIGNANI & KLIGFIELD, 1990) or, alternatively, to a progressive deformation during the late stage of the collisional event (CAROSI et alii, 2002). Concerning the D3 deformational event, there is a general consensus to refer such deformational event to an extensional framework. The rocks encountered by the PN8 borehole show some peculiarities: i) they were not affected by D3 deformation event; ii) the D2 deformational event developed without blastesis (fig. 8A). This microstructural feature differs with respect to that documented for the other analysed boreholes. Similar features were recognised in the Piancastagnaio Geothermal field (ELTER & PANDELI, 1991), where two different structural domains were described. Particularly, we can underline that the microstructural setting recognised in the PN8 borehole (Poggio Nibbio area) agrees with the Domain I described by ELTER & PANDELI (1991). According to these authors, we can hypothesise that the succession drilled by the PN8 borehole was deformed in a crustal level where the D2 deformational event was weakest than in the crustal level drilled by BG20, BG3, BG13bis and BG25 (Bagnore area).


The occurrence of the post-kinematic chloritoid recognised in the PN8 samples attests a static metamorphic event following the D2 deformational event (fig. 8B). No clear relationships allow us to establish the relation between this metamorphic event and the D3 deformation. Therefore, this data agree with those described for the Piancastagnaio Geothermal field by ELTER & PANDELI (1991). These authors documented the blastesis of chloritoid (± albite) developed in two different static metamorphic events that affected the metamorphic rocks drilled in the Piancastagnaio Geothermal field. The authors referred the two metamorphic events to post-kinematic metamorphisms interposed between the S1-S2, and S2-S3 tectonic foliations development. The later chloritoid development was recognised only within the metamorphic rocks of the Verrucano Group (ELTER & PANDELI, 1991). In this view, the static metamorphism that gave rise to the chloritoid within the metamorphic rocks drilled by the PN8 borehole can be compared with the later static event described by ELTER & PANDELI (1991). The calcite and quartz veins crosscutting both the Triassic and Palaeozoic rocks drilled by the PN8 borehole can be considered of hydrothermal origins. In fact, the PN8 borehole was drilled within a widespread mineralised area (Hg sulphide Quaternary epithermal mineralization) where mining (Siele, Solforate and Abetina mines) was mainly realised during the last centuries. The hydrothermal veins derived by fluids circulating within damaged rocks in the Poggio Nibbio area. Hydrothermal fluids circulation was enhanced by the superheating of the shallow crustal levels due to a magmatic intrusion emplaced at shallow depth in the crust, as hypothesised by MARINELLI et alii (1993). The emplacement of this magmatic body is considered responsible for the Present anomalous heat flow occurring in the Mt. Amiata area (BATINI et alii, 2003 cum bibl.).

CONCLUDING REMARKS

Following the main results and considerations from the analysed borehole logs, the following main conclusion can be proposed: – The Early-Middle Triassic and Palaeozoic successions drilled in the Bagnore and Poggio Nibbio areas show stratigraphic features similar to those characterising the successions exposed in the Middle Tuscan Ridge, in the Mt. Romani area, and drilled in the Piancastagnaio geothermal field. Concerning the Palaeozoic rocks, I can hypothesise that this phyllite succession may be referred to the Farma Fm. – The Palaeozoic succession was probably affected by tectonic repetitions but their occurrence cannot be documented with certainty due to the discontinuous sampling along the borehole logs. – The Verrucano Group is discontinuous below the Mt. Amiata geothermal area, probably due to tectonic elision induced by Middle-Late Miocene low-angle normal faulting. The Verrucano Group lacks in the northern part of the Mt. Amiata area (Seggiano 1) and is characterised by sudden change in thicknesses. – Both the Triassic and Palaeozoic succession was affected by the same tectonic history that took place during the Northern Apennines tectonic evolution. The

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microstructural setting is mainly defined by the occurrence of three superposed tectonic foliations (S1, S2 and S3). S1 and S2 normally developed under metamorphic conditions. S2 affecting the rocks drilled by the PN8 borehole was not accompanied by blastesis. The S3 foliation is discontinuously developed, and never accompanied by metamorphism. – In the Poggio Nibbio area, post-kinematic chloritoid developed after the S2 foliation during a static metamorphic event, similarly to the Piacastagnaio geothermal field. – Quartz and calcite hydrothermal veins crosscut all the tectonic foliations, and can be related to the Quaternary hydrothermal circulation that affected the Mt. Amiata region and produced the widespread epithermal mineralization (Hg and As sulphides). ACKNOWLEDGEMENTS ENEL GreenPower (Dr. Batini and Dr. Bertini) allowed the permission to analyse the samples (thin sections of cuttings and cores) of the geothermal boreholes drilled in the Bagnore and Poggio Nibbio areas. The reviewers Prof. Perotti (Pavia) and Prof. Pandeli (Firenze) are thanked for their comments and very constructive criticisms that improved the original version of the manuscript. Prof. Cassinis is also thanked for its editorial support, and careful reading of the text.

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Received 14 May 2007; revised version accepted 15 February 2008.