The metamorphic units of the eastern side of Monte Leoni (Northern Apennines, Italy)

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1 Boll. Soc. Geol. It., 124 (0), , 16 ff. The metamorphic units of the eastern side of Monte Leoni (Northern Apennines, Italy) MAURO ALDINUCCI (*), ANDREA BROGI (*) & FABIO SANDRELLI (*) ABSTRACT (*) Dipartimento di Scienze della Terra, Università di Siena, Via Laterina 8, Siena. This work deals with the stratigraphic and structural setting of the metamorphic succession exposed in the eastern side of the Monte Leoni, the southern extension of the Monticiano-Roccastrada Ridge. In the study area, three main tectonic units crop out, which are, from top to bottom: the Ophiolitic Unit, the Tuscan Nappe and the epimetamorphic Monticiano-Roccastrada Unit. They are unconformably overlain by post-orogenic, Neogene marine and Quaternary continental deposits. The Monticiano-Roccastrada Unit consists of Palaeozoic deposits overlain by the Triassic Verrucano Group, represented by the Civitella Marittima Fm. (Early?-Middle? Triassic), the Anageniti minute Fm. (Late Ladinian) and the Tocchi Fm. (Carnian). The Verrucano Group of the eastern side of Monte Leoni lacks the Monte Quoio Fm., which is extensively exposed in the northern part of the Monticiano-Roccastrada Ridge and pinches out southward being progressively replaced by the Anageniti minute Formation. The Palaeozoic deposits comprise the Falsacqua Fm., mainly consisting of normally graded metasandstones and metapelites, locally overlain conformably by a decametre-thick horizon of metacarbonates alternating with subordinate metapelites. The whole Palaeozoic succession exposed in the eastern side of Monte Leoni shows stratigraphic and sedimentological features which typically characterise the Farma Fm., considered in the type-area (Farma Valley, in the central part of the Monticiano-Roccastrada Ridge) as a Moscovian turbiditic deposits. As a consequence these pre-verrucano deposits represent the southernmost outcrops of the Farma Formation. The Civitella Marittima Fm. unconformably overlies the Palaeozoic deposits and is represented by three members, which are: the basal mainly conglomeratic member, the intermediate mainly arenaceous member and the upper pelitic-arenaceous member. The Civitella Marittima Fm. records a retrogradational fluvial fan setting, possibly represented by coalescing terminal fans. The Anageniti minute Fm. erosionally overlies the fine-grained siliciclastics of the upper member of Civitella Marittima Fm. and consists of finingupwards successions represented by quartzose metaconglomerates, quartzose metasandstones and metapelites. These record ephemeral stream and flood-plain sedimentation, whereas the carbonates and siliciclastic fines of the overlying Tocchi Fm. documents the Carnian marine transgression over such an alluvial plain. In the Monte Leoni the Verrucano Group is formed by two sedimentary cycles. The lower one, stratigraphically corresponding to the lower sedimentary cycle of the Punta Bianca area (La Spezia), is represented by the Civitella Marittima Fm., whereas the upper cycle, comprising the Anageniti minute Fm. and the Tocchi Fm., is referable to the Verrucano succession (Verruca Fm. and Monte Serra Fm.) of the Monti Pisani. Four deformational events (D 1, D 2, D 3 and D 4 ) related to the Northern Apennines tectonic evolution affected the metamorphic units. D 1 is represented by meso- to map-scale south- and east-verging F 1 folds, with associated penetrative S 1 axial planar tectonic foliation. S 1 consists of a pervasive schistosity, particularly well developed in the metapelites, ranging from parallel to strongly inclined with respect to the bedding (S 0 ). D 2 is represented by asymmetric east-verging folds (F 2 ), characterised by overturned to vertical shorter limbs. The F 2 are typified by NE-SW striking axes and subhorizontal to gently NW/SE dipping axial surfaces. A penetrative, axial-planar tectonic foliation (S 2 ) occurs in the metapelites. D 3 is represented by map-scale gentle folds (F 3 ) and by disjunctive and widely spaced S 3 tectonic foliation. Finally, high-angle normal faults (D 4 ) dissect all the previous structures. KEY WORDS: Stratigraphy, Structural geology, Tuscan Metamorphic Basement, Monticiano-Roccastrada Unit, Northern Apennines. RIASSUNTO Le unità metamorfiche affioranti nell area orientale di Monte Leoni (Appennino Settentrionale). In questo lavoro sono riportati i risultati derivati da un nuovo rilevamento geologico di dettaglio e dai nuovi studi di carattere stratigrafico-strutturale condotti nel versante orientale del nucleo metamorfico di Monte Leoni, prolungamento meridionale della Dorsale Monticiano-Roccastrada. Quest ultima ed il Monte Leoni rappresentano un tratto della Dorsale Medio Toscana, culminazione strutturale ove affiorano le unità più profonde dell Appennino Settentrionale e che attraversa la Toscana dalle Alpi Apuane, a nord, fino al Monte Argentario, a sud. Nell area di Monte Leoni affiorano tre principali unità tettoniche, che sono, a partire dall alto dell edificio strutturale: l Unità Ofiolitifera, la Falda Toscana e l Unità Monticiano-Roccastrada. Su queste unità tettoniche giacciono discordanti i depositi post-orogenici marini e continentali di età neogenico-quaternaria. L Unità Ofiolitifera, rappresentata unicamente dalla Formazione delle Argille a Palombini, compare in limitati affioramenti nel margine orientale di Monte Leoni, mentre la Falda Toscana è costituita prevalentemente dal Calcare Cavernoso e da modesti affioramenti delle sovrastanti formazioni carbonatiche e terrigene, come i Calcari e marne a Rhaetavicula contorta, il Calcare Massiccio, la Scaglia Toscana ed il Macigno. La sottostante Unità di Monticiano-Roccastrada è costituita dal Gruppo del Verrucano e dal Gruppo delle formazioni paleozoiche, entrambi interessati da metamorfismo alpino attribuito alla facies degli Scisti verdi da FRANCESCHELLI et alii (1986) e da CAROSI et alii (1999), di HP/LT retrogrado alla facies degli Scisti verdi secondo THEYE et alii (1997), GIORGETTI et alii (1997, 1998) e BRUNET et alii (00). Nell area di Monte Leoni l Unità Monticiano-Roccastrada è stata suddivisa in due sottounità tettoniche: la sottounità Monte Quoio-Montagnola Senese, più interna e geometricamente più alta, e la sottounità Monte Leoni-Farma, più esterna e profonda (LAZZAROTTO et alii, 03). Nel versante orientale di Monte Leoni affiora la sottounità Monte Leoni-Farma, costituita alla base da una successione paleozoica, su cui poggia in unconformity il Gruppo del Verrucano di età triassica. I depositi paleozoici sono rappresentati dalla Formazione di Falsacqua («Arenarie e scisti del Fosso di Falsacqua» in GELMINI, 1969), costituita da alternanze ritmiche di metarenarie quarzoso-micacee gradate e metapeliti, passanti localmente al tetto ad un alternanza decametrica di strati calcareo-dolomitici e subordinate metapeliti. Nell insieme, l intera successione paleozoica mostra caratteri stratigrafico-sedimentologici comparabili con quelli della Formazione del Farma, interpretata nell area-tipo (Valle del Torrente Farma, parte centrale della Dorsale Monticiano-Roccastrada) come un flysch ercinico di età moscoviana (COSTANTINI et alii, 1988). Il Gruppo del Verrucano,

2 314 M. ALDINUCCI ET ALII rappresentato dalla Formazione di Civitella Marittima, dalla Formazione delle Anageniti minute e dalla Formazione di Tocchi, è caratterizzato dall assenza della Formazione di Monte Quoio, particolarmente sviluppata, invece, nella parte centro-settentrionale della Dorsale Monticiano-Roccastrada e sostituita nell area in esame dalla Formazione delle Anageniti minute. La Formazione di Civitella Marittima (Triassico?inferiore-?medio: LAZZAROTTO et alii, 03 cum bibl.) comprende tre membri (CO- STANTINI et alii, 1989): un membro inferiore prevalentemente conglomeratico, uno intermedio costituito in gran parte da arenarie ed uno superiore pelitico-arenaceo. Nell insieme tale formazione, caratterizzata da un trend fining-upwards, è riferibile ad un ambiente di conoide fluviale (sensu STANISTREET & MCCHARTY, 1993) retrogradante, probabilmente rappresentato da più apparati di terminal fan. La Formazione delle Anageniti minute (Ladinico superiore: LAZZA- ROTTO et alii, 03 cum bibl.), costituita da una alternanza di metaconglomerati, metarenarie quarzose e metapeliti, spesso organizzati in cicli fining-upwards, è riferibile ad un ambiente alluvionale caratterizzato da fiumi effimeri e piane di esondazione. La Formazione di Tocchi (Carnico: COSTANTINI et alii, 19) è prevalentemente formata da brecce a clasti carbonatici e filladici (Breccia di Tocchi Auct.), mentre la tipica alternanza di metacalcari e metapeliti, caratteristica della parte basale della formazione, affiora localmente ed è interessata da intensa deformazione. La Formazione di Tocchi rappresenta un deposito costiero (COSTANTINI et alii, 19) che documenta la trasgressione marina carnica riconosciuta anche in altre aree della Toscana (PANDELI, 02 cum bibl.). Considerazioni stratigrafiche, effettuate sia nell area di studio, sia più a nord nella Dorsale Monticiano-Roccastrada, fanno ipotizzare che la Formazione di Civitella Marittima costituisca un ciclo sedimentario a sé stante (ALDINUCCI et alii, 03), separato mediante superfici di unconformity sia dalla successione paleozoica, sia dalla Formazione delle Anageniti Minute. Pertanto, tale ciclo potrebbe essere correlato con il primo ciclo sedimentario di età medio-triassica della successione di Punta Bianca (MARTINI et alii, 1986). Lo studio strutturale ha permesso di riconoscere una tettonica polifasata costituita dalla sovrapposizione di quattro principali eventi deformativi. Il primo (D 1 ) ha permesso lo sviluppo di pieghe sub-isoclinali, riconoscibili alla scala cartografica e, raramente, osservabili anche alla scala dell affioramento. Ad esse è associata una scistosità di piano assiale (S 1 ), localmente molto pervasiva, quasi sempre parallela alla stratificazione (S 0 ), oppure a basso angolo ( e 1 ). In quest ultimo caso la S 1 è principalmente riconoscibile nei livelli a dominante pelitica. I rapporti S 0 /S 1, come la geometria delle pieghe, indicano una vergenza orientale e meridionale delle strutture. Il secondo evento (D 2 ) ha generato pieghe (F 2 ) rovesciate, visibili a tutte le scale, con vergenza orientale ed assi orientati mediamente SW-NE. La loro geometria varia in funzione dei livelli interessati: si osservano pieghe sub-isoclinali nei livelli pelitici e più aperte in quelli arenacei. Ad esse è associata una foliazione tettonica (S 2 ), particolarmente pervasiva in corrispondenza dei nuclei delle pieghe e discontinua nei fianchi. Essa tuttavia è ben espressa soprattutto nei livelli a dominante pelitica, in corrispondenza dei quali, talvolta, ha obliterato le due foliazioni precedenti (S 0 ed S 1 ), delle quali soltanto raramente si riconoscono relitti. In sezione sottile la S 1 è descrivibile come continuos schystosity (sensu PASSCHIER & TROUW, 1996) in corrispondenza della quale è osservabile un evidente appiattimento dei granuli di quarzo e la rotazione di minerali fillosilicatici clastici. La S 2 è descrivibile come crenulation-cleavage, parallela e discreta con il % dei domini di clivaggio e come spaced-schystosity (sensu PASSCHIER & TROUW, 1996). Pieghe molto aperte (F 3 ) che hanno deformato il sistema S 0 /S 1 /S 2 suggeriscono la presenza di un successivo evento deformativo (D 3 ). L esistenza delle pieghe è deducibile soltanto alla scala cartografica, mentre, in affioramento ed in sezione sottile, è riconoscibile una foliazione tettonica (S 3 ) probabilmente ad esse associata. In affioramento quest ultima si presenta come clivaggio disgiuntivo, molto spaziato e sviluppato soltanto localmente; in sezione sottile la S 3 è sviluppata soltanto entro films a dominante fillosilicatica ed è descrivibile come crenulation cleavage parallelo e gradazionale con il % dei domini di clivaggio (sensu PASSCHIER & TROUW, 1996). Le strutture precedentemente descritte sono state dislocate da faglie dirette ad alto angolo di età Pliocenico-quaternaria (CARMIGNANI et alii, 1994; LIOTTA et alii, 1998) collegate con lo sviluppo dei bacini neogenico-quaternari della Toscana meridionale (DECANDIA et alii, 01 cum bibl.). TERMINI CHIAVE: Stratigrafia, Geologia strutturale, Basamento Metamorfico Toscano, Unità Monticiano-Roccastrada, Appennino Settentrionale. INTRODUCTION The deepest tectonic units of the Northern Apennines are mainly exposed in the Middle Tuscan Ridge, a discontinuous geomorphologic feature ranging from Alpi Apuane to Monte Argentario. This ridge, to which Monte Leoni belongs, is also a regional structural culmination in which the Tuscan Metamorphic Basement crops out (fig. 1). That makes Monte Leoni a key area for unravelling both the stratigraphy and the structural evolution of the Tuscan Metamorphic Units. Recent publications have described the stratigraphy and the structural setting of the metamorphic successions (Verrucano and Palaeozoic Groups) exposed north and south of Monte Leoni (fig. 1) (e.g. Monticiano-Roccastrada Ridge: COSTANTINI et alii, 1988, 02; CORSI et alii, 01; LAZZAROTTO et alii, 03; Iano: COSTANTINI et alii, 1998; PANDELI, 1998; Monti dell Uccellina: CAMPETTI et alii, 199, 1999; Monte Argentario: CIRILLI et alii, 02), whereas few studies deal with the geology of this area. The metamorphic succession of Monte Leoni has been subdivided stratigraphically in different ways by previous Authors (GELMINI, 1969; MORETTI, 1991) and different interpretations of the structural setting were given (MEC- CHERI et alii, 1987; MORETTI, 1991). New field work, including 1:.000 scale geological mapping of the eastern side of the Monte Leoni (fig. 2), together with stratigraphic-sedimentological and structural analysis, has allowed the stratigraphy and the structural setting of the metamorphic succession to be refined. GEOLOGICAL FRAMEWORK The Monte Leoni area is mainly characterised by Palaeozoic to Triassic metamorphic rocks and by sedimentary rocks from Triassic to Early Miocene in age. In the eastern side of this area, these rocks belong to three main tectonic units, which are, from top to bottom (fig. 3): a) Ophiolitic Unit: only represented by Lower Cretaceous, alternating shales and siliceous limestones referable to the Argille a Palombini Fm.; b) Tuscan Nappe: herein mainly represented by the basal Late Triassic Calcare Cavernoso Fm., locally overlain by the Late Triassic Calcari e marne a Rhateavicula contorta Fm. and by the Early Jurassic Calcare Massiccio Formation. Furthermore, small outcrops of a Cretaceous- Early Miocene succession, referable to the Scaglia Toscana and to the Macigno Fms. are present in the southeasternmost study area; c) Monticiano-Roccastrada Unit: meta-siliciclastics with subordinate meta-carbonates belonging to the Group of the Palaeozoic formations and to the Triassic Verrucano Group. These tectonic units are unconformably overlain by post-orogenic deposits mainly represented by Pliocene marine sands and clays with interbedded gravels and Quaternary continental gravels, gravelly sands and silty clays (BOCCALETTI et alii, 1987). STRATIGRAPHIC FRAMEWORK OF THE METAMORPHIC SUC- CESSION Two main lithostratigraphic units, the Palaeozoic and the Verrucano Groups (fig. 3), were recognised by previ-

3 THE METAMORPHIC UNITS OF THE EASTERN SIDE OF MONTE LEONI 31 North 0 km a Magmatic rocks Neogene rocks Pre-Neogene rocks Larderello Geothermal area Elba Island Tyrrhenian Sea Piombino Giglio Island Middle Tuscan Ridge Volterra Larderello Grosseto Giannutri Island Siena Mt. Leoni Monte Argentario Mts. CHIANTI b Mt. Amiata Mt. Amiata Geothermal area Fig. 1 - Location of the study area: the black rectangle in fig. b indicates the area represented in fig. 2a. Ubicazione dell area studiata: il rettangolo nero riportato nella fig. b indica l area rilevata riportata in fig. 2a. Magmatic Magmatic rocks rocks Neogene-Quaternary deposits Ligurian Ligurian Units Units Subligurian Subligurian Units Units Tuscan Tuscan Nappe Nappe Tuscan Metamorphic Basement Tuscan Metamorphic Basement 0 2, 7, Km 0 2, 7, Km Monticiano Monticiano Boccheggiano Mt. Quoio Rosia Roccastrada - Roccastrada Ridge Murlo Study area Sticciano Paganico Mt.Leoni Montepescali Cinigiano ous Authors in the metamorphic succession of Monte Leoni, although different internal subdivisions of these groups were given (GELMINI, 1969; MECCHERI et alii, 1987; COSTANTINI et alii, 1988; MORETTI, 1991). However, it is generally agreed, on the basis of stratigraphic and sedimentological evidence, that an unconformity surface separates the Verrucano Group from the underlying Palaeozoic succession (GELMINI, 1969; MECCHERI et alii, 1987; COSTANTINI et alii, 1988), although at the outcropscale no angular unconformity has been clearly recognised. The Palaeozoic succession comprises the siliciclastic deposits of the «Arenarie e scisti del Fosso di Falsacqua» introduced by GELMINI (1969) and hereafter Falsacqua Formation, overlain by a discontinuous horizon, some ten of metres thick, consisting of metacarbonates with interbedded metapelites, first documented by MECCHERI et alii (1987). According to COSTANTINI et alii (1988), the Verrucano Group consists of three formations, which are, from base to top: the Civitella Marittima Fm. (Early?-Middle? Triassic), the Anageniti minute Fm. (Late Ladinian) and the Tocchi Fm. (Carnian). STRUCTURAL FRAMEWORK OF THE METAMORPHIC SUCCES- SION In the Middle Tuscan Ridge, the Monticiano-Roccastrada Unit is composed of three main subunits (LAZ- ZAROTTO et alii, 03), stacked during the Northern Apennines syn-collisional stage (fig. 4); they are, from top to the bottom: the innermost Iano subunit 3, the Monte Quoio- Montagnola Senese subunit 2 and the outermost and deepest Monte Leoni-Farma subunit 1. The Monte Quoio-Montagnola Senese subunit 2 and the Monte Leoni-Farma subunit 1 occur respectively in the western and eastern sides of the Monte Leoni (COSTANTINI et alii, 1988) (fig. 4). b GROSSETO Istia d Ombrone These embricated metamorphic successions record Northern Apennine tectonic evolution (COSTANTINI et alii, 1988; MECCHERI et alii, 1987; CONTI et alii, 1991; ELTER & PAN- DELI, 1993; CAMPETTI et alii, 199; 1999; CORSI et alii, 01; LIOTTA, 02). Hercynian deformation, consisting of a relic schistosity, appears only in the pre-visean rocks, exposed northward of Monte Leoni (COSTANTINI et alii, 1988; CONTI et alii, 1991; PANDELI et alii, 1994) and in the island of Elba (PANDELI & PUXEDDU, 1990). Three main superimposed Alpine deformational events have been documented by previous Authors in the post-visean successions: the first (D 1 ) produced eastward verging close folds and related pervasive schistosity (S 1 ), developed in high pressure/low temperature (HP/LT) (THEYE et alii, 1997; GIORGETTI et alii, 1997, 1998; BRUNET et alii, 00) or in green-schist facies (FRANCESCHELLI et alii, 1986) conditions. The second event (D 2 ) is characterised by map-scale to mesoscopic scale folds (CONTI et alii, 1991; LIOTTA, 02) with both eastward and westward vergences (CAMPETTI et alii, 1999; CORSI et alii, 01), whose schistosity (S 2 ) developed in low green-schist facies metamorphism (FRANCESCHELLI et alii, 1986; GIORGETTI et alii, 1998). A static metamorphism (HT/LP), occurring between D 1 and D 2, has been documented in the Tuscan Metamorphic Basement drilled in the Tuscan Geothermal

4 316 M. ALDINUCCI ET ALII Geological sketch-map of the eastern side of Monte Leoni CMS 4 PISA FIRENZE SIENA lake CMS 240 Materazzo 213 FAA CMM 1 FAA 8 CMM CMS CMS 1 CMM Poggio 6 Loreta CMS CMM 6 FA CMM Poggio della Poggio 1 Navaccia Cannicci FA 366 CMS CMS Monte Leoni Normal bedding 1 Overturned bedding Vertical bedding 1 S1 tectonic foliation Vertical S1 tectonic foliation 4 1 S2 tectonic foliation Horizontal F1 axes Horizontal axes of F1 folds. Lateral arrow indicates the sense of fold asymmetry Dipping F1 axes Dipping F1 axes. Lateral arrow indicates the sense of fold asymmetry Horizontal F2 axes Horizontal axes of F2 folds. Lateral arrow indicates the sense of fold asymmetry Dipping F2 axes Dipping F2 axes. Lateral arrow indicates the sense of fold asymmetry Stratigraphic or tectonised boundary Thrust surface reactivated as low-angle normal fault High-angle normal fault 0 1 km CMS Fosso 378 della Poggio del Moscoso CCA Nave DQ1 Scopaiola 2 CMS 3 CCA CMM Poggio del Massani Castone Nord Poggio Vaccherecce lake lake 328 MONTORSAIO MAS RCO CM DQ1 18 Case Ontaneta 227 Poggio l Acquaviva SECT. 4 CCA DQ1 SECT SECT DUD MAC RCO d 4 Detritus Neoautochthonous DQ1 DN Continental deposits (Holocene) Continental deposits (Pleistocene) Marine deposits (Pliocene) Ophiolitic Unit Argille a Palombini APA (Early Cretaceous) Tuscan Nappe Monticiano-Roccastrada Unit Verrucano Group Tocchi Fm. FTO (Carnian) Anageniti Minute Fm. (Late Ladinian) Civitella Marittima Fm. CMS upper member (Early?-Middle? Triassic) Palaeozoic Group metacarbonates FAC alternating with minor metapelites (Late Palaeozoic) Falsacqua Fm. (Late Palaeozoic) PIETRATONDA 0 DQ1 lake SECT. 1 MAC Macigno (Oligocene-Early Miocene) Scaglia Toscana DUD (Cretaceous-Oligocene) MAS Calcare Massiccio (Liassic) Calcari e Marne a RCO Rhaetavicula contorta (Rhaetian) Calcare Cavernoso CCA (Norian-Rhaetian) Civitella Marittima Fm. CMM intermediate member (Early?-Middle? Triassic) Civitella Marittima Fm. CMI basal member (Early?-Middle? Triassic) FAA Fig. 2a - Geological sketch-map of the eastern side of Monte Leoni. Its location is shown in fig. 1b. Schema geologico del settore orientale dell area di Monte Leoni la cui ubicazione è riportata in fig. 1b.

5 THE METAMORPHIC UNITS OF THE EASTERN SIDE OF MONTE LEONI 317 Sect. 1 0 CMM CMS d d CCA d DQ Sect NW SE 0 CMM FAA CMI CMM FAA Fosso di Falsacqua d FAA CMI CMM CMS d CMM d FAC d CMS d d d d NW Monte Leoni SE Sect. 3 0 CMM CMS Montorsaio d CCA FTO d CMS CCA d NW SE 0 Sect. 4 CCA FTO FTO CCA 0 MAS DN 0 NW SE 0 0 m Fig. 2b - Geological cross-sections. Their traces are shown in fig. 2a. Sezioni geologiche. Le loro tracce sono riportate in fig. 2a. 0 0

6 318 M. ALDINUCCI ET ALII Tectono-stratigraphic relationships among the Tectonic Units in the Monte Leoni area Anageniti minute metaconglomerates unconformity Post- orogenic deposits OPHIOLITIC UNIT ARGILLE A PALOMBINI Fm. (Early Cretaceous) "CALCARI E MARNE A RHAETAVICULA CONTORTA" -"MACIGNO" Fms. (Rhaetian - Early Miocene) CALCARE CAVERNOSO Auct. (Norian - Rhaetian) TUSCAN NAPPE a metapelites (a) b with caliche (b) thin-bedded quartzose metasandstones and metapelites planar-laminated and cross-bedded quartzose metasandstones quartzose metaconglomerates Upper member Intermediate member Basal member CIVITELLA MARITTIMA FORMATION VERRUCANO GROUP unconformity TOCCHI Fm. (Carnian) ANAGENITI MINUTE Fm. (Late Ladinian) CIVITELLA M.MA Fm. (Early?-Middle? Triassic) MONTICIANO - ROCCASTRADA UNIT unconformity FALSACQUA Fm. (Late Palaeozoic) P ALAEOZOIC GROUP Fig. 3 - Tectonic and stratigraphic relationships among the tectonic units exposed in the eastern side of the Monte Leoni. Relazioni tettoniche e stratigrafiche tra le unità affioranti nel settore orientale di Monte Leoni. areas (DEL MORO et alii, 1982; PUXEDDU, 1984; ELTER & PANDELI, 1991; VILLA & PUXEDDU, 1994). The D 3 deformational event is locally documented and mainly characterised by a discontinuous tectonic foliation associated with gentle folds characterised by sub-vertical axial planes (CORSI et alii, 01; COSTANTINI et alii, 02). Later normal faults dissected all the previous structures (CARMIGNANI et alii, 1994; LIOTTA et alii, 1998). STRATIGRAPHY AND SEDIMENTOLOGICAL DATA This study has refined the stratigraphy and the depositional features of the metamorphic succession exposed in the eastern side of Monte Leoni, in order to provide constraints to unravel the tectono-sedimentary evolution of this part of the Tuscan Domain from Late Palaeozoic to Carnian. Unfortunately, Alpine tectonic deformation makes it difficult to reconstruct both the original thickness and the geometry of these deposits, preventing more accurate sedimentological analysis. The metamorphic succession consists, from bottom to top, of the following lithostratigraphic units (fig. 3). FALSACQUA FORMATION This unit, with a minimum estimated thickness of about 1 m, is characterised by a monotonous alternation of grey, medium/coarse- to fine-grained, micaceous/quartzose metasandstones and dark-grey, graphiterich metapelites. These lithotypes are typically arranged in normally graded beds, up to 1, m thick, which can be described using the Bouma sequence, although the distinction between the pelitic d-division and the hemipelagic pelite of the e-division can not always be made, due to the weathering and to their commonly similar appearance. Beds show incomplete, mainly T a/de Boumatype sequences, with minor T ab/de sequences characterised by a well developed T a division. At the outcrop scale the bedding surfaces are even and parallel; evidence of erosion of seafloor are represented by occasional small-scale scours. A decametric alternation of dark-grey, unfossiliferous calcareous-dolomitic beds with minor metapelites (fig. a), overlies conformably the Falsacqua Fm. in a creek extending between «Poggio Cannicci» and «Poggio la Navaccia». The metacarbonate beds consist of decimetre-

7 Elsa R. THE METAMORPHIC UNITS OF THE EASTERN SIDE OF MONTE LEONI 319 thick, microsparitic limestones and dolostones with scattered mica and silt- to fine sand-sized quartz. CIVITELLA MARITTIMA FORMATION In the Monticiano-Roccastrada Ridge the Civitella Marittima Fm. («Green quartzite» lithofacies of the Verrucano Group: AZZARO et alii, 1976) has been subdivided by COSTANTINI et alii (1989) into three members, corresponding to lithofacies associations (ALDINUCCI et alii, 03). They are: i) the lower, mainly conglomeratic member; ii) the intermediate, mainly arenaceous member; iii) the upper, mainly pelitic member. In the study area all the three members crop out and reach a maximum thickness of about 0 m as a whole. An Early? Middle? Triassic age has been attributed to the Civitella Marittima Fm. owing to its stratigraphic position (LAZZAROTTO et alii, 03). Lower member This is characterised by a lens-shaped geometry, with maximum thickness of about metres. The lower member is mainly represented by erosive-based, fining-upwards successions up to 1 m in thickness. The type-succession starts with a metres-thick mainly conglomeratic body, which grades to a light-grey decimetres-thick quartzose metasandstones, in turn overlain by green/purple metapelites up to 2 m thick. Thin quartzose metasandstones are present in the conglomeratic bodies, whereas discontinuous pelitic intercalations occur in both the conglomeratic and the arenitic bodies. Geometries of the conglomeratic-arenitic basal part of the fining-upwards successions are not completely observable in the field, although channelled geometries with fairly high width/depth ratio are suspected from the sporadic good exposures available. The metaconglomerates (fig. b), light-grey to palegreen in colour and poorly sorted with frequent amalgamation surfaces, range from generally massive to crudely, coarse-tail normally graded, although often only in the topmost part of the bed. Sometimes a decimetre-thick inversegraded interval is present at the base of some beds (fig. b). Metaconglomerates are polymodal with common granules and pebbles (sensu BLAIR & MCPHERSON, 1999) in sparse to abundant sand- and silt-sized quartz matrix with minor amount of clay. The clast composition show a dominance of white to light-grey quartz, with minor amounts of pink quartz, acidic volcanic rocks (fig. c) (Permian volcanics?), cherts, tourmalinites (CAVARRETTA et alii, 1992) and dark-grey to green metapelites, the latter interpreted as mud intraclasts. Quartzose-metasandstones, light-grey to pale-green in colour and poorly- to medium-sorted, are dominantly massive, with granules and pebbles both scattered and concentrated in thin pebble-size laminae. Metapelites consist of massive silty clays with minor sandy beds. Carbonate nodules are locally present in the purple metapelites (fig. d), especially concentrated at the top of the fining-upwards successions. Iano Massa M.ma Gavorrano S. Gimignano Sub Unit 3 (Iano) Sub Unit 2 (Mt. Quoio - Montagnola Senese) Sub Unit 1 (Mt. Leoni - Farma) Palaeozoic rocks Main thrust Boccheggiano Cetine Monticiano Roccastrada Montagnola Senese Mt.Quoio Intermediate member This, up to 1 m thick, is represented by finingupwards successions similar to those of the underlying member, although typically thinner (max 7 m) and finergrained. In the topmost part of the intermediate member, metaconglomerates disappear and the fining-upwards successions start with quartzose, locally pebbly, metasandstones, which grade upwards to metapelites. Metaconglomerates and metapelites display sedimentological features similar to the same lithotypes of the lower member. Quartzose metasandstones, generally medium- to coarse-grained and medium to poorly sorted, range from massive (fig. 6a) to crudely parallel-laminated with occasional cross sets. Upper member This member, whose maximum thickness is about 1 m, is made up of metapelites with interbedded fine- to medium-grained quartz-metasandstones. Four lithofacies N Civitella M. ma Mt. Leoni SIENA Monticiano - Roccastrada Ridge Farma R. Ombrone R. Fig. 4 - Tectonic sketch-map of the Monticiano-Roccastrada Ridge showing the relationships among the stacked subunits belonging to the Monticiano-Roccastrada Unit (after LAZZAROTTO et alii, 03). Schema tettonico della Dorsale Monticiano-Roccastrada che illustra i rapporti tra le sottounità che costituiscono l Unità di Monticiano-Roccastrada (da LAZZAROTTO et alii, 03).

8 3 M. ALDINUCCI ET ALII Fig. - a) Alternating metacarbonates and metapelites representing the uppermost part of the Palaeozoic succession exposed in a creek between Poggio la Navaccia and Poggio Cannicci. The metapelites are characterised by a more pervasive tectonic foliation (S 2 ) with respect to the metacarbonate beds; b) Clast-supported quartzose metaconglomerates in the lower member of Civitella Marittima Formation. The bed above the pencil is characterised by a weakly erosive basal surface and shows a crude inverse- to coarse-tail normal-grading; c) Microphotograph showing a portion of an acidic Permian? volcanite clast (V) in a mainly coarse-grained quartz matrix (on the left) (quartzose metaconglomerate of the lower member of Civitella Marittima Formation); d) Carbonate nodules interpreted as caliche in purple metapelites in the lower member of Civitella Marittima Formation. a) Alternanza di metacarbonati e metapeliti della parte superiore della Formazione del Farma affiorante lungo un fosso senza nome compreso tra Poggio la Navaccia e Poggio Cannicci; le metapeliti sono caratterizzate da una pervasiva foliazione tettonica (S 2 ) meno evidente nei metacarbonati; b) Metaconglomerati quarzosi clasto-sostenuti del membro inferiore della Formazione di Civitella Marittima. Lo strato al di sopra della matita è caratterizzato da una superficie basale debolmente erosiva e mostra una grossolana gradazione, da inversa a normale; c) Microfotografia nella quale è visibile sulla destra parte di un clasto di vulcanite acida (vulcanite permiana?, V) in una matrice quarzosa grossolana; d) Noduli carbonatici interpretati come caliche in metapeliti viola del membro inferiore delle Formazione di Civitella Marittima. have been distinguished: 1) massive, purple to rarely green clayey metasiltstones (max m thick) locally including carbonate nodular concretions; 2) bodies (max m thick) of alternating quartzose metasandstones and clayey metasiltstones green/purple in colour. Beds range from a few millimetres up to few decimetres in thickness with sheet- to lens-like geometry. Quartzose metasandstones are sharp- to erosive-based with small pelitic intraclasts (fig. 6b). Sedimentary structures, such as parallel laminations, cross-bedding and cross-laminations, are frequent (fig. 6c). Quartzose metasandstones grade rapidly into the overlying clayey metasiltstones over a few millimetres, although sharp contacts, sometimes with wave-rippled top surfaces, are locally present; 3) bodies (up 2 m thick) represented by alternating, millimetre-thick clayey metasiltstones and fine-grained, well-sorted quartzose metasandstones with even and sharp, basal and top surfaces; 4) light grey/purple to pale green, medium/fine-grained, well-sorted quartzose metasandstones in bodies up to 6 m thick. Internally these metasandstones show parallel lamination and cross-bedding (fig. 6d), with small amounts of phyllosilicates marking the sandy laminae and/or occurring scattered among quartz grains. Most of the upper member consists of lithofacies 1 and lithofacies 2, with minor horizons represented by lithofacies 3 and 4. ANAGENITI MINUTE FORMATION The Anageniti minute Fm., about 0 m thick, consists of whitish-pink to purple quartzose metaconglomerates («microanagenite» Auct.), quartzose metasandstones

9 THE METAMORPHIC UNITS OF THE EASTERN SIDE OF MONTE LEONI 321 Fig. 6 - a) Massive, medium-grained quartzose metasandstone erosionally overlying purple metapelites in the intermediate member of Civitella Marittima Formation; b) Mudstone intraclasts in a fine-grained quartzose metasandstone (lithofacies 2) of the upper member of Civitella Marittima Formation; c) Fine-grained quartzose metasandstones and metapelites (lithofacies 2) of the upper member of Civitella Marittima Formation. The lower part of this picture shows a parallel- to cross-laminated quartzose metasandstone with preserved rippleforms at its top. The metapelite resting on this bed is overlain by a parallel-laminated fine-grained quartzose metasandstone with soft-sediment deformation at its base; d) Well-sorted, locally disrupted, medium-grained quartzose metasandstones (lithofacies 4) showing parallellamination and cross-bedding in the upper member of Civitella Marittima Formation. a) Metarenaria quarzosa massiva che poggia con contatto erosivo su metapeliti viola (membro intermedio della Formazione di Civitella Marittima); b) Intraclasti pelitici in una metarenaria quarzosa fine (litofacies 2, membro superiore della Formazione di Civitella Marittima); c) Metarenarie quarzose fini e metapeliti del membro superiore (litofacies 2) della Formazione di Civitella Marittima. La porzione inferiore della foto mostra una metarenaria con laminazione piano-parallela e incrociata, con ripple al tetto. Il livello metapelitico che poggia su questo strato è a sua volta ricoperto da una metarenaria con laminazione piano-parallela e soft-sediment deformation alla base; d) Metarenarie quarzose ben cernite a grana media del membro superiore della Formazione di Civitella Marittima (litofacies 4), localmente fratturate, con laminazione piano-parallela ed incrociata. and purple metapelites. Although bad exposure conditions often hinder the observation of the vertical arrangement of such lithotypes, fieldwork has highlighted that the main body of the Anageniti minute Fm. is represented by metres-thick (max m) fining-upwards successions. These are characterised by basal metaconglomerates and/or pebbly metasandstones passing upwards to metasandstones, in turn overlain by metapelites, the latter locally bearing carbonate nodules. Towards the top of the formation these fining-upwards successions show increasing textural and compositional maturity accompanied by a minor content of metaconglomerates. The uppermost part of the Anageniti minute Fm., well exposed in the Montorsaio village, consists of a decametres-thick alternation of varicoloured, millimetre- to centimetre-thick, fine-grained quartzose metasandstones and metapelites (fig. 7a), marking the transition to the overlying Tocchi Formation. Metaconglomerates are typically clast-supported with common granules and pebbles in a variable amount of silt- to sand-sized quartz matrix with subordinate phyllosilicates. Clasts consist of white and pink quartz with minor acidic volcanic rocks (Permian?), tourmalinites (CAVARRETTA et alii, 1992) and sedimentary lithics, such as pelites and purple to light grey quartzites, the latter particularly abundant in the basal metaconglomerates. Internally the metaconglomerates, which become finer and better sorted towards the top of the formation, range from massive to coarse-tail normally graded; locally a crude parallel lamination, marked by pebbles alignment, occurs (fig. 7b). Metasandstones are massive to parallel laminated (fig. 7c), less often with faint traces of cross-bedding. The Anageniti minute Fm. is commonly referred to the Late Ladinian (LAZZAROTTO et alii, 03) for its stratigraphic position below the Carnian Tocchi Formation. TOCCHI FORMATION The Tocchi Fm. mainly consists of yellowish, vacuolar breccias made up of carbonate clasts with scattered phyllitic clasts (fig. 7d), commonly referred to as «Breccia di Tocchi» (COSTANTINI et alii, 19). Alternating greenish to purple phyllites and yellowish carbonate beds representing the basal part of the formation are locally exposed in discontinuous and tectonised outcrops. In the typearea (Tocchi village, in the northern part of the Monticiano-Roccastrada Ridge), the Tocchi Fm. has a Carnian age on the basis of foraminifers found within basal carbonate beds (COSTANTINI et alii, 19).

10 322 M. ALDINUCCI ET ALII Fig. 7 - a) Alternating, fine-grained, quartzose metasandstones and minor metapelites in the uppermost part of the Anageniti minute Fm. (Montorsaio village); b) Quartzose metaconglomerate with crude parallel-lamination of the Anageniti minute Formation; c) Parallel-laminated medium-grained quartz-metasandstone of the Anageniti minute Fm.; d) A typical outcrop of the Tocchi Breccia consisting of metapelite and carbonate clasts in a carbonate matrix. a) Alternanze di metarenarie quarzose a grana fine e metapeliti nella parte sommitale della Formazione delle Anageniti minute affioranti presso Montorsaio; b) Metaconglomerato quarzoso caratterizzato da una mal definita laminazione piano-parallela della Formazione delle Anageniti minute; c) Metarenaria quarzosa a laminazione piano-parallela della Formazione delle Anageniti minute; d) Un tipico affioramento della Breccia di Tocchi, costituita da clasti pelitici e carbonatici in una matrice carbonatica. STRUCTURAL DATA Polyphase tectonics affected the metamorphic succession. The structural setting is defined by four main superimposed deformational events. STRUCTURES OF THE D 1 EVENT The oldest tectonic event is mainly represented by a pervasive tectonic foliation (S 1 ) related to folding (F 1 folds) (fig. 8). Due to the scarcity of large exposures, the F 1 folds have been rarely observed, even though their occurrence has been deduced from the alternating variation of S 0 /S 1 angular relationships. F 1 are meso- to mapscale asymmetric recumbent folds with shapes and amplitudes corresponding to the B 4, B, C 4, C, D 4 and D classes of Hudleston s classification (HUDLESTON, 1973). The S 0 /S 1 relationships and the F 1 asymmetry, as well as their facing combined with the stratigraphic polarity, suggest south- or east-vergence for the D 1 structures. A map-scale F 1 fold, with the Falsacqua Fm. at its hinge, has been recognised in the Falsacqua creek (henceforth Falsacqua fold). This fold shows a vertical attitude as suggested by both the sub-vertical plunge of the fold axis and the vertical dip of the fold axial surface (figs. 9a and 9b). This geometrical setting highlights the profile (sensu RAMSAY & HUBER, 1987) of the fold at the intersection of the topographic surface. The fold profile shows a fold

11 THE METAMORPHIC UNITS OF THE EASTERN SIDE OF MONTE LEONI 323 Fig. 8 - S 0 /S 1 /S 2 relationships observed in the uppermost part of the Anageniti minute Fm. exposed in the Montorsaio area (in all the pictures, west is on the left and east on the right); a) Very pervasive S 1 foliation at a high angle with respect to S 0 ; b) S 1 is locally transposed by S 2 which is particularly evident in the metapelites; c) F 1 fold and related S 1 convergent tectonic foliation, with a west dipping S 2 oblique foliation superimposed; d) overturned F 1 fold limb with related oblique east-dipping S 1 tectonic foliation. S 0 and S 1 are cut by a discontinuous west-dipping S 2 foliation. Relazioni tra le superfici S 0 /S 1 /S 2 riconosciute in alcuni affioramenti appartenenti alla Formazione delle Anageniti minute esposta nell area di Montorsaio. Nelle foto il lato sinistro coincide con l ovest, mentre il lato destro con l est. a) La foliazione S 1, localmente molto pervasiva, è ad alto angolo rispetto alla superficie S 0 ; b) La foliazione S 1 è localmente trasposta dalla foliazione S 2, ben sviluppata in corrispondenza dei livelli a dominante pelitica; c) Piega F 1 e relativa foliazione (S 1 ) convergente all intradosso, entrambe attraversate dalla foliazione S 2 obliqua ed immergente verso ovest; d) Fianco rovesciato di una piega F 1 e relativa foliazione S 1 a basso angolo rispetto alla S 0 ed immergente verso est. La S 0 e la S 1 sono successivamente attraversate obliquamente dalla S 2, immergente verso ovest. interference pattern, ascribable to the Type2 (sensu RAM- SAY & HUBER, 1987), which suggests the superposition of two folding events, at least. Parasitic metre- to decimetre-size F 1 folds have been typically observed in the metapelites occurring in the vertical limbs of the Falsacqua fold. The asymmetry of these minor folds, together with the S 0 /S 1 angular relationships, all suggest, at present, the southward vergence of the Falsacqua fold (figs. 9a and 9b). The F 1 fold axes, measured or inferred by L 1 lineations (S 0 /S 1 intersection), show scattered orientation (fig. ). The S 1 foliation (fig. ) ranges from parallel to weakly inclined ( -1 ) with respect to the bedding surface (S 0 ). S 1 consists of a pervasive schistosity, well developed in the fine-grained lithotypes. A syn-kinematic metamorphic mineral paragenetic association (M 1 ), made up of fine grained muscovite, paragonite, chlorite, quartz, carbonates and oxides (fig. 11), is associated to the S 1. In thin section, the latter can be described as continuous schistosity (sensu PASSCHIER & TROUW, 1996) mainly defined by elongate quartz grains and non-layered homogeneous distribution of platy minerals grains with a preferred orientation. Planar shape fabrics are mainly defined by flattened crystals, such as quartz, and by rotation/reorientation of detrital phyllosilicates, such as white mica and chlorite (fig. 12). Pressure solution and solution transfer (RUTTER, 1976; SPIERS et alii, 1990) are the mechanisms which produced the flattening, mainly of the quartz grains, enhancing preferential grain growth in the direction of the tectonic foliation. In the metaconglomerates the S 1 has been locally developed and produced spaced domains consisting of millimetre-sized phyllosilicate rich layers and centimetre-sized quartz domains mainly composed of quartz clasts (fig. 13). Rotation of fabric elements, such as quartz clasts and detrital phyllosilicates, suggests the occurrence of bulk simple shear which mainly affected the quartz rich domain (fig. 13). Top to south-east sense of shear in the normally graded beds is indicated by σ-type quartz clasts (fig. 13). The tectonic transport direction during the D 1 event is shown by quartz-grain and phyllosilicate alignments on S 1.

12 324 M. ALDINUCCI ET ALII Detritus Verrucano Group Quaternary continental deposits Anageniti minute Fm Civitella M.ma Fm. upper member Civitella M.ma Fm. intermediate member Civitella M.ma Fm. lower member Palaeozoic Group Falsacqua Fm. Traces of F1 axial planes Traces of F2 axial planes 0 0 m Bedding D1 fold axes D2 fold axes S1 tectonic foliation S2 tectonic foliation 6 Fig. 9a - Geological sketch-map showing the superimposition of F 1 and F 2 fold systems. The Falsacqua F 1, with the Falsacqua Fm. at its hinge, was deformed by an asymmetric east-verging F 2 fold. In particular along the Falsacqua Creek the F 1 fold appears in the shorter sub-vertical limb of the later asymmetric F 2 fold. Consequently the F 1 is characterised by a vertical attitude, as attested by both the plunge of the fold axes and the dip of the axial surface. Schema geologico che mostra i rapporti osservabili alla scala cartografica tra i due sistemi di pieghe F 1 e F 2. In questo schema è riconoscibile una principale piega F 1 (piega del Fosso di Falsacqua), al cui nucleo compare la Formazione di Falsacqua, deformata da una piega F 2 asimmetrica vergente verso est. In particolare lungo il Fosso di Falsacqua la piega omonima compare in corrispondenza del fianco corto sub-verticale di una piega asimmetrica F 2. Ne consegue che in quest area la piega F 1 mostra il suo piano assiale e l asse con giacitura sub-verticale. Fig 9b - Geological cross-section along the trace shown in fig. 9a. Sezione geologica lungo la traccia riportata in fig. 9a.

13 THE METAMORPHIC UNITS OF THE EASTERN SIDE OF MONTE LEONI 32 STRUCTURES OF THE D 2 EVENT The D 1 structures have been later deformed by microto map-scale asymmetric folds (F 2 ) with a locally pervasive tectonic foliation (S 2 ). F 2 folds are frequently recumbent and characterised by overturned shorter limbs and axial directions oriented mainly NE-SW (fig. 9). Their axial surfaces range from sub-horizontal to gently SE or NW dipping. The asymmetry of the folds, as well as the way-up of the bedding together with the S 0 /S 1 /S 2 angular relationships (fig. 14), point to an eastward vergence of F 2 folds. The meso- and macro-scale folds show different shapes and amplitudes in different lithotypes, such as tight and isoclinal folds (sensu WILLIAMS & CHAPMAN, 1979) in the metapelites, close and rarely tight folds (sensu WILLIAMS & CHAPMAN, 1979) in the metarenites and metaconglomerates. According to Hudleston s classification the F 2 folds occurring in the metapelites are referable to the B 4, B, C 4 and C classes (HUDLESTON, 1973), whereas those present in the metarenites and metaconglomerates correspond to the B 2, B 3, C 2, C 3, D 2 and D 3 classes. S 2, which is very pervasive in the metapelites and discontinuous in the metarenites, is represented by a sub-horizontal to gently NW or SE dipping crenulation-cleavage. In some cases, S 2 affecting the metapelites has transposed the oldest foliations. Microstructural analysis highlighted a syn-kinematic lowgrade metamorphism (M 2 ), occasionally developed and associated with S 2, which consists of phyllosilicates (mainly muscovite) and oxides (fig. 12). Consequently, the S 2 tectonic foliation can be described as spaced schistosity (sensu PASSCHIER & TROUW, 1996). Otherwise S 2 is not associated with a blastesis, but the reorientation of pre-existing minerals occurs along it. In this case the S 2 tectonic foliation can be described as a smooth crenulation-cleavage, parallel and discrete, with a % volume percentage of cleavage domains (sensu PASSCHIER & TROUW, 1996). STRUCTURES OF THE D 3 EVENT The S 0 /S 1 /S 2 pattern and the F 1 and F 2 folds are locally deformed by later, map-scale gentle folds (F 3 ) with axial directions mainly oriented NE-SW, as suggested by the orientation of the related spaced tectonic foliation (disjunctive cleavage) which is only recognisable in small outcrops (S 3 ) (fig. ). From microstructural analysis of some oriented samples, the S 3 foliation can be described as a very discontinuous tectonic foliation developed only in the phyllosilicates-rich layers (fig. 1). The S 3, never accompanied by blastesis, can be described as smooth crenulation-cleavage, parallel and gradational with a % volume percentage of cleavage domains (sensu PASS- CHIER & TROUW, 1996). STRUCTURES OF THE D 4 EVENT High angle normal faults dissect all the previous structures. The faults have been recognised by mapscale observations, but fault surfaces and related kinematic indicators are never observed. The faults are small in extent and characterised by decametres displacements. Their orientation ranges from N1 to N1 (fig. 2a). Fig. - Stereonets (Schmidt diagram, lower hemisphere) showing: A) and B) axes and contouring of F 2 folds (84 data); C) and D) axes and contouring of F 1 folds, directly measured or inferred by S 0 /S 1 relationships (32 data); E), F) and G) cyclographic, poles and contouring of S 2 tectonic foliation (67 data); H), I) and L) cyclographic, poles and contouring of S 1 tectonic foliation (41 data). Diagrammi stereografici (diagramma di Schmidt, emisfero inferiore): A) e B) mostrano rispettivamente gli assi delle pieghe F 2 ed il loro contouring (84 dati); C) e D) mostrano rispettivamente gli assi delle pieghe F 1 ed il loro contouring (32 dati); E), F) e G) mostrano rispettivamente le ciclografiche, i poli ed il contouring della foliazione tettonica S 2 (67 dati); H), I) e L) mostrano rispettivamente le ciclografiche, i poli ed il contouring della foliazione tettonica S 1 (41 dati).

14 326 M. ALDINUCCI ET ALII Fig SEM microphotos of a metapelitic sample belonging to the upper member of Civitella Marittima Formation; A) S 1 /S 2 relationships: S 1 is a continuous, very pervasive schistosity, while S 2 occurs as a discontinuous schistosity; S 2 locally causes the rotation and reorientation of the S 1 ; B) detail of the inset area in A showing in more detail the rotation and reorientation of the oldest tectonic foliation; C) microphoto of the inset area in B showing the mineralogical association, mainly consisting of phyllosicates (mainly white mica), developed along the S 2 foliation. Symbols: quartz (Qtz), white mica (Wm), chlorite (Cl), rutile (Rut). Foto al microscopio elettronico (SEM) di un campione di metapeliti appartenenti al membro superiore della Formazione di Civitella Marittima. A) relazioni tra S 1 e S 2 : la S 1 è caratterizzata da una scistosità pervasiva, mentre la S 2 è discontinua. Lo sviluppo della superficie S 2 causa localmente la rotazione della foliazione S 1. B) dettaglio di A che illustra la rotazione della S 1 in prossimità della S 2. C) dettaglio di B ove è evidente l associazione mineralogica costituita principalmente da fillosilicati (soprattutto mica bianca), sviluppati lungo la S 2 ; spiegazione dei simboli: quarzo (Qtz), mica bianca (Wm), clorite (Cl), rutilo (Rut). DISCUSSION AND CONCLUSION The metamorphic succession of Monte Leoni comprises deposits of the Variscan sedimentary cycle (Palaeozoic succession) and the unconformably overlying Triassic siliciclastics (Civitella Marittima Fm. and Anageniti minute Fm.) and metacarbonates and metapelites (Tocchi Fm.) of to the Alpine sedimentary cycle. The lithological and sedimentological features of the Falsacqua Fm. led us to refer it to the Farma Fm., as previously suggested by GELMINI (1969), which is typically exposed in the Monticiano-Roccastrada Ridge and is also documented in the subsurface of the Monte Amiata geothermal field (PANDELI et alii, 1988). In this framework, the dominantly carbonate horizon with minor metapelites, which conformably overlies the Falsacqua Fm., still probably belongs to the Farma Formation. Indeed, carbonate components have been frequently recognised in the upper part of the Farma Fm. exposed in the Monticiano-Roccastrada Ridge (COSTANTINI et alii, 1988). As a consequence, the Palaeozoic succession exposed in the eastern side of Monte Leoni represents the southernmost outcrops of such a formation. In the type-area (Farma Valley, central part of the Monticiano-Roccastrada Ridge) the Farma Fm. has been referred to the Moscovian (COSTANTINI et alii, 1988) due to the finding of Fusulinids within carbonate clasts and occasionally in the matrix of a metaparaconglomerate bed (PASINI, 19). The Farma Fm. is thought to record a Variscan, syn-tectonic turbiditic sedimentation (COSTANTINI et alii, 1988; LAZZAROTTO et alii, 03), similar to the Culm facies of the Southern Alps (e.g. Hochwipfel flysch in the Carnian Alps: SPALLETTA et alii, 19) and Central Europe. After the Farma Fm. deposition, an intense erosional phase preceded the deposition of the Verrucano metasediments. They filled new accommodation space related to the Triassic continental rifting (CASSINIS et alii, 19) that led to the Middle Jurassic spreading of the Piedmont-Ligurian Ocean (ABBATE et alii, 19, 1994). In the eastern side of Monti Leoni, the Verrucano succession lacks the Monte Quoio Fm. (COSTANTINI et alii, 1988), typically represented by metapelites and quartzose metasandstones with associated lens-shaped, lithic-rich (mainly carbonate pebbles and quartzite pebbles and boulders) quartzose metaconglomerates. The Monte Quoio Fm. crops out extensively in the northern Monticiano-Roccastrada Ridge, where it unconformably overlies the Civitella Marittima Fm. (ALDINUCCI et alii, 03); southward it pinches out, being progressively replaced by the Anageniti minute Formation. However,

15 THE METAMORPHIC UNITS OF THE EASTERN SIDE OF MONTE LEONI 327 Fig S 0 /S 1 /S 2 relationships in the purple metapelites of the upper member of Civitella Marittima Fm. a) Outcrop photograph showing the east dipping S 2 tectonic foliation superimposed on the west-dipping S 1. The «illusory» angle between the bedding surface (S 0 ) and S 1 is due to the rotation induced by the S 2 development. b) Photomicrograph (plane polarised light, 2X magnification) showing the enlarged white rectangle in a: S 1 is sub-parallel to S 0, both deformed by microfolds (F 2 ) with associated S 2 foliation; c) Photomicrograph (plane polarised light, 0X magnification) showing the S 0 /S 1 relationships with the strong flattening of quartz-grains along the S 1. Rapporti tra S 0, S 1 e S 2 entro le metapeliti viola del membro superiore della Formazione di Civitella Marittima. a) foto di un affioramento ove è riconoscibile la foliazione S 1 sub-parallela alla superficie di stratificazione (S 0 ) ed immergente verso ovest, entrambi attraversate dalla foliazione S 2 immergente verso est. L angolo «apparente» che la foliazione S 1 forma con la superficie di stratificazione S 0 è dovuto alla rotazione della S 1 durante lo sviluppo della S 2 ; b) Dettaglio di a (N.I., 2X) dove è riconoscibile il sistema S 0 /S 1 deformato da micropieghe (F 2 ) con associata la foliazione S 2 ; c) Microfoto (N.I., 0X) che illustra con maggior dettaglio i rapporti angolari tra S 0 ed S 1 e dove sono riconoscibili granuli di quarzo fortemente appiattiti lungo la superficie S 1. lithic-rich metaconglomerates, strictly resembling those of the Monte Quoio Fm., although finer grained and devoid of carbonate clasts, occur in the lowermost part of the Anageniti minute Formation. The depositional environments of the Verrucano Group range from alluvial to coastal marine. The Civitella Marittima Fm. was deposited in a retrogradational fluvial fan setting (sensu STANISTREET & MCCARTHY, 1993), possibly represented by coalescing terminal fans (ALDINUCCI et alii, 03). In this framework the fining-upwards successions of the lower and intermediate members represent deposition within distributaries of the fluvial fan system (FRIEND, 1978; GRAHAM, 1983; KELLY & OLSEN, 1993). The purple metapelites at the top of these successions represent interchannel deposits following the avulsion of distributaries. The carbonate nodules, locally

16 328 M. ALDINUCCI ET ALII Fig Outcrop photographs and related line-drawings of rotated quartz-clasts along the S 1 tectonic foliation (quartzose metaconglomerates of to the lower member of the Civitella Marittima Fm.). The sense of shear inferred by the rotation of the quartz clasts is only coherent with respect to the location of the F 1 hinge folds. Foto e relativi line-drawing di clasti quarzosi ruotati lungo la foliazione tettonica S 1 in metaconglomerati appartenenti al membro inferiore della Formazione di Civitella Marittima. Il senso di taglio dedotto dalla rotazione dei clasti è sempre coerente con la posizione del nucleo delle pieghe F 1. included within purple metapelites, have been interpreted as caliche, attesting to pedogenic processes in a semi-arid climate (GOUDIE, 1973). The coarse-grained bodies of fining-upwards successions record ephemeral stream deposition by mean of high-density flows, mainly hyperconcentrated flows (NEMEC & STEEL, 1984; SMITH & LOWE, 1991; MUTTI et alii, 00; BENVENUTI & MARTINI, 02), which were triggered by heavy rains (flash floods). After a first phase of en-masse deposition, such flows became more dilute (i.e. stream flows: sensu COSTA, 1988) and deposited pebbly sands, mainly under upper-stage planebed regime (parallel-laminated beds). The transition from the basal to the intermediate member reflects the transition from the proximal to the intermediate part of the fluvial fan system, whereas the upper member records the distal end of such a system, corresponding to a floodbasin. There, sheet-floods (lithofacies 2 p.p.) spread in a flood-plain (lithofacies 1), giving rise to occasional ephemeral shallow lakes (lithofacies 2 p.p.), locally subjected to wind-blown sand deposition (lithofacies 3, see also ALDINUCCI et alii, 03). In this framework, the wellsorted quartzose metasandstones of lithofacies 4 (fig. 6d) could represent either short-lived marine transgressions over the coastal flood-plain, where alluvial sediments winnowed by waves probably accumulated into coastal bars or, alternatively, eolian deposits. Unfortunately, the S0//S1 W S2 S2 S2 S0 S1 S1 Younging Fig Line-drawing of an outcrop belonging to the Anageniti minute Fm. exposed in the Montorsaio area. The F 2 east-verging folds deform the S 0 /S 1 system, with S 1 at a low angle with respect to S 0. The angular S 0 /S 1 relationships and the younging of the beds point to an east-vergence for the D 1 structures as well. Line-drawing di un affioramento appartenente alla Formazione delle Anageniti Minute osservabile nell area di Montorsaio. Le pieghe F 2 a vergenza orientale deformano il sistema S 0 /S 1, con la S 1 a basso angolo rispetto alla superficie di stratificazione S 0. I rapporti angolari tra S 1 e S 0, unitamente alla polarità stratigrafica degli strati, indicano una vergenza orientale anche delle strutture collegate con l evento D 1. E

17 THE METAMORPHIC UNITS OF THE EASTERN SIDE OF MONTE LEONI 329 metamorphism and the rare exposures of this lithofacies prevent accurate sedimentological analyses. The fine-grained lithofacies of the upper member of the Civitella Marittima Fm. are erosively overlain by the metaconglomerates of the Anageniti minute Formation. This contact (fig. 16) has been interpreted as an unconformity (disconformity) on the basis of both regionalscale stratigraphic considerations and sedimentological evidence (ALDINUCCI et alii, 03). Consequently the Civitella Marittima Fm. has been correlated to the Punta Bianca lower cycle (MARTINI et alii, 1986), whereas the Anageniti minute Fm. and the Tocchi Fm. would be referable to the Punta Bianca upper cycle, which corresponds to the Verrucano Group (Verruca Fm. and Monte Serra Quartzite Fm.) of the Monti Pisani (RAU & TONGIORGI, 1974; TONGIORGI et alii, 1977). The sedimentological features of the Anageniti minute Fm., such as the abundance of massive to crude normally graded metaconglomerates and pebbly metasandstones, combined with their overall moderate to poor sorting, all point to deposition by mean of flash floods. These heavily sediment-charged flows were typically represented by hyperconcentrated flows to upper-plane bed stream flows, the latter indicated by crude, discontinuous parallel laminations and rare imbricated clasts. Channel-forms have not been recognised, even though field work gives suggestions of channels with low-angle perimeters (sensu FRIEND, 1983) and high width/depth ratio (sheet-like channels sensu MIALL, 1996), encased within the floodplain purple metapelites. These were locally subjected to pedogenic processes and to sand input by major floods. On the whole, the Anageniti minute Fm. was deposited within shallow ephemeral streams surrounded by floodplains. Finally, the alternating varicoloured quartzose metasandstones and metapelites, at the top of the Anageniti minute Fm., probably record coastal flood-plains and/or lagoons, which heralded the lagoonal, mainly carbonate deposition of the Carnian Tocchi Formation. The structural study revealed a tectonic setting grossly coherent with previous studies on the Monticiano- Fig. 1 - S 0 /S 1 /S 2 /S 3 relationships in metapelites of to the upper member of Civitella Marittima Fm. (north-west on the left and south-east on the right). a) Outcrop photo showing F 2 folds deforming the S 0 /S 1 system with S 0 sub-parallel to S 1. b) enlarged zone of the inset area in a showing S 2 axial plain foliation associated with the F 2 folds, defined as crenulation cleavage; S 2 foliation is recognisable at the hinge of the fold, whereas in the fold limbs it is parallel to the S 0 /S 1 system. c) Photomicrographs (plane polarised light, magnification 2X) of the inset area in b, showing S 2 foliation crenulated by later kinks (C 3 ) with associated S 3 discontinuous tectonic foliation clearly evident only in the phyllosilicate films. d) Photomicrograph (plane polarised light, magnification 0X) of the inset area in c showing detail of the S 0 /S 1 /S 2 relationships. Relazioni tra S 0, S 1, S 2 ed S 3 entro le metapeliti del membro superiore della Formazione di Civitella Marittima. In tutte le foto il nord-ovest corrisponde con il lato sinistro, mentre il sud-est con il lato destro. a) Foto di affioramento ove sono riconoscibili pieghe F 2 che deformano il sistema S 0 /S 1, con la superficie S 1 sub-parallela alla S 0. b) dettaglio del riquadro in a che mostra la foliazione S 2 di piano assiale rispetto alla piega F 2, descrivibile come clivaggio di crenulazione; la S 2 è ben riconoscibile in corrispondenza della cerniera della piega, ma diviene parallela in corrispondenza dei fianchi, laddove si sovrappone al sistema S 0 /S 1. c) Microfoto (N.I., 2X) dettaglio del riquadro in b dove sono riconoscibili pieghe C 3 che deformano la foliazione S 2 e danno luogo ad una foliazione S 3 discontinua, sviluppata esclusivamente entro i livelli fillosilicatici. d) dettaglio di c (N.I., 0X), ove sono evidenti i rapporti tra le superfici S 0, S 1 e S 2.

18 3 M. ALDINUCCI ET ALII Fig Quartzose metaconglomerates of the Anageniti minute Fm. (a) which erosively overlies alternating quartzose metasandstones and metapelites of the upper member of the Civitella Marittima Formation (b). Metaconglomerati quarzosi delle Anageniti minute (a) che poggiano con contatto erosivo su metarenarie quarzose e metapeliti del membro superiore della Formazione di Civitella Marittima (b). Roccastrada Ridge (MECCHERI et alii, 1987; COSTANTINI et alii, 1988; CONTI et alii, 1991; MORETTI, 1991; CORSI et alii, 01). Nevertheless, the inferred tectonic evolution of the Monte Leoni metamorphic succession partially differs from that proposed by MECCHERI et alii (1987) and MORETTI (1991). A D 1 non-metamorphic deformational event is claimed by MECCHERI et alii (1987) in order to explain the stratigraphic superimposition of the basal Anageniti minute Fm. metaconglomerates onto the finegrained upper member of the Civitella Marittima Fm. In fact, such a superimposition was linked by MECCHERI et alii (1987) to thrusting associated to the syn-collisional compressive tectonics. Moreover, according to MECCHERI et alii (1987), MORETTI (1991) supposed the existence of a D 1 tectonic event which gave rise to thrusts without development of either folds or tectonic foliation. However, the existence of this tectonic event is not supported by either structural evidence or stratigraphic constraints. As a consequence the D 2 and D 3 tectonic events of MEC- CHERI et alii (1987) correspond respectively to the D 1 and D 2 of this paper. The petrographic studies carried out in the metamorphic units exposed in the inner part of the Northern Apennines (Giglio Island: ROSSETTI et alii, 1998; Monte Argentario: THEYE et alii, 1997, BRUNET et alii, 00) and particularly in the southern and northern extension of the Monticiano-Roccastrada Ridge (GIORGETTI et alii, 1998; BRUNET et alii, 00) revealed a mineralogical paragenesis indicating HP/LT metamorphism associated with the collisional tectonics. In particular, Mg-carpholite occurring in syn-foliation quartz veins and the mineralogical assemblages consisting of chlorite, chloritoid, pyrophyllite and quartz led GIORGETTI et alii (1998) to refer the peak metamorphism of the metasediments of the Verrucano Group to temperatures ranging from to 4 C and pressure ranging from 8- Kbar. The observed mineralogical assemblage related to the different tectonometamorphic events fit well with the paragenesis documented by previous Authors in the eastern side of Monte Leoni. The S 3 tectonic foliation, associated to the gentle folds deforming the D 1 and D 2 structures, highlights, for the first time, a D 3 deformational event, as previously documented in the Tuscan Metamorphic Basement exposed in other areas of southern Tuscany (CORSI et alii, 01; COSTANTINI et alii, 02; CIRILLI et alii, 02) and also in the subsurface of the Mt. Amiata geothermal field (ELTER & PANDELI, 1991; BROGI, 03). According to the tectonic history of the Monticiano- Roccastrada Ridge (COSTANTINI et alii, 1988; CONTI et alii, 1991), the D 1 event is referable to the Northern Apennines syn-collisional tectonics. The D 2, according to CARMIGNANI et alii (1994) and LIOTTA (02), could be related to the conjugate extensional shear zones which characterised the evolution of the Tuscan crust during the Early-Middle Miocene extensional stage. D 3 and D 4 events represent the later evolution of the extensional tectonics which developed during the Late Miocene Quaternary time interval (CARMIGNANI et alii, 01 with references therein). ACKNOLEDGEMENTS The authors thankfully acknowledge Gianluca Cornamusini, Anna Gandin, Antonio Lazzarotto and Domenico Liotta for their comments and fruitful discussions. This research is part of the Phd theses of Mauro Aldinucci (stratigraphic-sedimentological data) and Andrea Brogi (structural data) and benefit of the supervision of Fabio Sandrelli. Barbara Terrosi and Antonella Mancini are thanked for the electronic editing of the geological map. The work is financially supported by P.A.R. 04 grants (coordinator Fabio Sandrelli). Enrico Pandeli and an anonymous referee are thanked for their remarks and critical reading of the previous version of the manuscript.