Fossiliferous high-pressure metasediments from the Western Alps (Petit-Saint-Bernard Pass,Subbriançonnais Unit,Italy)

International Journal of Earth Sciences -     

Metamorphic evolution of the Briançonnais units along the ECORS-CROP profile (Western Alps): new data on metasedimentary rocks

Briançonnais units are squeezed between two Mesozoic eclogitic belts (Piemont-Ligurian ocean and Valaisan ocean) along the ECORS-CROP seismic line in the Italian-French Western Alps (France, Italy). The metamorphic evolution of this area plays a key role for understanding the evolution of the Western Alps and is discussed on the basis of detailed petrographic investigations carried out on weathered sediments issued from the erosion of the Hercynian belt, especially on lower Permian to Mesozoic sediments. In the Zone Houillère, as well in the Permo-Triassic cover of the Briançonnais basement, the index metamorphic mineral assemblage is mainly composed of white micas with varying chemical composition, chloritoid and garnet. This same assemblage occurs within different lithologies (metaarkose, metapelite, metasandstone). Consequently, equilibrium phase diagrams were computed for different whole rock compositions using DOMINO software. The results of the P-T investigations clearly show that each unit underwent a different sequence of metamorphic reactions. An increase in metamorphic grade from greenschist facies conditions in the Northwest (Zone Houillère) to the transition between blueschist and eclogite facies conditions in the Southeast (Internal Briançonnais) is observed. A major discontinuity in metamorphic grade is located at the contact between Zone Houillère and Ruitor unit, as documented by a pressure gap of ~ 7 kbar. In general, the observed metamorphic field gradient is inverted and is interpreted to represent different depths of burial during subduction, which correlates with the paleogeographic position of the different units.     

Middle-Late Jurassic syndepositional tectonics recorded in the Ligurian Briançonnais succession (Marguareis–Mongioie area,Ligurian Alps,NW Italy)

The Middle–Upper Jurassic succession of the Marguareis–Mongioie area (Ligurian Briançonnais Domain), developed in a protected shelf environment evolving into a pelagic plateau, bears clear evidence of synsedimentary tectonics such as: growth fault-related structures; neptunian dykes; marked lateral variations in stratigraphic thicknesses testifying to the juxtaposition of sectors characterized by different sedimentation and subsidence rates; discordant, anomalous stratigraphic contacts corresponding to paleoescarpments; nodular beds showing evidence of fluidification interpreted as seismites; and the occurrence of sand-sized quartz grains pointing to denudation of Permo-Triassic quartz-rich rocks. Such evidence documents an important Middle-Late Jurassic post-breakup tectonic activity, which was more effective in controlling the basin topography than the Early Jurassic syn-rift tectonic phase. Two main tectono-sedimentary stages, one occurring during the Bathonian, the other falling within the Callovian–Kimmeridgian interval, were reconstructed. The first stage can be referred to a fault-related activity occurring shortly after the initial stages of oceanic spreading of the Ligurian Tethys; the second can be genetically related to the far effects of the first rifting stage of the Bay of Biscay and the Valais basin.     

Contrast in stress–strain history during exhumation between high- and ultrahigh-pressure metamorphic units in the Western Alps: Microboudinage analysis of piemontite in metacherts

Our analyses of microboudinage structures of piemontite grains embedded within six samples of metachert, one collected from an ultrahigh-pressure (UHP) metamorphic unit at Lago di Cignana in Italy of the Western Alps, and the other five from surrounding high-pressure (HP) metamorphic units in Italy and France, have revealed that the structures are all symmetrical in type, and were presumably produced in coaxial strain fields. Stress–strain analyses of the microboudinaged grains revealed significant contrasts in the stress and strain histories of the UHP and HP metamorphic units, with the differential stress recorded by the UHP sample being unequivocally lower than that recorded by the five HP samples. In addition, our analyses showed that the UHP sample underwent stress-relaxation during microboudinage, whereas the five HP samples did not. On the basis of these observations and analyses we discuss the mechanical decoupling of the UHP and HP units that led to different histories in differential stress between the units during exhumation of the Western Alps.     

Structural evolution of a briançonnais cover nappe,the caprauna-armetta unit (ligurian alps,Italy)

The Caprauna-Armetta Unit (CAU) is a Briançonnais cover nappe emplaced on the external margin of the Ligurian Briançonnais Zone. A structural analysis of the nappe indicates that there are four superposed deformations (D₁-D₄). D₁ produced large recumbent isoclinal folds associated with a strong axial-plane cleavage and a SW-trending lineation. These folds can be related to a SW-directed overthrust shear. D₂ produced open to moderately tight folds with subvertical axial planes, overturned towards the northeast. D₃ and D₄ are represented by large wavelength open folds affecting only the large-scale setting of the nappe.A finite strain map of the nappe has been compiled using data from an oolitic limestone layer. The measured strains appears to be essentially the product of the D₁ phase. The measured ellipsoids are generally triaxial. The trend of the finite strain X axes is towards the southwest. Prolate ellipsoids with very high R_xz ratios occur on the inverted limbs and sometimes near the hinge zones of the anticlinal F₁ folds. Oblate ellipsoids are prevalent on the normal limbs. This pattern of finite strain resulted from deformation in a ductile shear zone generated within the tectonic units trailed at the base of the huge Helminthoid Flysch Nappe during its motion towards the foreland.     

Stratigraphy and palaeoenvironment of the Lower-“middle” Oligocene units in the northern part of the Western Taurides (İncesu area,Isparta, Turkey)

This study describes the stratigraphic and palaeoenvironmental significance of the Lower-“middle” Oligocene sediments based on the fauna from the Delikarkas? Formation and the microflora from the ?ncesu Formation of the ?ncesu area (northern part of the western Taurides, Isparta province, Turkey). In the area, the Oligocene sediments show a regressive succession, which begins with the limestones of the Delikarkas? Formation indicating marine conditions followed by conglomerates, sandstones and coaly mudstones of alluvial and fluvial (shallow marine-continental) origin. A well preserved foraminiferal assemblage including Nummulites intermedius, Nummulites vascus and Halkyardia maxima proves an Early Oligocene age for the Delikarkas? Formation. Due to palynological markers such as Boehlensipollis hohli, Slowakipollis hippophaëoides, Dicolpopollis kockelii, Magnolipollis neogenicus ssp. minor, Plicapollis pseudoexcelsus, Caryapollenites simplex and Intratriporopollenites instructus the ?ncesu Formation, which concordantly rests on the Delikarkas? Formation, may be assigned to the Early-“middle” Oligocene. From the palynomorph assemblage, three zones have been recognised according to abundance of species. Zone 1 is characterized by predominance of C. simplex and Momipites punctatus and rarely presence of tricolpate and tricolporate pollen. Zone 2 consists mainly of Inaperturopollenites dubius, Leiotriletes maxoides ssp. maximus, Verrucatosporites favus, Verrucatosporites alienus and infrequently marine dinoflagellate cysts. Zone 3 is characterized by a high percentage of ferns such as Echinatisporis?chattensis and Polypodiaceoisporites saxonicus. The presence of marine dinoflagellate cysts like Apectodinium sp. and Cleistosphaeridium sp., back-mangrove elements such as Acrostichum aureum and lepidocaryoid palms (e.g. Longapertites discordis, Longapertites punctatus and Longapertites psilatus) in the sediments of the ?ncesu Formation imply coastal or near-coastal conditions. Terrestrial palynomorphs in more inland settings were transported by running water towards the sea. Conifers are represented by poorly preserved and rare pollen grains of Pityosporites, Cathayapollis and Piceapollis which may have been transported by wind. In this study, the terrestrial climate of the ?ncesu Formation is also discussed on the basis of the Coexistence Approach method. The climate was warm at the coast (over 20 °C), as evidenced by A. aureum and lepidocaryoid palms, whereas there was a mean annual temperature of 17.2–17.4 °C must be assumed for the upland environment(s).     

Fault slip analysis in the Koralm Massif (Eastern Alps) and consequences for the final uplift of “cold spots” in Miocene times

The Paleogene and Neogene evolution of Austroalpine basement units east of the Tauern Window is characterised by the formation of two major sets of faults: (1) ESE–WNW- to E–W-trending faults, associated with ENE- and NNW-trending conjugate structures and (2) N–S to NNE-SSW striking structures, mainly acting as high-angle normal faults, often associated with E-dipping low-angle normal faults along the western margin of the Styrian Basin.Together with the stratigraphic evolution of the Styrian and Lavanttal Basins and the related subsidence histories a tectonic evolution may be reconstructed for this part of the Eastern Alps. In the southern part of the Koralm Massif, WNW-trending fractures were activated as dextral strike-slip faults, associated with the evolution of WNW-trending troughs filled up with coarse block debris. W- to WNW-trending fractures were reactivated as normal faults, indicating N–S extension. It is assumed that these phases resulted in subsidence and block debris sedimentation in Karpatian and Badenian times (ca. 17–13 Ma).In the Western Styrian Basin no Sarmatian (13–11.5 Ma) sediments are observed; Pannonian (11.5 to 7.1 Ma) sediments are restricted to the Eastern Styrian Basin. This indicates, that the Koralm basement and the Western Styrian Basin were affected by post-Sarmatian uplift, coinciding with a re-activation of N-trending normal faults along the eastern margin of the Koralm Massif. Therefore, we suggest that the final uplift of the Koralm Complex, partly together with the Western Styrian Basin, occurred during the early Pannonian (at approximately 10 Ma). The elevation of clastic deposits indicates that the Koralm Complex was elevated by approximately 800 m during this phase, associated with an additional phase of E–W-directed extension accommodated by N–S striking normal faults.     

Late Triassic-Early Jurassic Paleokarst from the Ligurian Alps and its geological significance (Siderolitico Auct., Ligurian Briançonnais domain)

In the Ligurian Briançonnais domain, the thick Middle Triassic carbonateplatform units, formally known as the Costa Losera Fm. and S. Pietro dei Monti Dolomites, are followed by a significant unconformity separating them from the Rio di Nava neritic limestones of Bathonian age. According to the evolution of a passive continental margin, the end of subsidence and subsequent uplift led to the progressive erosion of the Triassic carbonate platform from outer to inner units, i.e., in the direction of the Ligurian Tethys. This erosion, possibly acting on a fault-controlled block system, created sedimentary sequences of differential composition, and the virtual absence of the entire Triassic complex and even the underlying Permian rocks. In the study area (and in many other locations belonging to the external Ormea unit), the so-called “briançonnais sedimentary gap” shows more than a nondepositional surface: the associated deposits (known as “Siderolitico” Auct.) include both red pelites between the above-mentioned units, and deep-penetrating karstic red breccia within the Ladinian carbonates. We performed detailed stratigraphic, microfacies and compositional analyses on the karsts and paleosoils associated with the unconformity in order to define their character, genesis and age. In addition, we discuss the regional context and importance of these deposits in a large-scale comparison with the classic Briançonnais domain and other locations of the Western Alpine Arch with the same unconformity. In light of these data, we propose an Upper Triassic to Early Jurassic age (until the Upper Bajocian?) for the karstic event in the Ligurian Alps.     

Geometry and impact of transpressional faulting in polyphasic metamorphic orogenic belts: the Viù Deformation Zone (inner Western Alps)

This article describes the geometry and structural architecture of the Viù Deformation Zone (VDZ), a brittle-ductile to brittle structure affecting the metamorphic units of the inner Western Alps, and its role in modifying the pre-existing syn-metamorphic structural setting. The VDZ reactivates and displaces the contact between two different oceanic units, the Lanzo Ultramafic Complex and the Lower Susa–Lanzo Valleys Unit, characterized by polyphase syn-metamorphic deformation. It shows a strike-slip duplex geometry, constituted by N–S reverse-dextral faults linked by NW–SE antithetical sinistral-reverse faults, and represents a contractional step-over zone along a N–S regional dextral-reverse structure, the Col del Lis-Trana Deformation Zone. Formation of these transpressional structures steepened the Lanzo Ultramafic Complex during the last stages of its exhumation. The 3D geometry of the VDZ was strongly controlled by the reactivation of pre-existing structures, such as the buried western edge of the Ivrea body and metamorphic foliations. Brittle reactivation also induced block rotation along the VDZ, causing anomalous kinematic relations between the VDZ-associated faults. This study, hence, shows that in metamorphic orogens the mechanisms generating strike-slip duplexes may be different from those classically provided by the literature, with brittle reactivation and block rotation strongly prevailing on newly formed faults. In such orogens, moreover, rotations induced by transpressional faulting may sometimes be mistaken for steep syn-metamorphic shear zones. Underestimating the effects of later brittle deformation and associated rotations may cause erroneous interpretations of the tectonic evolution of orogens.     

Structural Styles of the Longmenshan Thrust Belt and Evolution of the Foreland Basin in Western Sichuan Province, China

The Longmenshan thrust system consists of two major groups of structural styles according to the depth of their involvement: basement thrusts-compressional fault blocks; fold-thrust system in the cover. In cross-section, the Longmenshan structural belt is divided into 5 zones. The propagation of the Longmenshan thrust system is piggy-back due to pushing at the early stage and overstep due to gravity sliding at the late stage. Balanced cross-sections and palinspastic reconstruction reveal that the total sliding displacement of the thrust system amounts to 120 km. The tectonic evolution of the Tethys domain in western Sichuan has experienced 5 stages: continental break-up; ocean-continent subduction ; continent-arc collision; orogenic thrusting; uplift of western Sichuan.     

Geochronologic-petrochemical studies of the Hongshishan mafic–ultramafic intrusion,Beishan area,Xinjiang (NW China): petrogenesis and tectonic implications

The Hongshishan mafic–ultramafic intrusion (SIMS zircon U–Pb age 286.4 ± 2.8 Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni–Cu mineralization. Mass balance estimates and Sr–Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting.

Southward subduction of the Palaeo-Tianshan–Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic–ultramafic intrusions distributed in the eastern Tianshan–Beishan area. Integrating the regional positive ?_Nd(t) granites, high-Mg and low-Ti basaltic magmas (mafic–ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.     

Geochronology and geochemistry of late Jurassic adakitic intrusions and associated porphyry Mo–Cu deposit in the Tongcun area,east China: Implications for metallogenesis and tectonic setting

The genesis of adakites and associated Mo–Cu mineralization in non–arc settings in China is poorly constrained. Here, we present geochronology, geochemistry and Sr–Nd–Pb isotopes for the Tongcun intrusive complex, and report Pb isotopes and ⁴⁰Ar–³⁹Ar age for the Tongcun Mo–Cu deposit. The Tongcun intrusive complex is composed mainly by granodiorite and monzogranite (phase 1 and phase 2), with emplacement age of 160 Ma to 148 Ma. The Tongcun complex can be classified as typical high–K calc–alkaline I–type granitoid and also shows adakitic geochemical features. Moreover, the negative Nb, Ta, Ti, and P anomalies and enriched initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7083–0.7092 of the Tongcun intrusive complex are consistent with those of the subduction–related magmatism. The ⁴⁰Ar–³⁹Ar dating of sericite, which is intergrown with chalcopyrite, indicates that the late Cu mineralization event occurred at ~ 155.5 Ma. The early Mo (Cu) and the late Cu mineralization events in this deposit were temporally, spatially and genetically associated with the emplacement of monzogranite (phase 1). There are no obvious linear correlation between SiO₂ and most of the major and trace elements, and all rock samples fall within the fields of unfractional crystallization felsic granites in Zr + Nb + Ce + Y versus FeO^T/MgO and (K₂O + Na₂O)/CaO diagrams, indicating that partial melting rather than fractional crystallization has played an important role for the formation of the Tongcun intrusive complex. Magmatic inherited zircons from the Tongcun granitoids with the age peaked at 780–812 Ma, imply that the Neoproterozoic igneous rocks in the lower crust have been incorporated into the magma source. The uniform ε_Nd(t) (− 6.3 to − 7.3), initial ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁴Pb (15.596–15.621), and ²⁰⁸Pb/²⁰⁴Pb (38.374–38.650), as well as high K₂O contents (3.36–4.10 wt.%) and relatively high Mg# values (35.40 to 40.30) suggest the Tongcun intrusive complex was derived from partial melting of the thickened lower continental crust triggered by basaltic magma underplating plus additional input from the EM II mantle-derived basaltic melts. The Tongcun area was controlled by a compression setting related to the subduction of the Paleo–Pacific Plate in Mesozoic period.     

Geochemistry and petrology of the lower Miocene bimodal volcanic units in the Tunçbilek–Domaniç basin,western Anatolia

ABSTRACT

The lower Miocene (~22–19 Ma) volcanic units in the NE–SW-trending Tunçbilek–Domaniç basin, located in the northeastern-most part of the Neogene successions in western Anatolia, are composed of (1) high-K, calc-alkaline dacitic to rhyolitic volcanic rocks of the Oklukda?? volcanics; (2) calc-alkaline low-MgO (evolved) basalts; and (3) high-MgO mildly alkaline basalts of the Karaköy volcanics. Sr isotopic ratios of the volcanic units increase from high-MgO (~0.7055–0.7057) to low-MgO basaltic rocks (~0.7066–0.7072) and then to dacitic-rhyolitic rocks (0.7081–0.7086). Geochemical features of the volcanic rocks reveal that the calc-alkaline evolved basalts were formed by mixing of basic and acidic magmas.

Geochemical studies in the last decade show that the Miocene mafic volcanic rocks in western Anatolia are mainly composed of high-MgO shoshonitic-ultrapotassic rocks (SHO-UK), of which mantle sources were variably, but also intensely metasomatized with crustally derived materials during collisional processes in the region. However, geochemical comparison of the high-MgO basalts of the Karaköy volcanics with the SHO-UK rocks in this region reveal that that the former has too low ⁸⁷Sr/⁸⁶Sr_(i) and high ¹⁴³Nd/¹⁴⁴Nd_(i) ratios, with lower LILE and LREE abundances, which are firstly described here. These features are interpreted to be derived from more slightly enriched lithospheric mantle sources than that of the SHO-UK. Accepting the SHO-UK rocks in the region were derived from mantle sources that had been metasomatized by northward subduction of crustal slices during Alpine collisional processes, it is proposed that the imbrication and direct subduction of crustal slices were not reached to, and were limited in the mantle domains beneath the basin. The dacites of the Oklukda?? volcanics might be formed either by high-degree melting of the same sources with the SHO-UK, or by melting of the lower crustal mafic sources as previously proposed, and then evolved into the rhyolites via fractional crystallization with limited crustal contribution.     

Globular Phyllosilicates of the Glauconite–Illite Series in the Cambrian and Ordovician Rocks of the Eastern Baltica (Northern Estonia,Western Lithuania,and Western Latvia)

Lithology and Mineral Resources - The mineralogical, structural and crystal-chemical features of seven samples of globular phyllosilicates of the glauconite–illite series (GPS) from the Lower...     

The Taraklı Flysch in the Boyali area (Sakarya Terrane,northern Turkey): Implications for the tectonic history of the IntraPontide suture zone

In the Boyal? area, northern Turkey, the tectonic units of the ?stanbul–Zonguldak Terrane and the IntraPontide suture zone are thrust over the deposits at the top of the Sakarya Terrane, known as Tarakl? Flysch. It consists of Early Maastrichtian–Middle Paleocene turbidite and mass-gravity deposits, whose source mainly corresponds to the ?stanbul–Zonguldak Terrane, and, with a lesser extent, to the IntraPontide suture zone. These deposits were sedimented in a foredeep basin developed during the convergence between Sakarya and Eurasian continental microplates. In the Late Paleocene–Early Eocene time span, the Tarakl? Flysch was deformed (D1 phase) during the closure of the foredeep basin. In the Miocene time, the strike-slip tectonics (D2 phase) related to the North-Anatolian fault produced further deformations of the Tarakl? Flysch.     

Characterization of non-equilibrium and equilibrium occurrences of paragonite/muscovite intergrowths in an eclogite from the Sesia–Lanzo Zone (Western Alps, Italy)

 Coexisting muscovite and paragonite have been observed in an eclogite from the Sesia–Lanzo Zone (Western Alps, Italy). The P-T conditions of this eclogite reached 570–650 °C and 19–21 kbar and the rocks show several stages of mineral growth during their retrograde path, ranging from the subsequent lower-P eclogite facies to the blueschist facies and then the greenschist facies. Muscovite and paragonite are very common in these rocks and show two texturally different occurrences indicating equilibrium and non-equilibrium states between them. In one mode of occurrence they coexist in equilibrium in the lower-P eclogite facies. In the same rock muscovite ± albite also replaced paragonite during a greenschist-facies overprint, as evidenced by unique across – (001) layer boundaries. The chemical compositions of the lower-P eclogite-facies micas plot astride the muscovite – paragonite solvus, whereas the compositions of the greenschist-facies micas lie outside the solvus and indicate disequilibrium. The TEM observations of the textural relations of the greenschist-facies micas imply structural coherency between paragonite and muscovite along the layers, but there is a sharp discontinuity in the composition of the octahedral and tetrahedral sheets across the phase boundary. We propose that muscovite formed through a dissolution and recrystallization process, since no gradual variations toward the muscovite – paragonite interfaces occur and no intermediate, homogeneous Na-K phase has been observed. Because a solid-state diffusion mechanism is highly unlikely at these low temperatures (300–500 °C), especially with respect to octahedral and tetrahedral sites, it is assumed that H₂O plays an important role in this process. The across-layer boundaries are inferred to be characteristic of such non-equilibrium replacement processes. The characterization of these intergrowths is crucial to avoiding erroneous assumptions regarding composition and therefore about the state of equilibrium between both micas, which in turn may lead to misinterpretations of thermometric results. Received: 3 February 1999 / Accepted: 19 October 1999     

Redefinition of Early Mesoproterozoic (1800–1600 Ma) stratigraphy in the northern Kongling area,China: The nucleus of Yangtze Craton and its tectonic significance

The Wujiatai Formation, which is well exposed in Huangjiatai-Xichahe region of the northern Kongling area of central Yangtze Craton, is a suite of epimetamorphic conglomerates to pebbly sandstones to fine sandstone-dolostones deposited in littoral-carbonate platform facies. The formation has angular unconformity contacts with both the overlying Neoproterozoic Nantuo Formation and the underlying Paleoproterozoic Huanglianghe Formation complex. Detrital zircons from metafine sandstones of the lower Wujiatai Formation have ages ranging from 3377–1828 Ma, with the youngest zircons dating to about 1828 Ma. In addition, whole-rock Pb-Pb isochron ages from dolostones in the upper Wujiatai Formation yield an age of 1718±230 Ma. These dates constrain the depositional age of the Wujiatai Formation between 1800 Ma and 1600 Ma. These are the earliest Mesoproterozoic sedimentary records reported in the Kongling region, and fill the gaps in Early Mesoproterozoic stratigraphy in Yangtze Craton. Histograms of detrital zircon ages for the Wujiatai Formation reveal four major peaks at 2039 Ma, 2691 Ma, 2966 Ma and 3377 Ma, which is consistent with the ages of the basement rocks that underlie the center of Yangtze Craton, indicating that sediment provenance is mainly from the Kongling complex. The lower Wujiatai Formation mainly consists of clastic rocks, whereas the upper Wujiatai Formation consists of dolostones. This stratigraphic change implies a deepening sequence in an expanding basin with an initial cratonic rifting tectonic setting, corresponding to the initial breakup of the Columbia supercontinent in Yangtze Craton.© 2019 China Geology Editorial Office.     

Metamorphic consequences of Neogene thermal anomaly in the northern Apennines (Radicondoli-Travale area,Larderello geothermal field – Italy)

In the Radicondoli–Travale area of the Larderello geothermal field (Italy) new structural and petrologic data on some metamorphic units, cored from geothermal wells, reveal the existence of a wide complex of hornfels rocks. The development of the complex is related to the emplacement of Neogene magmatic rocks at a shallow depth within a Permo–Triassic terrigenous metasedimentary sequence of a low metamorphic grade. The sequence was subjected to alpine tectonic-metamorphic events. This reconstruction gives new insights in the tectonic setting of metamorphic units below the Tuscan Nappe, in southern Tuscany. Particularly, the medium and high grade metamorphic rocks seem strictly related to the development of Neogene thermal aureoles and do not represent remnants of a Paleozoic basement. Furthermore, in this sector of the Larderello geothermal system, the presence of deep geothermal reservoir at a depth of 3 000 m is mainly linked to the enhanced permeability caused by fracturing in these hornfels rocks. This fracturing is a consequence of the dehydration reaction occurring in the metapelites due to Neogene thermal metamorphism. This mechanism allowed the development of a long-lived hydrothermal system, shown by the secondary mineralogical assemblages. These testify the presence of at least two hydrothermal stages which are well preserved in the less permeable units.