Selected clast-poor mm-sized PST samples were crushed and studied by X-ray diffraction on a Phili... more Selected clast-poor mm-sized PST samples were crushed and studied by X-ray diffraction on a Philips® PW3710. The goniometer, using conventional Bragg-Brentano parafocusing geometry, was equipped with incident-beam and diffracted-beam Soller slits, a curved graphite diffracted-beam monochromator, and a scintillation counter. The normal-focus Cu-Ka 1 X-ray tube operated at 40 kV and 20 mA. Spectrograms were recorded in the 3–70° 2! range in continuous-scan mode with an increment of 0.02° and a counting time of 0.5 s for each step, and processed by the PC-APD software (Philips ®). 2! and d-space values were calculated using the second-derivative algorithm of SAVITZKY & GOLAY (1964). Rock and mineral analyses Bulk compositions of the host rocks and PST veins were acquired by the XRF technique, using a Philips® PW2400 instrument equipped with an Rh tube. Before analysis, rock powders were dried at 860 °C for 20 min and then at 980 °C for 2 h. Precision was better than 1%. The average compositions of phases, minerals and PST glassy groundmass were analysed using a Cameca SX50 electron microprobe (University of Paris), under the following conditions: beam diameter ~1 mm; accelerating voltage 15 kV; beam current 10 nA (micas and glassy groundmass) or 40 nA (other minerals). Compositions of the PTS glassy groundmass were obtained by averaging ca. 100 analyses of clast-free zones (15 µm " 15 µm) scanned by the electron beam. The technique does not allow reaching 100% of total weight, due to the scanning of surface defects (holes, cracks), hydrated glass and micaceous crystallites. 40 Ar-39 Ar dating Selected portions of PST veins were drilled (diameter 9 mm) from polished sections and then investigated by BSE at the SEM. Then, the same sections were slightly polished again, heated on a hot plate to remove the sample from the holding glass, and cleaned in methanol and deionised water in an ultrasonic bath. Samples were later wrapped in Al foil, loaded in quartz vials together with the Fish Canyon Tuff biotite standard (27.95 Ma; BAKSI et alii, 1996) and irradiated for 28 hours in the central thimble of the 250 kW TRIGA reactor (University of Pavia). The irradiated samples were placed inside an ultrahigh vacuum laser chamber and baked overnight at about 180°C to remove adsorbed atmospheric argon. In-situ analyses were performed
Pseudotachylyte veins of the Tonale nappe (NE Italy) occur along faults and shear zones which acc... more Pseudotachylyte veins of the Tonale nappe (NE Italy) occur along faults and shear zones which accommodated the deformation that shortened the Adriatic crust to the north of the Peria-driatic Lineament during the late Alpine orogeny. These veins are characterized by thin glass layers showing variable clast content and colours, coupled with chemical variations. They are derived from surrounding gneiss, but mass-balance studies show some quartz missing in the veins (i.e., 1 rock = a vein + b quartz, with b ranging from 0.1 to 0.3). Thermodynamic modelling of the melting process indicates that the energy necessary to melt 1 cm 3 of rock is 4-5 kJ. Temperature reached inhomogeneous values between ~1100°C (total melting of micas) and ~1700°C (partial melting of the quartz clasts). The modelling of the cooling process accounts well for the microlite blastesis in the glassy groundmass of the veins, provided that the pressure at which pseudotachylytes formed along faults/shear zones ranges between 4 and 8 kbar. 40 Ar-39 Ar laser-probe in-situ data show that vein zoning corresponds to a range of apparent ages. Step-heating analyses indicate the presence of components younger and older than the age proposed for the vein, due to alteration and clasts, respectively. Interplay between these two factors affects the in-situ data. The new 40 Ar-39 Ar ages of the Tonale nappe pseudotachylytes (~50-21 Ma), as compared to previously published data, testify a long-lived deformation lasting from Eocene to Early Miocene. The peak of this deformation is Late Eocene-Early Oligocene in age (~35-30 Ma), coeval with the emplacement of the Northern Adamello plutons and of several apophyses and dykes in the Adriatic crust.
In the Southern Aosta Valley, the St. Marcel Valley metaophiolites consist of mainly metavolcanic... more In the Southern Aosta Valley, the St. Marcel Valley metaophiolites consist of mainly metavolcanics and their sedimentary cover metamorphosed under HP subduction-related metamorphism. A detailed geological map carried out in the St. Marcel Valley reveals that the metasedimentary cover, although transposed by the Alpine tectonics, is essentially made of three main terms, that are Mn-rich metaquartzites, marble, and calcschists. The metasedimentary sequence is quite comparable with the unmetamorphosed sequence made of radiolarian cherts, Calpionella limestones and Palombini shales covering the Ligurian ophiolites. The St. Marcel Valley metaophiolites represent the upper crustal section of the Mesozoic Tethyan ocean and its pelagic sedimentary cover, overthrusting the serpentinite unit of the Mount Avic, to the East. Within the Piemonte nappe stack, the St.Marcel metaophiolites are located at the higher structural level.
La miniera di Lovignanaz (conosciuta anche come miniera di Molina o Mollère) è situata sulla sini... more La miniera di Lovignanaz (conosciuta anche come miniera di Molina o Mollère) è situata sulla sinistra idrografica della Val Clavalité (1350-1425 m s.l.m.), a sud del paese di Fénis (AO). Le gallerie tuttora visibili si sviluppano entro cloritoscisti, talcoscisti e metagabbri trasposti tettonicamente con i calcescisti appartenenti alla Piemontese dei calcescisti con pietre Le rocce ospitanti le mineralizzazioni a solfuri di ferro e rame mostrano associazioni mineralogiche tipiche delle facies scisti blu ed eclogitica, analogamente alle rocce del più noto complesso minerario di Saint-Marcel (Martin et al., 2008).
Selected clast-poor mm-sized PST samples were crushed and studied by X-ray diffraction on a Phili... more Selected clast-poor mm-sized PST samples were crushed and studied by X-ray diffraction on a Philips® PW3710. The goniometer, using conventional Bragg-Brentano parafocusing geometry, was equipped with incident-beam and diffracted-beam Soller slits, a curved graphite diffracted-beam monochromator, and a scintillation counter. The normal-focus Cu-Ka 1 X-ray tube operated at 40 kV and 20 mA. Spectrograms were recorded in the 3–70° 2! range in continuous-scan mode with an increment of 0.02° and a counting time of 0.5 s for each step, and processed by the PC-APD software (Philips ®). 2! and d-space values were calculated using the second-derivative algorithm of SAVITZKY & GOLAY (1964). Rock and mineral analyses Bulk compositions of the host rocks and PST veins were acquired by the XRF technique, using a Philips® PW2400 instrument equipped with an Rh tube. Before analysis, rock powders were dried at 860 °C for 20 min and then at 980 °C for 2 h. Precision was better than 1%. The average compositions of phases, minerals and PST glassy groundmass were analysed using a Cameca SX50 electron microprobe (University of Paris), under the following conditions: beam diameter ~1 mm; accelerating voltage 15 kV; beam current 10 nA (micas and glassy groundmass) or 40 nA (other minerals). Compositions of the PTS glassy groundmass were obtained by averaging ca. 100 analyses of clast-free zones (15 µm " 15 µm) scanned by the electron beam. The technique does not allow reaching 100% of total weight, due to the scanning of surface defects (holes, cracks), hydrated glass and micaceous crystallites. 40 Ar-39 Ar dating Selected portions of PST veins were drilled (diameter 9 mm) from polished sections and then investigated by BSE at the SEM. Then, the same sections were slightly polished again, heated on a hot plate to remove the sample from the holding glass, and cleaned in methanol and deionised water in an ultrasonic bath. Samples were later wrapped in Al foil, loaded in quartz vials together with the Fish Canyon Tuff biotite standard (27.95 Ma; BAKSI et alii, 1996) and irradiated for 28 hours in the central thimble of the 250 kW TRIGA reactor (University of Pavia). The irradiated samples were placed inside an ultrahigh vacuum laser chamber and baked overnight at about 180°C to remove adsorbed atmospheric argon. In-situ analyses were performed
Pseudotachylyte veins of the Tonale nappe (NE Italy) occur along faults and shear zones which acc... more Pseudotachylyte veins of the Tonale nappe (NE Italy) occur along faults and shear zones which accommodated the deformation that shortened the Adriatic crust to the north of the Peria-driatic Lineament during the late Alpine orogeny. These veins are characterized by thin glass layers showing variable clast content and colours, coupled with chemical variations. They are derived from surrounding gneiss, but mass-balance studies show some quartz missing in the veins (i.e., 1 rock = a vein + b quartz, with b ranging from 0.1 to 0.3). Thermodynamic modelling of the melting process indicates that the energy necessary to melt 1 cm 3 of rock is 4-5 kJ. Temperature reached inhomogeneous values between ~1100°C (total melting of micas) and ~1700°C (partial melting of the quartz clasts). The modelling of the cooling process accounts well for the microlite blastesis in the glassy groundmass of the veins, provided that the pressure at which pseudotachylytes formed along faults/shear zones ranges between 4 and 8 kbar. 40 Ar-39 Ar laser-probe in-situ data show that vein zoning corresponds to a range of apparent ages. Step-heating analyses indicate the presence of components younger and older than the age proposed for the vein, due to alteration and clasts, respectively. Interplay between these two factors affects the in-situ data. The new 40 Ar-39 Ar ages of the Tonale nappe pseudotachylytes (~50-21 Ma), as compared to previously published data, testify a long-lived deformation lasting from Eocene to Early Miocene. The peak of this deformation is Late Eocene-Early Oligocene in age (~35-30 Ma), coeval with the emplacement of the Northern Adamello plutons and of several apophyses and dykes in the Adriatic crust.
In the Southern Aosta Valley, the St. Marcel Valley metaophiolites consist of mainly metavolcanic... more In the Southern Aosta Valley, the St. Marcel Valley metaophiolites consist of mainly metavolcanics and their sedimentary cover metamorphosed under HP subduction-related metamorphism. A detailed geological map carried out in the St. Marcel Valley reveals that the metasedimentary cover, although transposed by the Alpine tectonics, is essentially made of three main terms, that are Mn-rich metaquartzites, marble, and calcschists. The metasedimentary sequence is quite comparable with the unmetamorphosed sequence made of radiolarian cherts, Calpionella limestones and Palombini shales covering the Ligurian ophiolites. The St. Marcel Valley metaophiolites represent the upper crustal section of the Mesozoic Tethyan ocean and its pelagic sedimentary cover, overthrusting the serpentinite unit of the Mount Avic, to the East. Within the Piemonte nappe stack, the St.Marcel metaophiolites are located at the higher structural level.
La miniera di Lovignanaz (conosciuta anche come miniera di Molina o Mollère) è situata sulla sini... more La miniera di Lovignanaz (conosciuta anche come miniera di Molina o Mollère) è situata sulla sinistra idrografica della Val Clavalité (1350-1425 m s.l.m.), a sud del paese di Fénis (AO). Le gallerie tuttora visibili si sviluppano entro cloritoscisti, talcoscisti e metagabbri trasposti tettonicamente con i calcescisti appartenenti alla Piemontese dei calcescisti con pietre Le rocce ospitanti le mineralizzazioni a solfuri di ferro e rame mostrano associazioni mineralogiche tipiche delle facies scisti blu ed eclogitica, analogamente alle rocce del più noto complesso minerario di Saint-Marcel (Martin et al., 2008).
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