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Pseudotachylytes of the Tonale Nappe (Italian Alps); petrogenesis, (super 40) Ar/ (super 39) Ar geochronology and tectonic implications

Silvana Martin, Gaston Godard, Marinella A. Laurenzi and Alfio Vigano
Pseudotachylytes of the Tonale Nappe (Italian Alps); petrogenesis, (super 40) Ar/ (super 39) Ar geochronology and tectonic implications
Italian Journal of Geosciences (June 2016) 135 (2): 217-235

Abstract

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 Periadriatic 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 (super 3) of rock is 4-5 kJ. Temperature reached inhomogeneous values between approximately 1100 degrees C (total melting of micas) and approximately 1700 degrees 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. (super 40) Ar- (super 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 (super 40) Ar- (super 39) Ar ages of the Tonale nappe pseudotachylytes ( approximately 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 ( approximately 35-30 Ma), coeval with the emplacement of the Northern Adamello plutons and of several apophyses and dykes in the Adriatic crust.


ISSN: 2038-1719
EISSN: 2038-1727
Coden: BOGIAT
Serial Title: Italian Journal of Geosciences
Serial Volume: 135
Serial Issue: 2
Title: Pseudotachylytes of the Tonale Nappe (Italian Alps); petrogenesis, (super 40) Ar/ (super 39) Ar geochronology and tectonic implications
Affiliation: Universita degli Studi di Padova, Dipartimento di Geoscienze, Padua, Italy
Pages: 217-235
Published: 201606
Text Language: English
Publisher: Societa Geologica Italiana, Rome, Italy
References: 85
Accession Number: 2016-062838
Categories: Igneous and metamorphic petrologyStructural geology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, geol. sketch maps
N46°10'00" - N46°15'00", E10°45'00" - E11°00'00"
Secondary Affiliation: Universite Paris-Diderot, FRA, FranceCNR, Istituto di Geoscienze e Georisorse, ITA, ItalyIstituto Nazionale di Oceanografia e di Geofisica Sperimentale, ITA, Italy
Country of Publication: Italy
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Societa Geologica Italiana. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201630
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