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GEOREF RECORD

A peculiar Ms-Pg textural association in a chloritoid-bearing micaschist recording a multistage P-T path

Claudio Mazzoli, Raffaele Sassi and Alain Baronnet
A peculiar Ms-Pg textural association in a chloritoid-bearing micaschist recording a multistage P-T path (in Mica 2000, Annibale Mottana (prefacer) and Paolo Francesco Sassi (prefacer))
European Journal of Mineralogy (December 2001) 13 (6): 1127-1138

Abstract

A peculiar textural association of muscovite, paragonite and chloritoid occurs in a micaschist bearing muscovite and garnet porphyroblasts, from the Greiner Unit (Eastern Alps, Italy). Three different tectonometamorphic stages can be recognised in this rock: 1) the early development of S (sub 1) preserved only as relic within garnet porphyroblasts; 2) a syn-kinematic crystallisation of chloritoid and muscovite along an early foliation S (sub 2) ; 3) the development of a crenulation cleavage (S (sub 3) ) associated with the growth of chlorite, muscovite and paragonite along S (sub 3) and porphyroblastic garnet and muscovite. Both garnet and muscovite porphyroblasts show inclusion trails of chloritoid, quartz and ores, which are parallel to S (sub 2) . Furthermore, the muscovite porphyroblasts include paragonite crystals which are parallel to S (sub 2) and display a rectangular to elongate "blocky shape" in thin section. The elongation is nearly perpendicular to the muscovite (001) basal plane (but with the same cleavage traces as those of the muscovite). An HRTEM/SAED/AEM study of the muscovite/paragonite single crystals reveals two different situations: 1) sharp boundaries for the "blocky shaped" paragonite crystal within muscovite porphyroblasts with the angle of the (001) basal plane across the boundary being that required for adjusting the different basal spacing; 2) the presence of K-rich paragonite micro-domains within Na-rich muscovite porphyroblasts; both are 2M (sub 1) polytypes and in structural continuity, with an angle between [00l]*paragonite and [00l] (super *) muscovite that varies from 0 to 5 degrees in order to accommodate the layer-thickness misfit. This results in a "tweed texture", suggesting a further decomposition inside the muscovite-paragonite solvus. This peculiar muscovite-paragonite textural association is a microstructural relic of a multistage metamorphism which affected these rocks with evidence of an isothermal increase of pressure due to the reaction: Fe-rich Cld+Na-rich Ms+Qtz-->Phe-rich Ms+Pg+Chl+ or - Mg-rich Cld+H (sub 2) O.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 13
Serial Issue: 6
Title: A peculiar Ms-Pg textural association in a chloritoid-bearing micaschist recording a multistage P-T path
Title: Mica 2000
Author(s): Mazzoli, ClaudioSassi, RaffaeleBaronnet, Alain
Author(s): Mottana, Annibaleprefacer
Author(s): Sassi, Paolo Francescoprefacer
Affiliation: Universita di Padova, Dipartimento di Mineralogia e Petrologia, Padua, Italy
Pages: 1127-1138
Published: 200112
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
Meeting name: Advances on micas; problems, methods, applications in geodynamics
Meeting location: Rome, ITA, Italy
Meeting date: 20001102Nov. 2-3, 2000
References: 13
Accession Number: 2002-012294
Categories: Igneous and metamorphic petrologyMineralogy of silicates
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, sketch map
N46°00'00" - N47°00'00", E11°00'00" - E12°00'00"
Secondary Affiliation: CRMC2, FRA, France
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200205
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