Clast rotation and the origin of thick ultramylonites

the El Pichao Shear Zone (Sierra de Quilmes), NW Argentina

Research output: Contribution to conferenceAbstractOther

Abstract

The El Pichao shear zone in the Sierra de Quilmes, NW Argentina is a 3-7km thick, ductile shear zone between high grade migmatites and low grade metamorphic sequences of an exhumed basement. In the low grade metasediments deformation fabrics vary from protomylonite via mylonite to ultramylonite. These two sequences are separated by a granitic intrusion and intensely intruded by pegmatite dykes. The shear zone overprints all three main rock sequences. The ultramylonite fabrics can reach extreme thicknesses of 1km, which has been seldom observed elsewhere. Thus, the El Pichao Shear Zone provides a unique opportunity to understand differences in strain accommodation at the mylonite-ultramylonite transition, and the nature of thick ultramlyonite sequences. The mylonite sequences contain an anisotropic matrix comprising Bt + Qtz + Ms + Plg + Kfs, with coarse Qtz ribbons, mica bands and feldspar porphyroclasts up to 5 cm in diameter. Qtz ribbons have undergone grain boundary migration recrystallization and have been folded around feldspar clasts. Feldspar clasts have been variably rotated, demonstrated by the orientation of dynamically recrystallized material around the clasts. Three types of deformation behaviour occur simultaneously amongst the feldspar clasts: (i) brittle fracturing, (ii) partial recrystallization, (iii) complete recrystallization. The ultramylonite sequences contain a fine-grained equigranular matrix of Qtz + Ms + Bt + Plg + Kfs with feldspar porphyroclasts. Rotation-induced strain accommodation is demonstrated by the presence of δ-style porphyroclasts and the orientation of micas around clast boundaries. Qtz ribbons or strong S-C fabrics are lacking and the matrix tends to be homogeneous with only weak foliation defined by the preferred orientation of micas. The bulk connectivity of phases in the matrix decreases toward ultramylonite, which suggests a homogenization of the matrix between mylonite and ultramylonite. Quartz c-axis data demonstrates a systematic variation in the accommodation of strain in quartz between the mylonite/ultramylonite sequences, and within different structural domains of the mylonites. This data suggests that the slip systems of quartz change as a result of the homogenization process between mylonite and ultramylonite. The data also supports our current arguments that quartz slip systems are only relevant to the structural domains from which they are derived, and cannot be used to make kinematic assumptions for the whole rock. We argue based on petrographic features that the formation of thick ultramlyonites can occur where strain is high enough to instigate intense clast rotation. The homogenization of the originally banded mylonite results from continual rotation of clasts, which disaggregated the anisotropic matrix and thus inhibited strain localisation. The relative rotation of clasts in the matrix was a function of their vorticity and geometry, which may have influenced the variable deformation behaviours of feldspars in the mylonites. Strain softening at the clast matrix interface may have also played a role in increasing the vorticity of clasts and promoting rotation-induced strain accommodation.
Original languageEnglish
Number of pages1
Publication statusPublished - 2012
EventFall Meeting of the American-Geophysical-Union 2012 - San Francisco, United States of America
Duration: 3 Dec 20127 Dec 2012

Conference

ConferenceFall Meeting of the American-Geophysical-Union 2012
CountryUnited States of America
CitySan Francisco
Period3/12/127/12/12

Keywords

  • porphyroclasts
  • mylonite
  • strain localisation

Cite this

Hunter, N. JR., Hasalova, P., Weinberg, R. F., & Finch, M. A. (2012). Clast rotation and the origin of thick ultramylonites: the El Pichao Shear Zone (Sierra de Quilmes), NW Argentina. Abstract from Fall Meeting of the American-Geophysical-Union 2012, San Francisco, United States of America.
Hunter, Nicholas JR ; Hasalova, Pavlina ; Weinberg, Roberto Ferrez ; Finch, Melanie Anne. / Clast rotation and the origin of thick ultramylonites : the El Pichao Shear Zone (Sierra de Quilmes), NW Argentina. Abstract from Fall Meeting of the American-Geophysical-Union 2012, San Francisco, United States of America.1 p.
@conference{74920220aaba47bd82fcdc01215ef92b,
title = "Clast rotation and the origin of thick ultramylonites: the El Pichao Shear Zone (Sierra de Quilmes), NW Argentina",
abstract = "The El Pichao shear zone in the Sierra de Quilmes, NW Argentina is a 3-7km thick, ductile shear zone between high grade migmatites and low grade metamorphic sequences of an exhumed basement. In the low grade metasediments deformation fabrics vary from protomylonite via mylonite to ultramylonite. These two sequences are separated by a granitic intrusion and intensely intruded by pegmatite dykes. The shear zone overprints all three main rock sequences. The ultramylonite fabrics can reach extreme thicknesses of 1km, which has been seldom observed elsewhere. Thus, the El Pichao Shear Zone provides a unique opportunity to understand differences in strain accommodation at the mylonite-ultramylonite transition, and the nature of thick ultramlyonite sequences. The mylonite sequences contain an anisotropic matrix comprising Bt + Qtz + Ms + Plg + Kfs, with coarse Qtz ribbons, mica bands and feldspar porphyroclasts up to 5 cm in diameter. Qtz ribbons have undergone grain boundary migration recrystallization and have been folded around feldspar clasts. Feldspar clasts have been variably rotated, demonstrated by the orientation of dynamically recrystallized material around the clasts. Three types of deformation behaviour occur simultaneously amongst the feldspar clasts: (i) brittle fracturing, (ii) partial recrystallization, (iii) complete recrystallization. The ultramylonite sequences contain a fine-grained equigranular matrix of Qtz + Ms + Bt + Plg + Kfs with feldspar porphyroclasts. Rotation-induced strain accommodation is demonstrated by the presence of δ-style porphyroclasts and the orientation of micas around clast boundaries. Qtz ribbons or strong S-C fabrics are lacking and the matrix tends to be homogeneous with only weak foliation defined by the preferred orientation of micas. The bulk connectivity of phases in the matrix decreases toward ultramylonite, which suggests a homogenization of the matrix between mylonite and ultramylonite. Quartz c-axis data demonstrates a systematic variation in the accommodation of strain in quartz between the mylonite/ultramylonite sequences, and within different structural domains of the mylonites. This data suggests that the slip systems of quartz change as a result of the homogenization process between mylonite and ultramylonite. The data also supports our current arguments that quartz slip systems are only relevant to the structural domains from which they are derived, and cannot be used to make kinematic assumptions for the whole rock. We argue based on petrographic features that the formation of thick ultramlyonites can occur where strain is high enough to instigate intense clast rotation. The homogenization of the originally banded mylonite results from continual rotation of clasts, which disaggregated the anisotropic matrix and thus inhibited strain localisation. The relative rotation of clasts in the matrix was a function of their vorticity and geometry, which may have influenced the variable deformation behaviours of feldspars in the mylonites. Strain softening at the clast matrix interface may have also played a role in increasing the vorticity of clasts and promoting rotation-induced strain accommodation.",
keywords = "porphyroclasts, mylonite, strain localisation",
author = "Hunter, {Nicholas JR} and Pavlina Hasalova and Weinberg, {Roberto Ferrez} and Finch, {Melanie Anne}",
year = "2012",
language = "English",
note = "Fall Meeting of the American-Geophysical-Union 2012 ; Conference date: 03-12-2012 Through 07-12-2012",

}

Hunter, NJR, Hasalova, P, Weinberg, RF & Finch, MA 2012, 'Clast rotation and the origin of thick ultramylonites: the El Pichao Shear Zone (Sierra de Quilmes), NW Argentina' Fall Meeting of the American-Geophysical-Union 2012, San Francisco, United States of America, 3/12/12 - 7/12/12, .

Clast rotation and the origin of thick ultramylonites : the El Pichao Shear Zone (Sierra de Quilmes), NW Argentina. / Hunter, Nicholas JR; Hasalova, Pavlina; Weinberg, Roberto Ferrez; Finch, Melanie Anne.

2012. Abstract from Fall Meeting of the American-Geophysical-Union 2012, San Francisco, United States of America.

Research output: Contribution to conferenceAbstractOther

TY - CONF

T1 - Clast rotation and the origin of thick ultramylonites

T2 - the El Pichao Shear Zone (Sierra de Quilmes), NW Argentina

AU - Hunter, Nicholas JR

AU - Hasalova, Pavlina

AU - Weinberg, Roberto Ferrez

AU - Finch, Melanie Anne

PY - 2012

Y1 - 2012

N2 - The El Pichao shear zone in the Sierra de Quilmes, NW Argentina is a 3-7km thick, ductile shear zone between high grade migmatites and low grade metamorphic sequences of an exhumed basement. In the low grade metasediments deformation fabrics vary from protomylonite via mylonite to ultramylonite. These two sequences are separated by a granitic intrusion and intensely intruded by pegmatite dykes. The shear zone overprints all three main rock sequences. The ultramylonite fabrics can reach extreme thicknesses of 1km, which has been seldom observed elsewhere. Thus, the El Pichao Shear Zone provides a unique opportunity to understand differences in strain accommodation at the mylonite-ultramylonite transition, and the nature of thick ultramlyonite sequences. The mylonite sequences contain an anisotropic matrix comprising Bt + Qtz + Ms + Plg + Kfs, with coarse Qtz ribbons, mica bands and feldspar porphyroclasts up to 5 cm in diameter. Qtz ribbons have undergone grain boundary migration recrystallization and have been folded around feldspar clasts. Feldspar clasts have been variably rotated, demonstrated by the orientation of dynamically recrystallized material around the clasts. Three types of deformation behaviour occur simultaneously amongst the feldspar clasts: (i) brittle fracturing, (ii) partial recrystallization, (iii) complete recrystallization. The ultramylonite sequences contain a fine-grained equigranular matrix of Qtz + Ms + Bt + Plg + Kfs with feldspar porphyroclasts. Rotation-induced strain accommodation is demonstrated by the presence of δ-style porphyroclasts and the orientation of micas around clast boundaries. Qtz ribbons or strong S-C fabrics are lacking and the matrix tends to be homogeneous with only weak foliation defined by the preferred orientation of micas. The bulk connectivity of phases in the matrix decreases toward ultramylonite, which suggests a homogenization of the matrix between mylonite and ultramylonite. Quartz c-axis data demonstrates a systematic variation in the accommodation of strain in quartz between the mylonite/ultramylonite sequences, and within different structural domains of the mylonites. This data suggests that the slip systems of quartz change as a result of the homogenization process between mylonite and ultramylonite. The data also supports our current arguments that quartz slip systems are only relevant to the structural domains from which they are derived, and cannot be used to make kinematic assumptions for the whole rock. We argue based on petrographic features that the formation of thick ultramlyonites can occur where strain is high enough to instigate intense clast rotation. The homogenization of the originally banded mylonite results from continual rotation of clasts, which disaggregated the anisotropic matrix and thus inhibited strain localisation. The relative rotation of clasts in the matrix was a function of their vorticity and geometry, which may have influenced the variable deformation behaviours of feldspars in the mylonites. Strain softening at the clast matrix interface may have also played a role in increasing the vorticity of clasts and promoting rotation-induced strain accommodation.

AB - The El Pichao shear zone in the Sierra de Quilmes, NW Argentina is a 3-7km thick, ductile shear zone between high grade migmatites and low grade metamorphic sequences of an exhumed basement. In the low grade metasediments deformation fabrics vary from protomylonite via mylonite to ultramylonite. These two sequences are separated by a granitic intrusion and intensely intruded by pegmatite dykes. The shear zone overprints all three main rock sequences. The ultramylonite fabrics can reach extreme thicknesses of 1km, which has been seldom observed elsewhere. Thus, the El Pichao Shear Zone provides a unique opportunity to understand differences in strain accommodation at the mylonite-ultramylonite transition, and the nature of thick ultramlyonite sequences. The mylonite sequences contain an anisotropic matrix comprising Bt + Qtz + Ms + Plg + Kfs, with coarse Qtz ribbons, mica bands and feldspar porphyroclasts up to 5 cm in diameter. Qtz ribbons have undergone grain boundary migration recrystallization and have been folded around feldspar clasts. Feldspar clasts have been variably rotated, demonstrated by the orientation of dynamically recrystallized material around the clasts. Three types of deformation behaviour occur simultaneously amongst the feldspar clasts: (i) brittle fracturing, (ii) partial recrystallization, (iii) complete recrystallization. The ultramylonite sequences contain a fine-grained equigranular matrix of Qtz + Ms + Bt + Plg + Kfs with feldspar porphyroclasts. Rotation-induced strain accommodation is demonstrated by the presence of δ-style porphyroclasts and the orientation of micas around clast boundaries. Qtz ribbons or strong S-C fabrics are lacking and the matrix tends to be homogeneous with only weak foliation defined by the preferred orientation of micas. The bulk connectivity of phases in the matrix decreases toward ultramylonite, which suggests a homogenization of the matrix between mylonite and ultramylonite. Quartz c-axis data demonstrates a systematic variation in the accommodation of strain in quartz between the mylonite/ultramylonite sequences, and within different structural domains of the mylonites. This data suggests that the slip systems of quartz change as a result of the homogenization process between mylonite and ultramylonite. The data also supports our current arguments that quartz slip systems are only relevant to the structural domains from which they are derived, and cannot be used to make kinematic assumptions for the whole rock. We argue based on petrographic features that the formation of thick ultramlyonites can occur where strain is high enough to instigate intense clast rotation. The homogenization of the originally banded mylonite results from continual rotation of clasts, which disaggregated the anisotropic matrix and thus inhibited strain localisation. The relative rotation of clasts in the matrix was a function of their vorticity and geometry, which may have influenced the variable deformation behaviours of feldspars in the mylonites. Strain softening at the clast matrix interface may have also played a role in increasing the vorticity of clasts and promoting rotation-induced strain accommodation.

KW - porphyroclasts

KW - mylonite

KW - strain localisation

M3 - Abstract

ER -

Hunter NJR, Hasalova P, Weinberg RF, Finch MA. Clast rotation and the origin of thick ultramylonites: the El Pichao Shear Zone (Sierra de Quilmes), NW Argentina. 2012. Abstract from Fall Meeting of the American-Geophysical-Union 2012, San Francisco, United States of America.