Densification and rheological changes to subducted felsic continental crust in the Sunnfjord area, Norwegian Caledonides: petrogenesis, preservation and implications for scandian collision dynamics

Research output: Contribution to conferenceAbstract

Abstract

Geodynamic models for continental collision require information on rock density and rheology as they evolve during the subduction-eduction cycle. The Western Gneiss Complex (WGC) in the Scandinavian Caledonides represents a continental margin that underwent transient subduction during the Scandian collision. Preservation of peak eclogite facies metamorphic parageneses in the predominant granitoid gneisses is rare due to amphibolite-facies overprinting and partial melting.
In the Dalsfjord area of the southern WGR the gneisses escaped the migmatisation prevalent in the higher-T, UHP, northern WGC. Grey bt+pl+qz+ep±grt±Ca-amp orthogneisses enclose areas of green, omphacite-bearing granitoid gneiss. These evolved from a Mesoproterozoic charnockitic precursor to omp+grt+czo+ky+qz+rt±pg±phe (omp extensively altered to amp+pl symplectite). They are L>S tectonites with a strong omphacite-aggregate shape fabric. Peak P-T was ~650oC at 2.3GPa.
Density of the omphacite gneiss at peak P&T was about 3.2 g.cm-3 based upon the estimated mineral mode and isochemical phase diagram sections. This would have been neutrally or slightly positively buoyant relative to anhydrous lithospheric mantle but negatively buoyant relative to a serpentinised mantle wedge. Phase diagram analysis shows that the HP assemblage of the grey gneisses would have been rich in phengite and poor in omphacite. Phengite spontaneously decomposes on decompression, while omphacite requires addition of an aqueous fluid. Hence the grey gneiss remained reactive so the HP paragenesis was lost, but the reactivity of the omphacite gneiss was constrained by the availability of a source of water so the HP paragenesis was preserved.
The evolution of density and rheology during the subduction cycle depended on an initial ingress of water followed by its differential consumption and release by different bulk compositions. The tectonite fabric indicates that transformation from charnockite resulted in a major change in rheology. However, the localised survival of pristine protolith shows that the transformation was inefficient.
Original languageEnglish
Publication statusUnpublished - 3 Jan 2018
EventTectonics Studies Group and Metamorphic Studies Group Joint Meeting 2018 - University of Plymouth, Plymouth, United Kingdom
Duration: 3 Jan 20185 Jan 2018
https://tectonicstudiesgroup.org/tsg-msg-joint-meeting-2018-university-of-plymouth-january-3rd-5th-workshops-on-the-6th/

Conference

ConferenceTectonics Studies Group and Metamorphic Studies Group Joint Meeting 2018
CountryUnited Kingdom
CityPlymouth
Period3/01/185/01/18
Internet address

Fingerprint

omphacite
petrogenesis
gneiss
continental crust
collision
paragenesis
rheology
phengite
subduction
granitoid
diagram
symplectite
mantle
metamorphic facies
charnockite
migmatization
overprinting
continental collision
decompression
eclogite

Cite this

@conference{6393445075ec49f5bb8fdd8159ce63d5,
title = "Densification and rheological changes to subducted felsic continental crust in the Sunnfjord area, Norwegian Caledonides: petrogenesis, preservation and implications for scandian collision dynamics",
abstract = "Geodynamic models for continental collision require information on rock density and rheology as they evolve during the subduction-eduction cycle. The Western Gneiss Complex (WGC) in the Scandinavian Caledonides represents a continental margin that underwent transient subduction during the Scandian collision. Preservation of peak eclogite facies metamorphic parageneses in the predominant granitoid gneisses is rare due to amphibolite-facies overprinting and partial melting. In the Dalsfjord area of the southern WGR the gneisses escaped the migmatisation prevalent in the higher-T, UHP, northern WGC. Grey bt+pl+qz+ep±grt±Ca-amp orthogneisses enclose areas of green, omphacite-bearing granitoid gneiss. These evolved from a Mesoproterozoic charnockitic precursor to omp+grt+czo+ky+qz+rt±pg±phe (omp extensively altered to amp+pl symplectite). They are L>S tectonites with a strong omphacite-aggregate shape fabric. Peak P-T was ~650oC at 2.3GPa. Density of the omphacite gneiss at peak P&T was about 3.2 g.cm-3 based upon the estimated mineral mode and isochemical phase diagram sections. This would have been neutrally or slightly positively buoyant relative to anhydrous lithospheric mantle but negatively buoyant relative to a serpentinised mantle wedge. Phase diagram analysis shows that the HP assemblage of the grey gneisses would have been rich in phengite and poor in omphacite. Phengite spontaneously decomposes on decompression, while omphacite requires addition of an aqueous fluid. Hence the grey gneiss remained reactive so the HP paragenesis was lost, but the reactivity of the omphacite gneiss was constrained by the availability of a source of water so the HP paragenesis was preserved. The evolution of density and rheology during the subduction cycle depended on an initial ingress of water followed by its differential consumption and release by different bulk compositions. The tectonite fabric indicates that transformation from charnockite resulted in a major change in rheology. However, the localised survival of pristine protolith shows that the transformation was inefficient.",
author = "Simon Cuthbert",
year = "2018",
month = "1",
day = "3",
language = "English",
note = "Tectonics Studies Group and Metamorphic Studies Group Joint Meeting 2018 ; Conference date: 03-01-2018 Through 05-01-2018",
url = "https://tectonicstudiesgroup.org/tsg-msg-joint-meeting-2018-university-of-plymouth-january-3rd-5th-workshops-on-the-6th/",

}

Densification and rheological changes to subducted felsic continental crust in the Sunnfjord area, Norwegian Caledonides : petrogenesis, preservation and implications for scandian collision dynamics. / Cuthbert, Simon.

2018. Abstract from Tectonics Studies Group and Metamorphic Studies Group Joint Meeting 2018, Plymouth, United Kingdom.

Research output: Contribution to conferenceAbstract

TY - CONF

T1 - Densification and rheological changes to subducted felsic continental crust in the Sunnfjord area, Norwegian Caledonides

T2 - petrogenesis, preservation and implications for scandian collision dynamics

AU - Cuthbert, Simon

PY - 2018/1/3

Y1 - 2018/1/3

N2 - Geodynamic models for continental collision require information on rock density and rheology as they evolve during the subduction-eduction cycle. The Western Gneiss Complex (WGC) in the Scandinavian Caledonides represents a continental margin that underwent transient subduction during the Scandian collision. Preservation of peak eclogite facies metamorphic parageneses in the predominant granitoid gneisses is rare due to amphibolite-facies overprinting and partial melting. In the Dalsfjord area of the southern WGR the gneisses escaped the migmatisation prevalent in the higher-T, UHP, northern WGC. Grey bt+pl+qz+ep±grt±Ca-amp orthogneisses enclose areas of green, omphacite-bearing granitoid gneiss. These evolved from a Mesoproterozoic charnockitic precursor to omp+grt+czo+ky+qz+rt±pg±phe (omp extensively altered to amp+pl symplectite). They are L>S tectonites with a strong omphacite-aggregate shape fabric. Peak P-T was ~650oC at 2.3GPa. Density of the omphacite gneiss at peak P&T was about 3.2 g.cm-3 based upon the estimated mineral mode and isochemical phase diagram sections. This would have been neutrally or slightly positively buoyant relative to anhydrous lithospheric mantle but negatively buoyant relative to a serpentinised mantle wedge. Phase diagram analysis shows that the HP assemblage of the grey gneisses would have been rich in phengite and poor in omphacite. Phengite spontaneously decomposes on decompression, while omphacite requires addition of an aqueous fluid. Hence the grey gneiss remained reactive so the HP paragenesis was lost, but the reactivity of the omphacite gneiss was constrained by the availability of a source of water so the HP paragenesis was preserved. The evolution of density and rheology during the subduction cycle depended on an initial ingress of water followed by its differential consumption and release by different bulk compositions. The tectonite fabric indicates that transformation from charnockite resulted in a major change in rheology. However, the localised survival of pristine protolith shows that the transformation was inefficient.

AB - Geodynamic models for continental collision require information on rock density and rheology as they evolve during the subduction-eduction cycle. The Western Gneiss Complex (WGC) in the Scandinavian Caledonides represents a continental margin that underwent transient subduction during the Scandian collision. Preservation of peak eclogite facies metamorphic parageneses in the predominant granitoid gneisses is rare due to amphibolite-facies overprinting and partial melting. In the Dalsfjord area of the southern WGR the gneisses escaped the migmatisation prevalent in the higher-T, UHP, northern WGC. Grey bt+pl+qz+ep±grt±Ca-amp orthogneisses enclose areas of green, omphacite-bearing granitoid gneiss. These evolved from a Mesoproterozoic charnockitic precursor to omp+grt+czo+ky+qz+rt±pg±phe (omp extensively altered to amp+pl symplectite). They are L>S tectonites with a strong omphacite-aggregate shape fabric. Peak P-T was ~650oC at 2.3GPa. Density of the omphacite gneiss at peak P&T was about 3.2 g.cm-3 based upon the estimated mineral mode and isochemical phase diagram sections. This would have been neutrally or slightly positively buoyant relative to anhydrous lithospheric mantle but negatively buoyant relative to a serpentinised mantle wedge. Phase diagram analysis shows that the HP assemblage of the grey gneisses would have been rich in phengite and poor in omphacite. Phengite spontaneously decomposes on decompression, while omphacite requires addition of an aqueous fluid. Hence the grey gneiss remained reactive so the HP paragenesis was lost, but the reactivity of the omphacite gneiss was constrained by the availability of a source of water so the HP paragenesis was preserved. The evolution of density and rheology during the subduction cycle depended on an initial ingress of water followed by its differential consumption and release by different bulk compositions. The tectonite fabric indicates that transformation from charnockite resulted in a major change in rheology. However, the localised survival of pristine protolith shows that the transformation was inefficient.

UR - https://tectonicstudiesgroup.org/tsg-msg-joint-meeting-2018-university-of-plymouth-january-3rd-5th-workshops-on-the-6th/

M3 - Abstract

ER -