Diversity among peridotites in the Western Gneiss Region, Norwegian Caledonides

Simon Cuthbert, H. L. M. Van Roermund

Research output: Contribution to conferencePaper

Abstract

Garnet-bearing peridotites in the Western Gneiss Complex (WGC) have been classed into two groups based upon bulk composition – the Mg-Cr type (generally accepted to be derived from subcontinental mantle) and the Fe-Ti type (metamorphosed ultramafic parts of layered intrusions into continental crust). The distribution of both types broadly coincides with the occurrence of UHP (coesite-stable) eclogites. In Sunnfjord, in the southern WGC, where only quartz-stable eclogites are known, major peridotite bodies of the Fe-Ti type are characterized by the assemblage ol + opx + chl + Ca-amp and lack garnet or any evidence that it has previously existed. While this paragenesis may be co-facial with the predominant amphibolite-facies mineral assemblages in the host gneisses, it is also consistent with estimated P-T conditions for the eclogites in this area at ~600oC and 2.3GPa, thus chlorite peridotites may have equilibrated at pressures under which eclogites were stabilized. Mg-Cr garnet-bearing peridotites in the WGC have often been grouped together and their features combined to determine a common evolution, but in fact two distinct groups appear to exist – a southern group including Almklovdalen, Aldalen and Tafjord, and a northern group including Fjortoft, Otroy and possibly Gurskoy. The northern group is dominated by harzburgites and less common dunites, with garnet-bearing varieties widely distributed throughout. Independent P-T estimates here are consistent with regional diamond stability. As in the southern group, the peridotites are porphyroclastic, but the neoblastic matrix has stable garnet. Porphyroclastic assemblages include relics of majoritic garnets, apparently formed first, during the mid-Proterozoic, then later during the Scandian continental subduction event, the latter associated with localized metasomatic microdiamond formation associated with development of strongly radiogenic Sr-isotopic compositions, presumably due to contamination by fluids or melts derived from adjacent, subducted continental crust. In contrast, southern group peridotites are predominantly dunites with only localized development of garnetiferous varieties, these resulting from localized Proterozoic refertilisation of pre-existing, severely depleted dunites. No strong evidence for a majoritic history has yet been published, and porphyroclast assemblages give a range of Proterozoic ages. Neoblastic matrix assemblages appear to lack stable garnet, consisting of ol + opx + cpx + Ca-amp +/- sp, and their age is unknown. Also, chlorite is abundant and forms a prominent foliation, in contrast to the northern group where it appears to be only locally developed along narrow late shear zones and fractures. No microdiamonds have yet been discovered, and Sr isotopic compositions are much less radiogenic than in the northern group. P-T conditions in the crust associated with the southern belt peridotites are ~650oC and 2.5GPa (Almklovdalen), and as in Sunnfjord this may be consistent with equilibration conditions for chlorite peridotites.
Original languageEnglish
Number of pages1
Publication statusUnpublished - Aug 2008
EventInternational Geological Congress - The Norway Convention Centre, Oslo, Norway
Duration: 6 Aug 200814 Aug 2008
Conference number: 33
http://iugs.org/33igc/coco/LayoutPage-c5002-p5001.html

Conference

ConferenceInternational Geological Congress
Abbreviated title33 IGC
CountryNorway
CityOslo
Period6/08/0814/08/08
Internet address

Fingerprint

gneiss
garnet
chlorite
Proterozoic
P-T conditions
continental crust
isotopic composition
layered intrusion
coesite
matrix
paragenesis
foliation
amphibolite facies
peridotite
diamond
shear zone
subduction
melt
crust
quartz

Cite this

Cuthbert, S., & Van Roermund, H. L. M. (2008). Diversity among peridotites in the Western Gneiss Region, Norwegian Caledonides. Paper presented at International Geological Congress, Oslo, Norway.
Cuthbert, Simon ; Van Roermund, H. L. M. / Diversity among peridotites in the Western Gneiss Region, Norwegian Caledonides. Paper presented at International Geological Congress, Oslo, Norway.1 p.
@conference{2620bbf162c347a69f52637e74cb82bc,
title = "Diversity among peridotites in the Western Gneiss Region, Norwegian Caledonides",
abstract = "Garnet-bearing peridotites in the Western Gneiss Complex (WGC) have been classed into two groups based upon bulk composition – the Mg-Cr type (generally accepted to be derived from subcontinental mantle) and the Fe-Ti type (metamorphosed ultramafic parts of layered intrusions into continental crust). The distribution of both types broadly coincides with the occurrence of UHP (coesite-stable) eclogites. In Sunnfjord, in the southern WGC, where only quartz-stable eclogites are known, major peridotite bodies of the Fe-Ti type are characterized by the assemblage ol + opx + chl + Ca-amp and lack garnet or any evidence that it has previously existed. While this paragenesis may be co-facial with the predominant amphibolite-facies mineral assemblages in the host gneisses, it is also consistent with estimated P-T conditions for the eclogites in this area at ~600oC and 2.3GPa, thus chlorite peridotites may have equilibrated at pressures under which eclogites were stabilized. Mg-Cr garnet-bearing peridotites in the WGC have often been grouped together and their features combined to determine a common evolution, but in fact two distinct groups appear to exist – a southern group including Almklovdalen, Aldalen and Tafjord, and a northern group including Fjortoft, Otroy and possibly Gurskoy. The northern group is dominated by harzburgites and less common dunites, with garnet-bearing varieties widely distributed throughout. Independent P-T estimates here are consistent with regional diamond stability. As in the southern group, the peridotites are porphyroclastic, but the neoblastic matrix has stable garnet. Porphyroclastic assemblages include relics of majoritic garnets, apparently formed first, during the mid-Proterozoic, then later during the Scandian continental subduction event, the latter associated with localized metasomatic microdiamond formation associated with development of strongly radiogenic Sr-isotopic compositions, presumably due to contamination by fluids or melts derived from adjacent, subducted continental crust. In contrast, southern group peridotites are predominantly dunites with only localized development of garnetiferous varieties, these resulting from localized Proterozoic refertilisation of pre-existing, severely depleted dunites. No strong evidence for a majoritic history has yet been published, and porphyroclast assemblages give a range of Proterozoic ages. Neoblastic matrix assemblages appear to lack stable garnet, consisting of ol + opx + cpx + Ca-amp +/- sp, and their age is unknown. Also, chlorite is abundant and forms a prominent foliation, in contrast to the northern group where it appears to be only locally developed along narrow late shear zones and fractures. No microdiamonds have yet been discovered, and Sr isotopic compositions are much less radiogenic than in the northern group. P-T conditions in the crust associated with the southern belt peridotites are ~650oC and 2.5GPa (Almklovdalen), and as in Sunnfjord this may be consistent with equilibration conditions for chlorite peridotites.",
author = "Simon Cuthbert and {Van Roermund}, {H. L. M.}",
year = "2008",
month = "8",
language = "English",
note = "International Geological Congress, 33 IGC ; Conference date: 06-08-2008 Through 14-08-2008",
url = "http://iugs.org/33igc/coco/LayoutPage-c5002-p5001.html",

}

Cuthbert, S & Van Roermund, HLM 2008, 'Diversity among peridotites in the Western Gneiss Region, Norwegian Caledonides' Paper presented at International Geological Congress, Oslo, Norway, 6/08/08 - 14/08/08, .

Diversity among peridotites in the Western Gneiss Region, Norwegian Caledonides. / Cuthbert, Simon; Van Roermund, H. L. M.

2008. Paper presented at International Geological Congress, Oslo, Norway.

Research output: Contribution to conferencePaper

TY - CONF

T1 - Diversity among peridotites in the Western Gneiss Region, Norwegian Caledonides

AU - Cuthbert, Simon

AU - Van Roermund, H. L. M.

PY - 2008/8

Y1 - 2008/8

N2 - Garnet-bearing peridotites in the Western Gneiss Complex (WGC) have been classed into two groups based upon bulk composition – the Mg-Cr type (generally accepted to be derived from subcontinental mantle) and the Fe-Ti type (metamorphosed ultramafic parts of layered intrusions into continental crust). The distribution of both types broadly coincides with the occurrence of UHP (coesite-stable) eclogites. In Sunnfjord, in the southern WGC, where only quartz-stable eclogites are known, major peridotite bodies of the Fe-Ti type are characterized by the assemblage ol + opx + chl + Ca-amp and lack garnet or any evidence that it has previously existed. While this paragenesis may be co-facial with the predominant amphibolite-facies mineral assemblages in the host gneisses, it is also consistent with estimated P-T conditions for the eclogites in this area at ~600oC and 2.3GPa, thus chlorite peridotites may have equilibrated at pressures under which eclogites were stabilized. Mg-Cr garnet-bearing peridotites in the WGC have often been grouped together and their features combined to determine a common evolution, but in fact two distinct groups appear to exist – a southern group including Almklovdalen, Aldalen and Tafjord, and a northern group including Fjortoft, Otroy and possibly Gurskoy. The northern group is dominated by harzburgites and less common dunites, with garnet-bearing varieties widely distributed throughout. Independent P-T estimates here are consistent with regional diamond stability. As in the southern group, the peridotites are porphyroclastic, but the neoblastic matrix has stable garnet. Porphyroclastic assemblages include relics of majoritic garnets, apparently formed first, during the mid-Proterozoic, then later during the Scandian continental subduction event, the latter associated with localized metasomatic microdiamond formation associated with development of strongly radiogenic Sr-isotopic compositions, presumably due to contamination by fluids or melts derived from adjacent, subducted continental crust. In contrast, southern group peridotites are predominantly dunites with only localized development of garnetiferous varieties, these resulting from localized Proterozoic refertilisation of pre-existing, severely depleted dunites. No strong evidence for a majoritic history has yet been published, and porphyroclast assemblages give a range of Proterozoic ages. Neoblastic matrix assemblages appear to lack stable garnet, consisting of ol + opx + cpx + Ca-amp +/- sp, and their age is unknown. Also, chlorite is abundant and forms a prominent foliation, in contrast to the northern group where it appears to be only locally developed along narrow late shear zones and fractures. No microdiamonds have yet been discovered, and Sr isotopic compositions are much less radiogenic than in the northern group. P-T conditions in the crust associated with the southern belt peridotites are ~650oC and 2.5GPa (Almklovdalen), and as in Sunnfjord this may be consistent with equilibration conditions for chlorite peridotites.

AB - Garnet-bearing peridotites in the Western Gneiss Complex (WGC) have been classed into two groups based upon bulk composition – the Mg-Cr type (generally accepted to be derived from subcontinental mantle) and the Fe-Ti type (metamorphosed ultramafic parts of layered intrusions into continental crust). The distribution of both types broadly coincides with the occurrence of UHP (coesite-stable) eclogites. In Sunnfjord, in the southern WGC, where only quartz-stable eclogites are known, major peridotite bodies of the Fe-Ti type are characterized by the assemblage ol + opx + chl + Ca-amp and lack garnet or any evidence that it has previously existed. While this paragenesis may be co-facial with the predominant amphibolite-facies mineral assemblages in the host gneisses, it is also consistent with estimated P-T conditions for the eclogites in this area at ~600oC and 2.3GPa, thus chlorite peridotites may have equilibrated at pressures under which eclogites were stabilized. Mg-Cr garnet-bearing peridotites in the WGC have often been grouped together and their features combined to determine a common evolution, but in fact two distinct groups appear to exist – a southern group including Almklovdalen, Aldalen and Tafjord, and a northern group including Fjortoft, Otroy and possibly Gurskoy. The northern group is dominated by harzburgites and less common dunites, with garnet-bearing varieties widely distributed throughout. Independent P-T estimates here are consistent with regional diamond stability. As in the southern group, the peridotites are porphyroclastic, but the neoblastic matrix has stable garnet. Porphyroclastic assemblages include relics of majoritic garnets, apparently formed first, during the mid-Proterozoic, then later during the Scandian continental subduction event, the latter associated with localized metasomatic microdiamond formation associated with development of strongly radiogenic Sr-isotopic compositions, presumably due to contamination by fluids or melts derived from adjacent, subducted continental crust. In contrast, southern group peridotites are predominantly dunites with only localized development of garnetiferous varieties, these resulting from localized Proterozoic refertilisation of pre-existing, severely depleted dunites. No strong evidence for a majoritic history has yet been published, and porphyroclast assemblages give a range of Proterozoic ages. Neoblastic matrix assemblages appear to lack stable garnet, consisting of ol + opx + cpx + Ca-amp +/- sp, and their age is unknown. Also, chlorite is abundant and forms a prominent foliation, in contrast to the northern group where it appears to be only locally developed along narrow late shear zones and fractures. No microdiamonds have yet been discovered, and Sr isotopic compositions are much less radiogenic than in the northern group. P-T conditions in the crust associated with the southern belt peridotites are ~650oC and 2.5GPa (Almklovdalen), and as in Sunnfjord this may be consistent with equilibration conditions for chlorite peridotites.

UR - http://iugs.org/33igc/coco/FilePool-c10728-p5002-e1-l0-p5001.html

M3 - Paper

ER -

Cuthbert S, Van Roermund HLM. Diversity among peridotites in the Western Gneiss Region, Norwegian Caledonides. 2008. Paper presented at International Geological Congress, Oslo, Norway.