Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H_2O at 15 kbar
We report the T-X(H_2O) phase relations for the trondhjemitic Nûk gneiss which comprises the principal component of the second phase of Archean (3.0–2.8 by) igneous activity in the Godthåb region of southwestern Greenland. A pressure of 15 kbar was chosen to place constraints on possible protoli...
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ftcaltechauth:oai:authors.library.caltech.edu:ys1eb-xzq45 2024-09-15T18:10:11+00:00 Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H_2O at 15 kbar Johnston, A. Dana Wyllie, Peter J. 1988-09 https://doi.org/10.1007/BF00399438 unknown Springer https://doi.org/10.1007/BF00399438 oai:authors.library.caltech.edu:ys1eb-xzq45 eprintid:64680 resolverid:CaltechAUTHORS:20160223-132250411 info:eu-repo/semantics/closedAccess Other Contributions to Mineralogy and Petrology, 100(1), 35-46, (1988-09) info:eu-repo/semantics/article 1988 ftcaltechauth https://doi.org/10.1007/BF00399438 2024-08-06T15:35:05Z We report the T-X(H_2O) phase relations for the trondhjemitic Nûk gneiss which comprises the principal component of the second phase of Archean (3.0–2.8 by) igneous activity in the Godthåb region of southwestern Greenland. A pressure of 15 kbar was chosen to place constraints on possible protoliths for trondhjemitic melts at lower crustal depths. Under H_2O-saturated conditions, a melting interval of ∼135° C separates the solidus at ∼610° C from the liquidus at 745° C. H_2O-saturation at 15 kbar occurs at approximately 15.5 wt % H_2O. The H_2O-undersaturated liquidus extends along a curved path from ∼745° C at 15.5 wt % H_2O to ∼1100° C at 2% H_2O. Lower H2O contents were not investigated. At low H_2O contents (<6%) sodic plagioclase (Pl, An_(32)) is the liquidus phase followed at lower but still near-liquidus temperatures by quartz (Qz) and then garnet (Ga). At 6% H_2O, Ga replaces Pl on the liquidus and is joined at slightly lower temperatures by Pl and hornblende (Hb). The field for liquidus Ga extends to only ∼7.5% H_2O where it is replaced by Hb which is the liquidus phase up to 13% H_2O. At all higher H_2O contents, epidote (Ep) is the first phase to crystallize, followed by biotite (Bi) at slightly lower temperatures. Following the standard inverse approach, the near-liquidus phase assemblages are interpreted as potential residues from which trondhjemitic melts could be extracted. At high melt H_2O contents (>7%), mafic residues consisting of some combination of Hb, Ga, Ep, and Bi are possible and could correspond to amphibolitic source rocks. At lower melt H_2O contents (< 5%), possible residues consist of Na-Pl+Qz±Ga and could correspond to an earlier generation of tonalitic-trondhjemitic rocks. However, such residues would not impart the highly fractionated REE patterns characteristic of Archean trondhjemites. If a first generation of tonalitic-trondhjemitic melts was generated by higher pressure partial fusion of eclogite and emplaced at 55 km depth, it would crystallize ... Article in Journal/Newspaper Greenland Caltech Authors (California Institute of Technology) Contributions to Mineralogy and Petrology 100 1 35 46 |
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Caltech Authors (California Institute of Technology) |
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description |
We report the T-X(H_2O) phase relations for the trondhjemitic Nûk gneiss which comprises the principal component of the second phase of Archean (3.0–2.8 by) igneous activity in the Godthåb region of southwestern Greenland. A pressure of 15 kbar was chosen to place constraints on possible protoliths for trondhjemitic melts at lower crustal depths. Under H_2O-saturated conditions, a melting interval of ∼135° C separates the solidus at ∼610° C from the liquidus at 745° C. H_2O-saturation at 15 kbar occurs at approximately 15.5 wt % H_2O. The H_2O-undersaturated liquidus extends along a curved path from ∼745° C at 15.5 wt % H_2O to ∼1100° C at 2% H_2O. Lower H2O contents were not investigated. At low H_2O contents (<6%) sodic plagioclase (Pl, An_(32)) is the liquidus phase followed at lower but still near-liquidus temperatures by quartz (Qz) and then garnet (Ga). At 6% H_2O, Ga replaces Pl on the liquidus and is joined at slightly lower temperatures by Pl and hornblende (Hb). The field for liquidus Ga extends to only ∼7.5% H_2O where it is replaced by Hb which is the liquidus phase up to 13% H_2O. At all higher H_2O contents, epidote (Ep) is the first phase to crystallize, followed by biotite (Bi) at slightly lower temperatures. Following the standard inverse approach, the near-liquidus phase assemblages are interpreted as potential residues from which trondhjemitic melts could be extracted. At high melt H_2O contents (>7%), mafic residues consisting of some combination of Hb, Ga, Ep, and Bi are possible and could correspond to amphibolitic source rocks. At lower melt H_2O contents (< 5%), possible residues consist of Na-Pl+Qz±Ga and could correspond to an earlier generation of tonalitic-trondhjemitic rocks. However, such residues would not impart the highly fractionated REE patterns characteristic of Archean trondhjemites. If a first generation of tonalitic-trondhjemitic melts was generated by higher pressure partial fusion of eclogite and emplaced at 55 km depth, it would crystallize ... |
format |
Article in Journal/Newspaper |
author |
Johnston, A. Dana Wyllie, Peter J. |
spellingShingle |
Johnston, A. Dana Wyllie, Peter J. Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H_2O at 15 kbar |
author_facet |
Johnston, A. Dana Wyllie, Peter J. |
author_sort |
Johnston, A. Dana |
title |
Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H_2O at 15 kbar |
title_short |
Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H_2O at 15 kbar |
title_full |
Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H_2O at 15 kbar |
title_fullStr |
Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H_2O at 15 kbar |
title_full_unstemmed |
Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H_2O at 15 kbar |
title_sort |
constraints on the origin of archean trondhjemites based on phase relationships of nã»k gneiss with h_2o at 15 kbar |
publisher |
Springer |
publishDate |
1988 |
url |
https://doi.org/10.1007/BF00399438 |
genre |
Greenland |
genre_facet |
Greenland |
op_source |
Contributions to Mineralogy and Petrology, 100(1), 35-46, (1988-09) |
op_relation |
https://doi.org/10.1007/BF00399438 oai:authors.library.caltech.edu:ys1eb-xzq45 eprintid:64680 resolverid:CaltechAUTHORS:20160223-132250411 |
op_rights |
info:eu-repo/semantics/closedAccess Other |
op_doi |
https://doi.org/10.1007/BF00399438 |
container_title |
Contributions to Mineralogy and Petrology |
container_volume |
100 |
container_issue |
1 |
container_start_page |
35 |
op_container_end_page |
46 |
_version_ |
1810447791727575040 |