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Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H_2O at 15 kbar

Johnston, A. Dana and Wyllie, Peter J. (1988) Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H_2O at 15 kbar. Contributions to Mineralogy and Petrology, 100 (1). pp. 35-46. ISSN 0010-7999. doi:10.1007/BF00399438.

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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 to an assemblage consisting almost entirely of Na-Pl+Qz with highly fractionated REE patterns. These rocks in turn could be partially melted to yield a second generation of trondhjemites which would inherit the highly fractionated REE patterns because neigher Pl nor Qz is capable of significantly fractionating HREE from LREE.

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Alternate Title:Constraints on the origin of Archean trondhjemites based on phase relationships of Nûk gneiss with H2O at 15 kbar
Additional Information:© 1988 Springer-Verlag. Received November 16, 1987; accepted May 16, 1988. This research was supported by the Earth Sciences Section of the National Science Foundation, Grant EAR83-41623, and the Early Crustal Genesis Project of NASA, NAG9-103. We thank V.R. McGregor, D. Bridgewater and the Geological Survey of Greenland for the rock samples and information, J.G. Arth and F. Barker for other trondhjemite samples and background information to guide our experiments, and A. Boudreau for help with the microprobe analysis. B.W. Evans, M.T. Naney, and an anonymous reviewer are thanked for their constructive reviews. Caltech Division of Geological and Planetary Sciences Contribution 4547.
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NSFEAR 83-41623
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Caltech Division of Geological and Planetary Sciences4547
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ID Code:64680
Deposited By: Tony Diaz
Deposited On:23 Feb 2016 21:50
Last Modified:10 Nov 2021 23:34

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