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Constraints on Archean Trondhjemite Genesis from Hydrous Crystallization Experiments on Nûk Gneiss at 10-17 Kbar

van der Laan, Sieger R. and Wyllie, Peter J. (1992) Constraints on Archean Trondhjemite Genesis from Hydrous Crystallization Experiments on Nûk Gneiss at 10-17 Kbar. Journal of Geology, 100 (1). pp. 57-68. ISSN 0022-1376. doi:10.1086/629571.

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We present crystallization experiments on Archean Nûk gneiss from West Greenland, and relate these to conditions for early continental crustal genesis. The studied composition is a trondhjemite from a voluminous felsic intrusive phase dated at 2,800-3,000 m.y. In a detailed set of experiments at 10 kbar, the liquidus mineralogy is observed to change with increasing water content from plagioclase above 1000°C and <3.5 wt % water, to clinopyroxene at 850-1000°C with 3.5-7 wt % water, to hornblende below 850°C with >7 wt % water. Water saturation of the melt occurs with 10.5 wt % water at 800°C. Mafic liquidus minerals at 15 kbar differ from those at 10 kbar in that garnet replaces pyroxene, and that epidote replaces hornblende below 690°C at >13 wt % water and at water saturation (15.5 wt %). Garnet crystallization is never observed at pressures below 13 kbar. Using additional experiments at 12.5, 13, and 17 kbar, the liquidus phase variation for trondhjemite with water is accurately defined between 8-18 kbars and 700-1200°C. Based on its liquidus mineralogy, this trondhjemite melt with 7-8 wt % water coexists with garnet, hornblende, clinopyroxene, and plagioclase at approximately 900°C at 14 kbar. At lower water contents than 7 wt % and higher temperatures than 900°C, plagioclase ± garnet crystallization is expected at pressures above 13 kbar and plagioclase ± pyroxene crystallization is expected at pressures below 13 kbar. REE patterns of Nûk gneiss prescribe a hornblende- or garnet-bearing residue from which the trondhjemite melts were extracted. For hornblende this requires high melt water contents (>8 wt %) and melting at temperatures below 900°C. For garnet, generally lower water contents and higher pressures and temperatures are needed. The experiments also indicate that trondhjemites are part of a narrow compositional interval in which water-bearing melts will form magmatic epidote at pressures of 8 kbar and higher. A near-solidus assemblage containing epidote + biotite + plagioclase (≈ 20-30%An) + quartz should be expected in such rocks.

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Additional Information:© 1992 University of Chicago Press. Manuscript received May 9, 1991; accepted September 7, 1991. This research was supported by the Earth Sciences section of the U.S. National Science Foundation, Grant EAR-8904375, and the Early Crustal Genesis program of NASA, NAG9-103. We thank V.R. McGregor and the Geological Survey of Greenland for the trondhjemite rock samples and background information. Critical comments by B. White and D. Sykes and thorough reviews by F. Barker, M.T. Naney, and an anonymous reviewer are much appreciated. SvdL thanks the University of Hawaii for a SOEST/HIG Fellowship. Caltech Division of Geological and Planetary Sciences Contribution 5056.
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Official Citation:Sieger R. van der Laan , and Peter J. Wyllie . "Constraints on Archean Trondhjemite Genesis from Hydrous Crystallization Experiments on Nûk Gneiss at 10-17 Kbar." The Journal of Geology 1992; 100(1), 57-68
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:65250
Deposited By: Tony Diaz
Deposited On:10 Mar 2016 19:15
Last Modified:10 Nov 2021 23:42

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