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

• Published in: The Journal of Geology
• Main Authors: van der Laan, Sieger R., Wyllie, Peter J.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: University of Chicago Press 1992
• Subjects: Greenland
• Online Access: https://doi.org/10.1086/629571

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 + ...