Constraints on the origin of Archean trondhjemites based on phase relationships of N�k gneiss with H2O at 15 kbar

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We report the T-X(H2O) phase relations for the trondhjemitic Nuk gneiss which comprises the principal component of the second phase of Archean (3.0–2.8 by) igneous activity in the Godthab 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 H2O-saturated conditions, a melting interval of ∼135° C separates the solidus at ∼610° C from the liquidus at 745° C. H2O-saturation at 15 kbar occurs at approximately 15.5 wt % H2O. The H2O-undersaturated liquidus extends along a curved path from ∼745° C at 15.5 wt % H2O to ∼1100° C at 2% H2O. Lower H2O contents were not investigated. At low H2O 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 H2O 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.