Mineralogical and thermodynamic constraints on the metasomatic origin of the York River nepheline gneisses, Bancroft, Ontario

Although these nepheline gneisses have often been interpreted as the products of Na metasomatism and 'nephelinization' of metasedimentary or metavolcanic precursors, only recently have sufficient data on minerals and fluids become available to allow such processes to be tested. One such model invokes NaCl brines from evaporites previously associated with the intercalated marbles to alter the anorthite component of plagioclase to nepheline by the reaction CaAl (sub 2) Si (sub 2) O (sub 8) + 2Na (super +) = 2NaAlSiO (sub 4) + Ca (super 2+) . The discovery of rare fluid inclusions containing NaCl daughter crystals supports the existence of strong brines at some time in the history of these rocks. Using log (a (super 2 ) (sub Na+ ) / a (sub Ca2+) ) vs log aSiO (sub 2) as axes and estimated activities of the mineral components, together with calculations making use of maximum and minimum values of X(CO (sub 2) ), demonstrates that because of the restricted size of the nepheline field, only solutions with concentrations from approximately 0.3 to 1 molal in NaCl could produce nepheline from plagioclase, without at the same time producing wollastonite or sodalite. Subject to the various assumptions and approximations made, it thus appears that 'nephelinization' by the exchange 2Na (super +) = Ca (super 2+) in plagioclase is unlikely to have produced much nepheline in these rocks.