RAPID HEATING AND PARTIAL MELTING OF THE LOWER CRUSTAL 2.6 GA FEHR GRANITE, ATHABASCA GRANULITE TERRANE: EVIDENCE FROM LEUCOSOME AND GARNET COMPOSITIONS

The 2.6 Ga Fehr granite, a lower crustal pluton in the Athabasca Granulite Terrane in northern Saskatchewan, partially melted as a result of the intrusion of water-oversaturated basalts associated with the 2000-km-long Chipman dike swarm. This regional event involved underplating of felsic lower continental crust by a thick layer of hot basalt, which rapidly heated the crust, leading to at least two melting reactions at different temperatures that resulted in distinct partial melt compositions. Leucosomes produced in different regions of the Fehr granite have different major, trace and rare earth element concentrations. In the northern and southern exposures of the Fehr granite, flux melting, described by the reaction ab + ksp + qtz + introduced H₂O = granitic melt, produced leucosomes that are less abundant, more potassic and less silicic. In central exposures, leucosomes were produced by biotite dehydration melting reactions such as: bt + qtz + ab = grt + ksp + melt or bt + ksp + ab + qtz + H₂O = grt + melt. Leucosomes that resulted from these peritectic reactions are more sodic, more siliceous, and more similar to un-melted Fehr granite than those produced by flux melting in northern and southern exposures. Because biotite dehydration melting occurs at higher temperature than water-fluxed eutectic melting, it seems likely that the central part of the Fehr granite attained a higher temperature, probably as a result of intrusion of larger volumes of melt or a shallower basaltic melt emplacement level. Rare earth element patterns of traverses in peritectic garnet have typical HREE-enriched patterns, but have LREE abundances that oscillate across crystals. This variety of concentrations of LREE, which are relatively incompatible in garnet, may indicate that diffusion was not rapid enough relative to the rate of garnet crystallization to remove LREE derived from reacting garnet growth surfaces to avoid incorporation of some LREE. Continued crystallization then sampled LREE-poor environments, leading to LREE-depleted zones. Oscillations in the REE concentrations of garnet are consistent with localized rapid heating and spatially distinct melting reactions resulting from large-scale underplating and invasion of the lower crust by the voluminous mafic magmas.