MAGMATIC EPIDOTE-HORNBLENDE RELATIONS IN MIGMATITIC BIOTITE TTG GNEISSES, NORTHERN GALLATIN RANGE, SW MONTANA

Migmatites in Archean TTG gneisses in the northern Gallatin Range, SW Montana, contain a) leucosomes of trondhjemitic composition, b) hornblende-rich melanosomes, and c) microcline-rich pegmatitic patches often containing hornblende. The migmatites are typically synkinematic, occupying ductile shear zones with varying degrees of isoclinal or disharmonic folding. Late- to post-kinematic nebulitic gneisses and pegmatitic K-spar patches occur throughout the gneisses. Peak metamorphic conditions are estimated to be in the range 670-730oC (gar-bio, gar-cpx) and 8-10 kbar (GASP, gar-cpx-plag); i.e. slightly higher temperature than the tonalite wet solidus, and significantly cooler than biotite or hornblende dehydration melt reactions. Textural relations in the leucosome and melanosome show biotite grains with irregular grain boundaries and corroded cleavage traces, euhedral epidote commonly with allanite cores, symplectic hornblende +/-epidote, and interstitial microcline that is concentrated in grain boundaries and coalescing hypidiomorphic patches. These textures are explained by the incongruent melting of biotite tonalite according to the reaction: biotite + plagioclase(1) +quartz=Kspar (melt) + hbld + plagioclase (2) +/-epidote. There is a slight calcic shift in plagioclase composition (~5% An) from groundmass to leucosome, and the hornblende composition is edenitic. Phase relations (e.g. Naney, 1983; Schmidt and Thompson, 1996;Gardien et al, 2000) require that a minimum of 2-4 wt % H2O must be added to the system to stabilize hornblende and epidote. External fluids may have been introduced via numerous generations of ductile shearing. Given the relatively low melt fraction in this system, trondhjemitic leucosomes are interpreted as part of the residuum. K-spar rich granitic melts aggregate in pegmatitic patches and low strain zones proximal to their source. These melts do not contribute greatly to crustal differentiation because they may decompress only a short way before intersecting the water-saturated solidus.