SILLIMANITE +QUARTZ REPLACEMENT OF K-FELDSPAR IN LATE TO POST OTTAWAN GRANITE BODIES IN AND ADJACENT TO THE CARTHAGE COLTON SHEAR ZONE, NW ADIRONDACK MTNS, NEW YORK STATE: A CASE OF FLUID DRIVEN AUTO METAMORPHISM?

Weakly to un-deformed bodies of granite exposed in and adjacent to the Carthage Colton Shear Zone in the vicinity of Fine, NY commonly show in-situ replacement of K-feldspar by masses of sillimanite plus or minus quartz. Sillimanite masses are also commonly observed in late-formed shear zones and in zones of brittle deformation within these granite bodies. Within deformed zones, sillimanite crystals are strongly aligned but in k-feldspar crystals outside of these zones, sillimanite shows random orientation. The sillimanite +/- quartz forming reactions appear to be a fluid driven incongruent dissolution of the k-feldspar resulting in the selective removal of potassium and silica as illustrated in the following reaction:

2 KAlSi₃O₈ + 21 H₂O=Al₂SiO₅ + 5 Si(OH)₄*2H₂O _aq + 2K(OH)_aq

The source for the fluid needed to drive these reactions is either meteoric in origin or derived from the parental granitic magma. In either case, fluid access to the granite is localized and facilitated by small shear zones cutting the granite or along grain boundaries. The granite samples studied are rich in fluid inclusions which is in stark contrast to the granulite facies metamorphic rocks into which these granite bodies intrude. Inclusion types in the host granite range from aqueous (saline) to three-phase H₂O-CO₂-salt inclusions in dense secondary arrays. Rare single-phase (CO₂-rich?) inclusions are present. The abundance of fluid inclusions trapped in the granite, coupled with the paucity of fluid inclusions in the surrounding country rock, suggest a fluxing of fluid through the solidified granite driving the sillimanite-forming reactions. The origin for this fluid may be locally derived from the parental granitic magma and released during crystallization. The affected granites may represent an intrusive cap through which this fluid was channeled. This type of auto-metasomatism may be the mechanism responsible for the formation of widespread sillimanite+quartz veins and selvages reported by McLelland for Lyon Mountain Granite exposures in the Moose River near the village of Lyons Falls, New York.