SCAPOLITE CORONAS ON CL-RICH HORNBLENDE: EVIDENCE FOR PERVASIVE FLUID INFILTRATION INTO 30 METER WIDE SHEAR ZONES: DANA HILL METAGABBRO ADIRONDACK MOUNTAINS, NYS

The mineralogy of veins and shear zones in the Dana Hill Metagabbro Body (DHMG) preserves a record of fluid infiltration during late-Grenville (Ottawan) upper amphibolite-granulite (650-680ºC; 600MPa) facies deformation. Textures in the 3+ km long DHMG range from ultra-mylonite to relict igneous (orthocumulate gabbro). Mineralogically, the gabbro consists of CPX + MAG /ILM + PLAG +/- QUARTZ while the deformed metagabbro consists of HORNBLENDE (high Cl) + PLAGIOCLASE + SCAPOLITE (Mizzonite) + TITANITE +/- QUARTZ +/- AUGITE. Infiltration of a C-O-H-salt fluid into shear zones during deformation drove the reaction:

9 PLAG + 5 CPX + 4 ILM + 3 H₂O + CO₂ = 3 HORNBLENDE + 4 TITANITE+ SCAPOLITE.

In the shear zones, scapolite coronas partially to completely separate hornblende from plagioclase. Backscattered imaging and EMP analyses show that both scapolite-plagioclase and scapolite-hornblende contacts are sharp, with little to no compositional variation across these boundaries. The formation of corona textures was controlled not by the lack of fluid, but by the abundance of a failing phase in the parent assemblage. In nearly all cases, the failing phases were either ILM or CPX. Fluid penetration into shear zones (up to 30 meters in width) during deformation was pervasive. The high Cl content of hornblende and growth of Mizzonite Scapolite requires the introduction of a H₂O-CO₂ -Cl bearing fluid(s) during reaction. There is no textural/mineralogic evidence to suggest that shearing/deformation occurred in the presence of immiscible fluids. This observation is born out by Thermodynamic modeling (Thermocalc, TWQ ) that shows at the P-T conditions of shear zone formation, the infiltrating fluid need only contain 5-15 mole percent CO₂ to drive the reaction keeping it well within the miscible field even for relatively high salt contents.