FLUID EVOLUTION DURING ACTIVE AND POST SHEAR ANNEALING AND THE FORMATION OF HIGH CL AMPHIBOLE AND SCAPOLITE: DANA HILL METAGABBRO, NW ADIRONDACK MOUNTAINS, NYS

The Dana Hill Metagabbro Body (DHMB) is located in the NE Adirondack Mts. in New York State and belongs to the Adirondack Highlands Terrane. The DHMB preserves several discrete deformational events during declining (granulite- upper-greenschist facies) P-T conditions during late collapse of the Ottawan Orogen that exhumes the meta-igneous dominated Adirondack Highlands Terrane against the meta-sediment/marble rich Adirondack Lowlands Terrane. Early large (30+m) wide zones of mylonitic gabbro contain the assemblage CPX+/- OPX+PLAG+Cl-rich AMPH+TITANITE. These zones are cross-cut by multiple generations of meter wide shear zones containing high Cl AMPH+/-OPX+PLAG+TITANITE followed by cm width shear zones with lower Cl AMPH +PLAG+SCAP+TITANITE. Polygonal textures indicate post-deformational annealing for all but the lowest grade shear zones. In high Cl-Amphibole +SCAP+PLAG+TITANITE shear zones, scapolite is only found in contact with amphibole. Polygonal contacts between Cl-rich AMPH and SCAP and between SCAP and PLAG are sharp and lack chemical zonation. Evidence for halite and extremely Cl-rich (1.3 Cl apfu) amphiboles occurring in narrow veins in the DHMB has been observed. The observed set of assemblages in the DHMB shear zones indicates an evolving fluid(s) during deformation. Early high temperature shearing produced AMPH at the expense of CPX and/or OPX. Hydration reactions indicate the presence of an aqueous or mixed H₂O-CO₂-NaCl fluid. Experimental evidence for the strong partitioning of Cl into aqueous solutions argues against the amphibole acquiring its high Cl content at this time. During reaction, consumption of H₂O may have driven the fluid toward NaCl saturation followed by brine extraction. Late deformation through post deformational annealing would then take place under dry halite saturated conditions driving Cl into the recrystallizing amphibole. The later/lower-grade scapolite-bearing shear zones require a CO₂ fluid in addition to a Cl source. Introduction of a CO₂ rich fluid into these active shear zones (already containing Cl rich amphibole) drives the scapolite forming reaction at plagioclase/amphibole contacts robbing the amphibole of some of its Cl content. The source for this CO₂ rich fluid may be derived from marbles in the newly juxtaposed Adirondack Lowlands Terrane.