Metamorphism in the Adirondacks: II. The role of fluids

Quantitative estimates of metamorphic fluid speciation, stable isotopic analyses, and studies of fluid inclusions all document the local complexity of fluids in the deep crustal rocks exposed in the Adirondack Mountains, NY. Estimates of the activity of H₂O in the granulite facies are substantially lower than in the amphibolite facies gneisses. The onset of low water activities in semi-pelitic gneisses generally correlates with migmatitic textures in the uppermost amphibolite facies, suggesting that partial melts absorbed H₂O at the peak of metamorphism. In granulite facies marbles and calc-silicates, conditions varied from extremely undersaturated in H₂O-CO₂ fluid to fluid saturated, and α_H2O and α_CO2 show sharp gradients within single outcrops. Low values of f_O2 and f_H2O, or of f_CO2, and f_H2O indicate fluid-absent conditions for some orthogneisses and marbles, which are inferred to have been ‘dry’ rocks before and during granulite facies recrystallization. Wollastonite is preserved from early contact metamorphism and serves as an index mineral for fluid-absent conditions in granulites where α_H2O is low. Values off_O2 range from near the hematite + magnetite buffer in metamorphosed iron formation to substantially below the quartz + magnetite + fayalite buffer in some orthogneisses. The anorthosite suite is more oxidized than some associated granitic gneisses. Halogens (F and Cl) substitute extensively for OH in micas and amphiboles, extending their stability, although F₂, Cl₂, HCl, and HF are minor components in any fluid. Oxybiotite-type exchanges involving O for OH are also important, extending the stability of biotite. Stable isotopic ratios of O and C demonstrate that premetamorphic whole-rock compositions are commonly preserved whereas mineral compositions generally reflect equilibration at the peak of metamorphism. The Marcy Anorthosite Massif was intruded as a high δ¹⁸O magma.

The combination of mineral equilibria, stable isotope data, and fluid inclusions is used to identify and to distinguish among pre-orogenic contact metamorphic/hydrothermal events, peak metamorphic events, and retrograde/postmetamorphic events. Polymetamorphism is documented at skarn zones adjacent to anorthosite, where large volumes of hydrothermal fluid were channeled during early, shallow contact metamorphism and where conditions were fluid poor during subsequent regional metamorphism. Peak metamorphic events are inferred to have been caused primarily by magmatic processes of intrusion and anatexis. Partial melting has caused low values of α_H2O in many rocks, but in other cases low values of α_H2O are recorded in orthogneisses derived from H₂O-poor magmas. Isotopic studies show that maximum fluid/rock ratios were <0·l and possibly 0·0 for infiltrating fluids at the peak of metamorphism in many localities. No evidence of pervasive, regional infiltration by a fluid at the peak of metamorphism has been substantiated in the Adirondacks. Fluid inclusions containing high-density CO₂ or CO₂ + H₂O represent conditions from after the peak of metamorphism and document isobaric cooling, in agreement with estimates from garnet zoning. Fine-scale retrograde veins are common and are associated with high-density CO₂-rich fluid inclusions.