Infiltration-Driven Metamorphism in Northern New England, USA

Mineral reactions at the biotite isograd were investigated inpelitic schists, micaceous sandstones, micaceous limestones,and metaigneous rocks from three stratigraphic units over anarea of 10000 km² in north-central New England. The biotiteisograd in north-central New England represents a metamorphicdecarbonation front that affected all major rock types in eachstratigraphic unit. Pressure at the isograd was near 3500 bat the northern end of the study area and near 5500 b in thesouth. Temperature was in the range 400–450?C. Equilibriummetamorphic fluids were approximately CO₂-H₂O mixtures withX_CO2=0?04–0?07. Volumetric fluid]-rock ratios were calculatedfor more than 70 samples of all major rock types from each formationusing measured progress of the prograde reactions and the estimatedP-T-X_CO2, conditions of metamorphism. Regardless of stratigraphicunit, limestones record low values of 0–0?2, pelites andmetaigneous rocks generally record high values of 1–3,and standstones record intermediate values of 0?2–1. With exception of the limestones, all samples from the biotitezone record fluid-rock ratios significantly greater than likelyrock porosity during metamorphism. The prograde decarbonationreactions therefore were driven by infiltration of rock by reactiveaqueous fluids. The observed correlations between fluid-rockratio and rock type indicate that significant permeability contrastsoccurred during low-grade metamorphsim with permeability increasingin the order: limestones<sandstones<pelites rocks. Asa corollary, reactive fluid flow must have been channelizedwith enhanced flow in pelites and metaigneous rocks relativeto sandstones and limestones. Results of this study in north-centralNew England taken together with studies of the biotite isogradin south-central Maine (Ferry, 1984, 1986a, 1988) demonstratethat low-grade metamorphism over much of the northern Appalachianorogen was infiltration-driven.