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Oscillatory zoning in garnet from the Willsboro Wollastonite Skarn, Adirondack Mts, New York: a record of shallow hydrothermal processes preserved in a granulite facies terrane
Author(s) -
Clechenko C. C.,
Valley J. W.
Publication year - 2003
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1046/j.1525-1314.2003.00478.x
Subject(s) - geology , granulite , metamorphism , skarn , geochemistry , grossular , metamorphic facies , facies , metamorphic rock , fluid inclusions , anorthosite , petrology , hydrothermal circulation , plagioclase , geomorphology , quartz , paleontology , structural basin
Oscillatory zoning in low δ 18 O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition ( X Adr = 0.13–0.36). The δ 18 O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with X Adr . The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ 18 O indicate that a high Fe 3+ /Al, high δ 18 O fluid mixed with a lower Fe 3+ /Al and δ 18 O fluid. The high δ 18 O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ 18 O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane.