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Dating Metasomatism: Monazite and Zircon Growth during Amphibolite Facies Albitization
Condit, Cailey B. Mahan, Kevin H. Curtis, Kelly C. Moller, Andreas
Condit, Cailey B.
Mahan, Kevin H.
Curtis, Kelly C.
Moller, Andreas
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Abstract
We present coupled textural observations and trace element and geochronological data from metasomatic monazite and zircon, to constrain the timing of high-grade Na-metasomatism (albitization) of an Archean orthogneiss in southwest Montana, USA. Field, mineral textures, and geochemical evidence indicate albitization occurred as a rind along the margin of a ~3.2 Ga granodioritic orthogneiss (Pl + Hbl + Kfs + Qz + Bt + Zrn) exposed in the Northern Madison range. The metasomatic product is a weakly deformed albitite (Ab + Bt + OAm + Zrn + Mnz + Ap + Rt). Orthoamphibole and biotite grew synkinematically with the regional foliation fabric, which developed during metamorphism that locally peaked at upper amphibolite-facies during the 1800–1710 Ma Big Sky orogeny. Metasomatism resulted in an increase in Na, a decrease in Ca, K, Ba, Fe, and Sr, a complete transformation of plagioclase and K-feldspar into albite, and loss of quartz. In situ geochronology on zoned monazite and zircon indicate growth by dissolution–precipitation in both phases at ~1750–1735 Ma. Trace element geochemistry of rim domains in these phases are best explained by dissolution–reprecipitation in equilibrium with Na-rich fluid. Together, these data temporally and mechanistically link metasomatism with high-grade tectonism and prograde metamorphism during the Big Sky orogeny.
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2018-04-29
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MDPI
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Dating Metasomatism.pdf
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metasomatism, albitization, geochronology, trace element geochemistry, monazite, zircon, SW Montana
Citation
Condit, C.B.; Mahan, K.H.; Curtis, K.C.; Möller, A. Dating Metasomatism: Monazite and Zircon Growth during Amphibolite Facies Albitization. Minerals 2018, 8, 187.