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Minerals 2018, 8(5), 187; https://doi.org/10.3390/min8050187

Dating Metasomatism: Monazite and Zircon Growth during Amphibolite Facies Albitization

1
Department of Earth, Environmental and Planetary Sciences, Rice University, Houston 77005, TX, USA
2
Department of Geological Sciences, University of Colorado, Boulder 80309, CO, USA
3
Department of Geology, The University of Kansas, Lawrence 66045, KS, USA
*
Author to whom correspondence should be addressed.
Received: 24 March 2018 / Revised: 20 April 2018 / Accepted: 24 April 2018 / Published: 29 April 2018
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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. View Full-Text
Keywords: metasomatism; albitization; geochronology; trace element geochemistry; monazite; zircon; SW Montana metasomatism; albitization; geochronology; trace element geochemistry; monazite; zircon; SW Montana
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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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.

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