Article

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.

Creator

• Curtis, Kelly
• Mahan, Kevin H.
• Condit, Cailey Brown
• Möller, Andreas

Date Issued

• 2018-01-01

Academic Affiliation

• Geology

Journal Title

• Minerals

Journal Issue/Number

• 5

Journal Volume

• 8

Last Modified

• 2019-12-05

Resource Type

• Article

Rights Statement

• In Copyright

DOI

•  http://dx.doi.org/10.3390/min8050187

ISSN

• 2075-163X

Language

• English [eng]

License

• Creative Commons BY Attribution 4.0 International

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