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.
Highlights
Fluids can significantly influence the chemical characteristics and rheological behavior of the deep crust and lithosphere
1750–1735 Ma, and establish a temporal link between albitization, metamorphism, and deformation during the Late Paleoproterozoic (~1800–1710 Ma) Big Sky orogeny [20,21]. This implies that local metasomatism and fluid infiltration was active during peak metamorphism and fabric development within this continental collision, rather than magmatic emplacement
Selected grains encompassing the range of sizes, textural occurrences, and morphologies were targeted for compositional Wavelength dispersive spectroscopy (WDS) U Mβ, Th Mα, Y Lα, and Si Kα maps collected on the JEOL 8600 Electron Microprobe (EMP) at the University of Colorado Boulder
Summary
Fluids can significantly influence the chemical characteristics and rheological behavior of the deep crust and lithosphere. Establishing the ambient conditions under which alteration occurs, the origin of the fluids, and the timing of metasomatism are all important aspects that facilitate understanding these geologic processes. These variables can be difficult to constrain. We consider both metasomatic and magmatic mechanisms of formation, and present field observations, geochemical data, and monazite and zircon morphology, textures, geochronology, and geochemistry. This implies that local metasomatism and fluid infiltration was active during peak metamorphism and fabric development within this continental collision, rather than magmatic emplacement
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