Metamorphic P–T paths of Archean granulite facies metasedimentary lithologies from the eastern Beartooth Mountains of the northern Wyoming Province, Montana, USA: constraints from quartz-in-garnet (QuiG) Raman elastic barometry, geothermobarometry, and thermodynamic modeling
Authors: Tuttle Larry , Henry Darrell J. 
Published: July 2023

Abstract. Metamorphic pressure and temperature (P–T) paths in late-Archean
high-grade rocks of the eastern Beartooth Mountains of Montana (USA), a
portion of the Wyoming Province, are established by a combination of
imaging, analytical, and modeling approaches. Garnet inclusion mechanical
and chemical thermobarometry, applied to several granulite-facies migmatites
and an iron formation, is particularly useful in constraining the prograde
P–T conditions. Quartz-in-garnet (QuiG) elastic Raman barometry was used on
quartz inclusions in garnet for all samples studied. For a smaller subset of
four representative samples, QuiG constraints were used in conjunction with
Ti-in-quartz (TitaniQ) and Ti-in-biotite (TiB) thermometry to establish
unique prograde inclusion entrapment P–T conditions. Ti measurements of
garnet hosts and cathodoluminescence (CL) imagery of inclusion and matrix
quartz grains were employed to check for Ti loss/diffusion. Lastly,
inclusion studies were supplemented with thermodynamic modeling and matrix
chemical thermobarometry to examine garnet nucleation temperatures and peak
metamorphic conditions. Disagreement between the volume strain and elastic tensor methods used to
calculate quartz inclusion pressures implies that quartz inclusions studied
are under strong differential strain. Prograde entrapment results from the
two inclusion thermobarometry pairs used are distinct: 0.55–0.70 GPa and
475–580 ∘C (QuiG–TitaniQ) versus 0.85–1.10 GPa and 665–780 ∘C (QuiG–TiB). Garnet modal isopleth modeling indicates that the
majority of garnet growth occurred at ∼ 450–600 ∘C,
implying that P–T conditions of garnet growth are interpreted to be most
reliably represented by QuiG–TitaniQ inclusion thermobarometry. Normal
distributions of calculated QuiG inclusion pressures and the concentration
of mineral inclusions in garnet cores suggest that the majority of garnet
inclusions were entrapped during a single stage of porphyroblast growth. A
general lack of evidence from CL imagery for post-entrapment mechanical or
chemical modifications to quartz inclusions suggests that quartz inclusions
used to calculate entrapment P–T largely preserve their initial entrapment
conditions. Biotite inclusions preserve higher temperatures than quartz
inclusions in the same garnets, likely due to Fe–Mg exchange with garnet
hosts that allowed Ti content of biotite to change after entrapment.
Pseudosection modeling and matrix chemical thermobarometry of multiple,
independent lithologies examined during inclusion studies suggest a range of
peak granulite facies conditions of ∼ 0.50–0.70 GPa and
730–800 ∘C. Peak metamorphic P–T modeling work from this study,
together with evidence of regional amphibolite facies overprinting in
immediately adjacent samples, indicates an overall clockwise metamorphic P–T
path with nearly isobaric prograde heating to peak temperatures. Interpreted
P–T path reconstructions are consistent with metamorphism developed in a
more modern-style continental arc subduction zone and are observed in
portions of the northern Wyoming Province as exemplified by metasupracrustal
lithologies of the eastern Beartooth Mountains.