TITANIUM IN QUARTZ THERMOMETRY OF QUARTZ INCLUSIONS IN GARNET AS A CONSTRAINT ON PROGRADE METAMORPHIC CONDITIONS IN ARCHEAN GRANULITE FACIES LITHOLOGIES FROM THE EASTERN BEARTOOTH MOUNTAINS OF MONTANA AND WYOMING, USA

Accurately establishing prograde metamorphic pressure-temperature (P-T) paths in Archean granulite facies lithologies of the eastern Beartooth Mountains is critical for reconstructing tectonometamorphic conditions that constrain the nature of nascent tectonic processes during the Archean. Portions of the prograde P-T path for several of the granulite facies lithologies have been broadly constrained by quartz-in-garnet (QuiG) barometry and Ti-in-biotite (TiB) mineral inclusion thermometry to be in the range of 675-775 °C and 9-11 kbar. However, Ti-in-quartz (TitaniQ) thermometry may be a more suitable thermometer to pair with QuiG barometry because these two approaches use a single mineral, quartz, for both P and T determinations. To obtain the optimal quartz grains for TitaniQ, quartz inclusions in garnet and matrix quartz were imaged with cathodoluminescence from electron microprobe (EM-CL) to establish analytical targets for Ti value measurements via electron microprobe analysis (EMPA). The EMPA Ti minimum detection limit is 1-2 ppm. Three quartzofeldspathic gneisses and one iron formation that each contain ilmenite as an inclusion phase in garnet were studied. Based on EM-CL imagery, quartz grains are texturally featureless except for recrystallization textures that are generally restricted to matrix grains. Average Ti values of quartz inclusions and quartz matrix grains range from 12-29 ppm and 8-20 ppm, respectively. Corresponding TitaniQ inclusion isopleths and QuiG entrapment isomekes predict prograde P-T conditions of approximately 450-550 °C and 4-6 kbar. The calculated TitaniQ entrapment T is significantly lower than TiB entrapment T estimates. It is possible that present Ti values of quartz inclusions approximate initial entrapment values due to 1) a near-constant activity of TiO₂ during garnet growth given by ilmenite inclusions and 2) a lack of evidence for extensive Ti loss in quartz (e.g. Ti diffusion profiles into garnet or extensive post-entrapment recrystallization textures of inclusions in EM-CL imagery). Therefore, non-quartz mineral inclusions in garnet such as biotite likely serve as a more reliable estimates of garnet mineral inclusion entrapment temperature during prograde metamorphism in the study area.