THE ROLE OF LATE FLUIDS IN THE TEXTURAL AND CHEMICAL EVOLUTION OF THE MESO- TO PALEOARCHEAN ROCKS OF THE BEARTOOTH MTNS, NORTHERN WYOMING PROVINCE, USA

In the 1950s-1960s the Beartooth Mountains of Montana/Wyoming of the northern Wyoming Province (USA) became one of the last bastions for the hypothesis of granitization i.e. conversion of sedimentary rocks to granitic rocks exclusively through subsolidus metasomatic processes. With the advent of newer analytical techniques, in the 1970s, Paul Mueller and his colleagues showed that most of the granitic rocks formed as a consequence of igneous processes. However, the role of metasomatic fluids does play a significant role in the textural and chemical evolution of the Archean rocks of the Beartooth Mtns. Volumetrically, much of the Beartooth Mountains, from the South Snowy Block (partially in Yellowstone National Park) to the Main Beartooth Block (~110 km distance), is dominated by 2.8 Ga magmatic rocks (tonalite-trondhjemite-granodiorite i.e. TTG suite). Within these rocks, there are xenoliths up to km scales containing a record of crustal evolution extending before 2.8 Ga (as early as ~4.0 Ga). The most common xenolithic lithologies are migmatitic quartzofeldspathic gneisses of tonalitic to granitic compositions. There are also extensive suites of high-grade (amphibolite to granulite facies) metasupracrustal rocks that include peraluminous gneisses, quartzites, banded iron formations, metabasites, and minor ultramafic rocks. These rocks are now tectonically mixed, and occur as meter-scale macrolithons, locally separated by high-grade ductile shear zones. Results of high-temperature fluid interactions in the included lithologies are preserved in many rocks, and are apparent with optical cathodoluminescence and backscattered-electron imaging. These imaging techniques illustrate the dynamic nature of the invasive fluids and microanalysis establishes the chemical nature of the resultant changes. Fluid interactions include partial hydration of anhydrous mineral assemblages (e.g. granulites and anhydrous igneous rocks), local dehydration and second generation of granulites from hydrous metamorphic rocks and/or introduction of K- and Ba-rich fluids to significantly metasomatize pre-existing rocks. This phenomenon should serve as a cautionary note in interpreting bulk chemical features of magmatic complexes, especially in terms of light-ion-lithophile and alkali-earth elements.