FORMATION OF TOPAZ-ENRICHED GNEISS IN THE EAST-CENTRAL COLORADO FRONT RANGE VIA CRYSTALLIZATION OF MESOPROTEROZOIC HALOGEN-RICH GRANITIC MAGMAS

Metamorphic rocks enriched in economically valuable minerals occur in all mountain belts worldwide, yet their processes of formation are often incompletely understood. Topaz [Al₂SiO₄(F,OH)₂] is a naturally occurring nesosilicate of great economic and industrial value, and is most commonly found in silica-rich igneous rocks. However, conspicuous 30-m-thick bands of topaz-, sillimanite-, rutile-, and quartz-rich (TSRQ) metamorphic gneiss occur in the Evergreen region of the east-central Front Range, Colorado, immediately adjacent to “normal” amphibolite and pelitic schist horizons. Bulk-rock geochemistry and isocon analysis shows that (e.g. Na, K, Mg, Ca) have been leached from these TSRQ rocks when compared to adjacent F-poor lithologies, indicating that the TSRQ gneiss must have interacted at some point with halogen-rich fluids. In addition, thermobarometry conducted via conventional and phase diagram-based techniques suggests pressure–temperature conditions of equilibration of ~700 °C and ~6.0 kbar, defining isotherms consistent with metamorphism in the middle continental crust during active mountain building. In-situ U–Pb dating of monazite within TSRQ units produced concordant ages mostly between representing the timing of F-rich fluid metasomatism. Together, these data are interpreted as recording metasomatic alteration in the middle crust by F-rich fluids released due to crystallization of nearby plutons, such as the Boulder Creek granodiorite at c. 1.65 Ga. As such, these topaz-rich horizons represent in-situ transformation of an older metamorphic assemblage, and not the metamorphosed products of weathered, pre-orogenic high-F tuffs or lava flows that were subsequently buried, as suggested by some previous studies. These results have far-reaching implications for constraining the behavior of halogens in the geological environment.