TAKING THE TECTONIC PULSE OF PALEOPROTEROZOIC OROGENESIS: U–TH–PB MONAZITE GEOCHRONOLOGY OF METASEDIMENTS IN THE COLORADO FRONT RANGE

Paleoproterozoic supracrustal rocks in Big Thompson Canyon, Colorado Front Range, are the oldest known regionally metamorphosed rocks that contain a coherent isograd sequence recording island arc accretion, crustal thickening, and burial metamorphism. Metapelites within this sequence allow for a rare opportunity to constrain the pressure–temperature–time (P–T–t) evolution of this region of the Front Range. By combing thermobarometry with in-situ U–Th–Pb monazite geochronology, the rate of prograde metamorphism of this accreted terrane can be quantified. Petrographic characterization of five metapelites collected from the garnet-, staurolite-, andalusite-, sillimanite-, and migmatite zones in the region, and bright-phase element mapping using automated mineralogy has confirmed the presence of monazite in each. Bulk-rock geochemical data will allow use of high-precision phase diagram-based thermobarometry to determine the conditions of prograde and peak metamorphism, and the relative stability of mineral assemblages as functions of P and T conditions for each zone. To better understand monazite intra-grain compositions, and their relationships to garnet growth, detailed element mapping using an electron probe microanalyzer (EMPA) will be performed. Determined stability fields will then be compared to associated monazite dates, obtained from in-situ monazite geochronology and textural relationships between garnet and monazite growth. These relationships will allow a maximum constraint to be place on P–T conditions at specific times. P–T–t points from each sample will then provide the necessary data for tectonic rates to be calculated. These field-based, laboratory, and computational datasets and results will be combined to produce a new model of the tectonothermal evolutions of this region will provide further insight to the paleo-crustal structure of northern Colorado Front Range and the long-term evolution of continental USA as a whole.