PETROCHRONOLOGIC CONSTRAINTS ON FOUR TECTONO-METAMORPHIC EVENTS IN WESTERN COLORADO

The Proterozoic tectonic evolution of the southwestern USA spans nearly 500 myr (1.8 to 1.3 Ga). Like many orogens, uncertainty exists in terms the timing, conditions, and progressive versus distinct nature of tectono-metamorphic phases. We applied multi-scale compositional mapping, petrologic modeling, in-situ monazite and xenotime petrochronology, and zircon geochronology to constrain pressure-temperature-time paths in western Colorado. Basement gneisses from the Needle Mountains, Pagosa Springs, and Gunnison record early 1.76 to 1.74 Ga metamorphism. Further evidence is provided by 1.76 to 1.74 Ga detrital monazite in the Uncompahgre Formation and an unnamed quartzite near Almont, CO. The Uncompahgre Formation was deposited at ca. 1.705 Ga, constrained by detrital monazite, xenotime, zircon, metamorphic xenotime, and ⁴⁰Ar/³⁹Ar data. Detrital monazite and xenotime compositions are consistent with derivation from a deep, garnet-bearing source in an unroofing Yavapai orogenic hinterland. Rocks near Gunnison and Almont record tectono-metamorphism at 0.3 GPa and 600°C at 1.72 to 1.69 Ga. Metamorphism at 1.68 to 1.60 Ga included greenschist facies conditions in the Needle Mountains and near Gunnison and Bedrock, CO and granulite facies (800°C at 1.1 GPa) near Pagosa Springs. Metamorphic conditions at ~1.4 Ga ranged from 270°C to >570°C. Collectively, in-situ geochronology of igneous, detrital, and metamorphic monazite and xenotime, igneous zircon, and quantitative thermobarometry indicate four tectono-metamorphic episodes. Greenschist to amphibolite facies metamorphism (M1) from 1760 to 1740 Ma may be related to collision(s) between arc terranes and/or backarc closure. The Uncompahgre Formation was deposited at ca. 1.705 Ga contemporaneous with the Yavapai orogeny (M2). Metamorphic conditions at 1.68 to 1.60 Ga during the Mazatzal orogeny (M3) are dominated by greenschist facies conditions but one drill core sample of uncertain structural context yielded high-pressure granulite facies conditions. Greenschist to amphibolite facies metamorphism at 1.47 to 1.40 Ga (M4) and syn-tectonic plutonism occurred during the protracted Picuris orogeny. These results emphasize the power of monazite and xenotime petrochronology to unravel the complex evolution of polymetamorphic orogenic belts.