Three Proterozic metamorphic episodes and 1.70-1.67 Ga age of the Uncompahgre quartzite resolved by in-situ monazite and xenotime petrochronology, Needle Mountains, Colorado

Ian Hillenbrand, Geosciences and Environmental Change Science Center, U.S. Geological Survey, Denver, CO
Michael Williams, Department of Geosciences, University of Massachusetts Amherst
Michael Jercinovic, Department of Geosciences, University of Massachusetts Amherst
Karl Karlstrom, University of New Mexico
Amy Gilmer, Geosciences and Environmental Change Science Center, U.S. Geological Survey, Denver, CO

Abstract

Constraints on the depositional age, provenance, and timing/conditions of metamorphism of metasedimentary rocks are critical for building tectonic models. However, these are difficult to determine in rocks that have experienced multiple tectono-metamorphic phases and where sedimentary basins lack fossil control, such as the 1.8-1.3 Ga orogenic belts of the Southwest U.S.A. We integrate multi-scale compositional mapping, metamorphic petrology, and in-situ monazite and xenotime petrochronology to constrain pressure-temperature-time paths from basement gneisses and cover quartzites in the Needle Mountains of southwestern Colorado, one of the most complete records of Proterozoic basement in the Southwest.

Garnet-staurolite-bearing Twilight Gneiss records peak metamorphic conditions of 650°C at 0.7 GPa at 1.764 ± 0.009 Ga with cooling to ~575°C 1.741 ± 0.010 Ga. Similarly, garnet-K-feldspar-bearing metagraywacke recovered from drill core near Pagosa Springs records M1 amphibolite-facies conditions of 700°C at 1.748 ± 0.009 Ga, followed by an M2 peak granulite facies metamorphism (1.1 GPa, 780°C) at ca. 1.68-1.65 Ga, and an M3 event, ~425°C at 1.424 ± 0.012 Ga. In the Needles, quartzites of the Uncompahgre Formation unconformably overly the basement gneisses and contain monazite and xenotime characterized by rounded detrital cores separated from overgrowths by a sharp compositional boundary. The maximum depositional age of the Uncompahgre Formation is constrained by detrital monazite (1.707 ± 0.008 Ga) and xenotime (1.704 ± 0.015 Ga) populations and the minimum depositional age is constrained by 1.67-1.60 Ga (weighted mean=1.644 ± 0.015 Ga) metamorphic xenotime. Textures in the 1.67-1.60 Ga xenotime grains, including micropores, inclusions, and the rare preservation of detrital cores, suggest greenschist facies conditions of ≤450°C during M2 metamorphism. M3 metamorphic temperatures at 1.47-1.44 Ga ranged from ~300°C in the Uncompahgre Gorge to ~575°C in the contact aureole of the ~1.44 Ga Eolus granite. These results, and published geochronology, indicate (1) 1.76-1.74 Ga amphibolite facies M1 metamorphism, (2) intrusion of 1.73-1.69 Ga granites, 3) deposition of the Uncompahgre Formation between 1.70 and 1.67 Ga, 4) M2 metamorphism that ranged from 1.68-1.65 Ga granulite facies metamorphism in Pagosa Springs to 1.67-1.60 Ga greenschist facies metamorphism in the cover quartzite sequence, and (5) 1.47-1.42 Ga greenschist to amphibolite facies metamorphism in cover and basement. Our results emphasize the importance of multi-scale compositional mapping and in-situ acquisition of monazite and xenotime compositional and geochronologic data. The workflow presented here may be applied to a wide range of rock types to constrain the depositional age, provenance, and complex tectono-metamorphic histories.

Session

Session 6: Advances in Geology, Geochronology, Geophysics