Paper No. 9
Presentation Time: 10:10 AM
MAKING THE CASE FOR A 1.48 TO 1.40 GA OROGENIC EVENT IN THE SOUTHWEST: THE PICURIS OROGENY
DANIEL, Christopher G., Department of Geology and Environmental Geosciences, Bucknell University, 1 Dent Drive, Lewisburg, PA 17837 and JONES III, James V., Department of Earth Sciences, University of Arkansas at Little Rock, 2801 S. University, Little Rock, AR 72204, cdaniel@bucknell.edu
Detrital zircon from the upper Marquenas Formation, Picuris Mountains, New Mexico indicate ca. 1.45 Ga sedimentation, the first recognized in the southwest US. Monazite inclusions within kyanite, sillimanite and andalusite from the Ortega Formation in the Picuris yield concordant ion microprobe ages between 1.44 Ga and 1.40 Ga. Electron microprobe chemical dating yields monazite core ages of ~1.47 Ga, with ~1.40 Ga rims. South of the Picuris, monazite rims from migmatites and upper amphibolite facies supracrustal rocks of the Santa Fe Range are ~1.40 Ga. Core ages are 1.65 to 1.7 Ga and may reflect detrital and/or metamorphic components. A hornblende-biotite tonalite that intrudes the migmatite complex yields a concordant zircon crystallization age of 1.434 Ga ± 0.006 Ga. Similar monazite ages, metamorphism and deformation occur to the east in the Rincon mountains. Km-scale north-vergent folds, fabrics, and ductile shear zones in the Picuris and adjacent mountain ranges are interpreted to be ca. 1.45 Ga. Overprinting south-vergent motion occurs locally. The occurrence of regional shortening, ductile shearing, metamorphism and likely syn-tectonic sedimentation are the hallmarks of an orogenic event. We suggest that this event be called the Picuris Orogeny.
We extend this event ~200 km north and ~200 km south of the Picuris range and from the Rincon range in eastern New Mexico, west into Arizona. Regional metamorphism and deformation at ca. 1.4 Ga are recorded in both the Needle and Wet Mountains of southern Colorado. Localized development of subvertical NE-striking fabrics and reactivation of pre-existing shear zones is documented across Colorado. In central New Mexico, ductile thrust systems such as the Monte Largo shear zone and the Vincent Moore thrust are interpreted to record ca. 1.4 Ga north-vergent shortening. Examination of local strain fields around 1.4 Ga plutons suggests early 1.44 to 1.48 N-S compression followed by 1.44 to 1.40 N-S extension, possibly corresponding to clockwise P-T paths that show heating and compression followed by near isothermal decompression in regional metamorphic rocks. The region of most intense ca. 1.45 Ga metamorphism and deformation corresponds to a broad area of 40Ar/39Ar thermal resetting and slow cooling.
