NEW U–PB ZIRCON AGE CONSTRAINTS FOR CA. 1520 TO 1450 MA SEDIMENTATION AND VOLCANISM IN THE SOUTHWESTERN UNITED STATES AND SOME THOUGHTS ABOUT THE PROPOSED CA. 1480–1400 MA PICURIS OROGENY

New U-Pb detrital zircon ages from Proterozoic rocks in the Santa Fe, Sandia, Manzano and Pedernal mountains of northern and central New Mexico are characterized by unimodal age distributions with peak ages between ca. 1680 and 1790 Ma. These age distribution patterns are similar to other Proterozoic quartzites across the Rocky Mountain region and largely reflect derivation of sediment from the local Proterozoic basement. Age distributions tend to become broader from north to south consistent with mixing of Yavapai age (ca. 1720 Ma) and older detritus with Mazatzal age (ca. 1650 Ma) detritus of the Mazatzal crustal province.

One exception to these age distribution patterns occurs in the Picuris Mountains of New Mexico. Detrital zircon age distributions from metaconglomerate and quartzite of the Marqueñas and Piedra Lumbre formations show evidence of both Paleoproterozoic and Mesoproterozoic detritus with detrital zircon grains as young as ca. 1450 Ma. New U-Pb crystallization ages from boulder– and cobble–sized rhyolite clasts from the lowermost Marqueñas Formation yield concordant ages of 1447 ± 8 Ma and 1456 ± 4 Ma. Centimeter- to half-meter-scale metatuff layers from the Pilar Formation exposed in the northern and southern Picuris mountains yield new crystallization ages of 1523 ± 13 Ma and 1518 ± 6 Ma, slightly older than a previously dated metatuff layer (1488 ± 6 Ma). These new data support earlier findings and constrain the timing of sedimentation and volcanism in the region to be ca. 1520–1450 Ma, overlapping in time with regional metamorphism and deformation related to the 1480–1400 Ma Picuris Orogeny.

The Picuris Orogeny is a proposed Mesoproterozoic orogenic event supported by the recently discovered Mesoproterozoic protolith ages and subsequent regional metamorphism and deformation across the southwestern United States. The timing, areal extent and tectonic framework of this orogenic event needs further resolution. Data sets related to the formation and emplacement of the Mesoproterozoic ferroan magmas across Laurentia that appear to suggest an extensional tectonic environment need to be reconciled with emerging data on the timing and areal extent of sedimentation, metamorphism and deformation in New Mexico, Arizona and southern Colorado that are more indicative of a compressional tectonic setting.