EVIDENCE FOR CA. 1357-1386 MA POST–TECTONIC MONAZITE GROWTH, PICURIS MOUNTAINS, NORTHERN NEW MEXICO, USA

Preliminary LA-ICPMS U-Pb monazite ages from the Picuris Mountains, northern New Mexico yield near-concordant monazite growth ages ranging from 1357 ± 6 Ma to 1386 ± 11 Ma. These ages are 20-50 m.y. younger than previously reported monazite ages for the area. The four analyzed samples represent a variety of bulk compositions and different stratigraphic and structural levels. Two of the samples are from south of the Plomo fault, in the southern Picuris Mountains, and include a metarhyolite clast from the Marqueñas Fm. And a micaceous quartzite from the underlying Vadito Formation that yield monazite growth ages of 1386 ± 11 Ma (n=4), and 1362 ± 3 Ma (n=13), respectively. Two additional samples from the Hondo synclinorium north of the Plomo fault, in the northern Picuris, include a garnet-biotite-staurolite schist from the Piedra Lumbre Fm. and a metatuff layer from the Pilar Fm. that yield ages of 1357 ± 6 Ma (n=11), and 1359 ± 19 Ma (n=4), respectively. The majority of monazite grains from the four samples are euhedral though some rounding or irregular boundaries is apparent due to the partial inclusion of matrix grains. Monazite within the metarhyolite clast show significant replacement by apatite.

Monazite grains in all samples are generally aligned parallel to the dominant regional foliation, though some crosscut the foliation at a high-angle. Monazite partially-to-completely overgrow matrix minerals and aligned inclusion are observed in all samples. Monazite show little evidence of internal deformation, no pressure shadows, and apatite pseudomorphs after monazite show no evidence of deformation. Based on these lines of evidence, the monazite is interpreted to generally post-date tectonic activity. These new data and observations allow us to conclude that major regional deformation associated with the ca. 1.45-1.40 Ga Picuris Orogeny ceased prior to ca. 1385 Ma.