Late Proterozoic (Grenville?) deformation in central New Mexico determined from single-crystal muscovite 40Ar/ 39Ar age spectra

Abstract

40Ar/ 39Ar age spectrum analyses on deformed and undeformed single-crystal muscovites from the Manzano Mountains in central New Mexico provide insights on the argon systematics of deformed muscovite, the post-1.4 Ga cooling history of the region, and the timing of low-temperature deformation. Microstructures in narrow, east-side-up shear zones in and around the 1427 Ma Priest pluton, indicate post-pluton greenschist facies (∼ 350°C) deformation. Single-crystal 40Ar/ 39Ar analysis of three samples containing undeformed muscovite yield plateau spectra with ages of 1390 ± 3, 1395 ± 2 and 1401 ± 3 Ma, suggests that by this time the pluton had cooled below ∼ 350–400°C. Single-crystal muscovites from within the shear zones have markedly different age spectra and reveal well-defined age gradients from ∼ 1.1 to 1.4 Ga. The age gradients are interpreted as resulting from deformation-induced argon loss or reduction of the effective radius for argon diffusion thereby decreasing the closure temperature of the originally pristine magmatic muscovites. These data, and previously published results, suggest a long period of crustal stability (ca. 1.4 to 1.1 Ga) in which average cooling rates were probably about 1°C/Ma. The ∼ 1.1 Ga ages in the deformed muscovite age spectra are interpreted as indicating that Grenvillian tectonism to the southeast affected central New Mexico. It can not be unambiguously determined if the consequences of the Grenville orogeny in New Mexico was uplift and erosion (cooling), a discrete thermal pulse (reheating), or if the shear zones record Grenville-age contractual deformation.