New Mexico Geological Society Annual Spring Meeting — Abstracts

Discrete, low-temperature shear zones, occur throughout the Precambrian Manzano Moutains of central New Mexico and provide an excellent site for detennining the timing of deformation and the argon systematics from deformed versus undeformed minerals.

Shear zones from within and around the 1427 Ma Priest pluton occur on a scale from a few cm to several meters wide and display microstructures indicating deformation at green-schist facies conditions. Temporally the shear zones post-date both the regional amphibolite metamorphism and the metamorphism associated with plutonism at ca. 1400 Ma, but are not yet linked to any know Precambrian deformational or thennal event.

A combination of single crystal and bulk sample furnace step-heating experiments were performed on mainly muscovite from deformed and undeformed samples within and surrounding the Priest pluton, as well as some additional hornblende, biotite and K-feldspar analyses from the region. Muscovites from undeformed samples yield flat age spectra with plateau ages of 1390 ±3, 1395 ±2 and 1390 ±5 Ma. This age represents the time which the pluton and country rock cooled through ~350-400ºC. Single crystal muscovites from shear zones within the pluton reveal age gradients from ~1.1-1.4 Ga but yield terminal ages of 1360-1400 Ma. A clear relationship exists between the degree of deformation, the total gas age, and the development of the age gradient.

The single crystal muscovite (~1 mm) analyses suggest that the age gradients are not related to a mixture of young and old crystal populations but rather reflect the intra-crystalline ⁴⁰Ar distribution. The muscovite age gradients extrapolate to ~1100-1200 Ma and may represent the timing of deformation. These shear zones may therefore reflect the effects of Grenville orogenesis. These data alone can not unambiguously determin if the consequences of Grenville tectonism in New Mexico was uplift and erosion (cooling), a discrete thermal pulse (reheating), or if the shear zones actually record Greenville-age deformation. The muscovites also reveal apparent ages less than 1 Ga for the initial 5% of argon released, and is consistent with a regional thermal event of 200-250ºC at ca. 850 Ma recorded by K-feldspars from the region.