New Mexico Geological Society Annual Spring Meeting — Abstracts

The timing of Proterozoic deformation in New Mexico is poorly constrained. In the southern Manzano Mountains two phases of deformation are constrained between the emplacement of the strongly deformed 1656±10 Ma Monte Largo pluton and the very weakly deformed 1427±10 Ma Priest pluton.

In Estadio Canyon, ca. 500 m west of the Priest pluton. a 20 cm wide, boudinaged granitic dike intruded quartzite. The dike contains microstructures indicative of deformation under amphibolite facies conditions. Two morphologically distinct popUlations of zircons were recovered from the dike. Anhedral crystals, interpreted to represent a xenocrystic population, are 1643 ± 10 Ma. Euhedral, prismatic crystals, interpreted to represent igneous zircon, are 1438 ±6 Ma: an age that is interpreted to represent the emplacement age of the dike. The timing of the dike and pluton suggests that the dike may be an early phase of the Priest pluton.

Microstructures and c-axis crystallographic preferred orientations in quartzites from the southern Manzano Mountains are indicative of syn-deformational formation at greenschist to amphibolite grade conditions. Such a microfabric is inconsistent with deformation occurring during accretion at ca. 1650 Ma followed by residency in the middle crust (~12 km depth) for 250 million years. If the quartzites had resided in the middle crust for 250 m.y. we would expect to see the effects of annealing; including features such as an increase in grain size. equant, strain-free quartz grains, and a weak to random c-axis crystallographic preferred orientation. Any quartz mylonites containing syn-deforrnational microstructures are therefore indicative of ca. 1400 Ma deformation.

These microstructural inferences and the geochronologic control indicate that significant deformation occurred between approx 1438 Ma and 1427 Ma, and that the effects of this defonnation are more widespread than previously thought.