Northeastern Section–41st Annual Meeting (20–22 March 2006)

Paper No. 5
Presentation Time: 7:00 PM-9:30 PM

EVIDENCE FOR 1.40-1.35 GA REGIONAL METAMORPHISM & DEFORMATION IN THE THOMPSON PEAK AREA, NEW MEXICO: AN APPLICATION OF MONAZITE THERMOCHRONOMETRY


SHORT, Emily J.1, DANIEL, Christopher G.1 and PYLE, Joseph M.2, (1)Department of Geology, Bucknell University, O'Leary Center, Lewisburg, PA 17837, (2)Earth and Environmental Sciences, Rensselaer Polytechnic Institute, 110 8th St, 110 8th Street, Troy, NY 12180, eshort@bucknell.edu

The Mesoproterozoic Thompson Peak metamorphic succession (~1.68-1.65 Ga) likely represents the northernmost extent of Mazatzal crust in north-central New Mexico. Previous work proposed that regional metamorphism and deformation in this area was related to the ~1.65 Ga Mazatzal Orogeny. However, preliminary results using monazite thermochronometry from 3 samples show that regional metamorphism and deformation occurred at 1.40-1.35 Ga.

BSE imaging and chemical mapping of monazite from all three samples reveal minor compositional zoning; however, xenotime from the plagioclase-muscovite schist samples show 2-3 distinct compositional domains suggesting a more complex reaction history than monazite. Monazite and xenotime are aligned parallel to a regional south-dipping schistosity (S1). S2 is a north-striking and moderately to steeply dipping crenulation that folds S1. Monazite and xenotime growth occurred prior to or synchronous with S1.

In situ electron microprobe analyses of monazite from two plagioclase-muscovite schists yield average U-Th-Pb chemical ages of 1400 ± 60 Ma (1 s.d., 15 grains, 49 spots) and 1365 ± 30 Ma (1 s.d., 7 grains, 50 spots). A garnet-biotite schist yields an average monazite age of 1345 ± 20 Ma (1 s.d., 10 grains, 16 spots). Xenotime from a plagioclase-muscovite schist and a garnet-biotite schist yields U-Th-Pb chemical ages of 1365 ± 40 Ma (1 s.d., 5 grains, 13 spots) and 1350 ± 20 Ma (1 s.d., 5 grains, 7 spots), respectively, consistent with monazite age determinations. Monazite-xenotime thermometery yields temperatures of 525 ± 10°C (1 s.d., 7 grains, 50 spots) and 550 ± 10°C (1 s.d., 10 grains, 18 spots) for two plagioclase schists and 530 ± 13°C (1 s.d., 10 grains, 16 spots) for a garnet-biotite schist. These estimates are 20 to 75°C lower than previous temperature estimates from garnet thermobarometry (570-600°C at 6-8 kbar).