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

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).