EVIDENCE FOR ~1.4 GA MONAZITE GROWTH, REGIONAL METAMORPHISM, AND DEFORMATION IN THE MESOPROTEROZOIC THOMPSON PEAK METAMORPHIC SUCCESSION, SANTA FE COUNTY, NEW MEXICO

The Mesoproterozoic Thompson Peak metamorphic succession is a sequence of metamorphosed mafic and felsic volcanic rock interlayered with metasedimentary rock that likely represents one of the northernmost exposures of the Mazatzal crustal province (1650-1670 Ma) in New Mexico. The supracrustal sequence is intruded along the northern and eastern margin by a felsic quartz porphyry with a reported age of ~1655 Ma. The southern margin of the supracrustal succession is bordered by gneissic granite of unknown age. Previous work in the region proposed that regional metamorphism and deformation were associated with the intrusion of the granites during the c. 1650 Ma Mazatzal Orogeny.

In-situ, electron microprobe analyses (n = 98) of fifteen metamorphic monazite from a biotite schist in the Thompson Peak metamorphic succession yield an average U-Th-Pb electron microprobe chemical age of 1400 Ma ± 60 Ma. X-ray mapping of monazite grains reveals minor compositional gradients in Y, Th, U, and Pb; several monazite grains are relatively unzoned. Monazite are abundant in this sample and are interpreted to reflect a single generation of monazite growth. Monazite-xenotime thermometry from this sample yields an average temperature (n=18) of 590 °C ± 15 °C.  Previous P-T estimates from petrogenetic grid constraints and grt-bt-ms-plag and grt-hbl thermobarometry are estimated at 570-600 °C and 6-8 kbar for samples collected in the central part of the metamorphic succession.

Locally, up to three foliations are recognized in the Thompson Peak area. Compositional layering (S₀) and the dominant regional foliation (S₁) are subparallel, strike from NW to NE and dip moderately S. S₂ is a locally developed, differentiated crenulation cleavage that strikes N-NE and dips moderately to steeply E. S₃ is an open to tight crenulation cleavage that strikes N and is near vertical. Only two foliations are observed in our samples, and monazite are aligned in S₀/S₁ and folded about S₃.

These preliminary monazite age and temperature estimates and the occurrence of monazite aligned in the earliest observed fabric are consistent with previous findings in the Rincon and Picuris range and show that the major regional metamorphism and deformation in north-central New Mexico is related to a c. 1400 Ma orogenic event.