U–PB MONAZITE AGES SUGGEST A COMPLEX MESOPROTEROZOIC METAMORPHIC HISTORY DURING THE PICURIS OROGENY RECORDED BY METAPELITES OF THE RINCON RANGE, NORTH–CENTRAL NEW MEXICO

Pelitic rocks from exposures of a preserved Proterozoic fold and thrust belt in the Rincon Range of north–central New Mexico were subject to upper amphibolite facies regional metamorphism during the ~1400 Ma Picuris Orogeny. Significant metamorphic garnet growth and ductile deformation overprint an intrusive contact with the granitic gneiss of the ~1680 Ma Guadalupita pluton. Outcrops of locally migmatitic schist from the northern fringes of this pluton preserve the assemblage Grt + Bt + Sil + Qtz + Ms + Pl [± St] + Ap + Mnz + Ilm [± melt]. Monazite shows patchy zoning patterns with generally complimentary Y and Th zoning. LA–ICPMS U–Pb analyses give a mean age of 1372 ± 5 Ma (n = 18). A Qtz-Ms schist from north of the Guadalupita pluton, locally with elongate garnet porphyroblasts up to 20 mm long, contains monazite in the matrix and as inclusions in garnet. U–Pb ages for these textural groups overlap within uncertainty, yielding 1368 ± 8 Ma for inclusions in garnet (n = 9) and 1362 ± 7 Ma for matrix monazite (n = 19). Both groups show no resolvable age difference across compositional domains. A sample of Grt-St schist collected from metasedimentary rocks along the interdigitated margin of the Guadalupita pluton contains the assemblage Grt + St + Ms + Qtz + Pl + Ilm ± Hc + Mnz + Zrn. Monazite occurs in the matrix and as inclusions in garnet and ilmenite porphyroblasts. Patchy zoning is observed in Y and Th, though age differences are not resolvable. Matrix monazite U–Pb analyses give a mean age of 1419 ± 10 (n = 20) Ma. Individual ages from monazite inclusions in garnet (1420 ± 22 and 1434 ± 23 Ma) and inclusions in ilmenite (1443 ± 22 and 1463 ± 28 Ma) suggest a possible phase of earlier monazite growth in this rock. No evidence is seen for monazite growth near the time of intrusion of the Guadalupita pluton, nor the ~1650 Ma Mazatzal Orogeny. Published monazite and garnet geochronology from other Proterozoic exposures in northern New Mexico record similar ages. However, the age range represented in the Rincon Range suggests that metamorphism and burial associated with the ~1400 Ma Picuris Orogeny may not have followed a simple northward propagation of crustal thickening/burial, but perhaps occurred episodically across the region in a more complex, out of sequence style.