EVIDENCE FOR MONAZITE GROWTH AT ~1.6 GA AND ~1.4 GA IN PROTEROZOIC MIGMATITES OF THE SANTA FE RANGE, NORTH-CENTRAL NEW MEXICO

Supracrustal Proterozoic migmatites exposed in the Aspen Basin area of the Santa Fe Range represent the northernmost exposure of the Mazatzal (~1.65 Ga) crustal province. Two migmatite samples (plag-bt-ms-qtz-hem±grt) yield preliminary in-situ electron microprobe U-Th-total Pb ages from monazite that are ~1.4 Ga; however, 1 grain yields a core age of ~1.6 Ga.

Monazite are generally aligned subparallel to a regional moderately, south-dipping foliation (S₁), indicating monazite growth was pre- or synkinematic. X-ray mapping of monazite shows variable U, Th, Pb, Y, and Ce zoning, interpreted to reflect three periods of monazite growth. Xenotime are relatively unzoned in most elements, but show minor discontinuous high-uranium rims.

Monazite grains analyzed in one sample show minor zoning in Th, U, and Y, and are interpreted to represent a single chemical domain. Individual grains yield ages (mean ± 1 standard deviation) of 1370 ± 17 Ma (n=8), 1376 ± 30 Ma (n=5), and 1334 ± 23 Ma (n=2). Intergrain ages are statistically indistinguishable at a level of 95% confidence.

A second sample contains monazite grains with variable zoning. The core of one grain is characterized by relatively high yttrium (3.96 wt % Y) and yields an age of 1471 ± 46 Ma (n=11). The rim of the same grain is slightly depleted in Y relative to the core (2.04 wt %), and yields an age of 1358 ± 36 Ma (n=9). Another monazite grain from the same rock has the opposite Y distribution (lower Y core (1.83 wt. %), higher Y rim (2.91 wt. %)). These chemical domains yield ages of 1601 ± 27 Ma (n=6) in the core and 1409 ± 53 Ma (n=3) at the rim. These ages may reflect polymetamorphism due to the ~1.65 Ga Mazatzal orogeny and a ~1.4 Ga intracratonic tectonic episode.