MONAZITE THERMOCHRONOMETRY IN MIGMATITES FROM NORTH-CENTRAL NEW MEXICO: CONTACT METAMORPHISM OR A REGIONAL MID-CRUSTAL MELT-RICH LAYER?

Monazite thermochronometry demonstrates that Proterozoic migmatites exposed in the central Santa Fe range and the Rincon range of north-central New Mexico preserve high-temperature monazite that crystallized near 1.4 Ga. No 1.4 Ga plutons are exposed in the Rincon range; however, undated plutons are present in the Santa Fe Range and a contact metamorphic setting is possible. Alternatively, partial melting in these two areas may represent a regional, mid-crustal, melt-rich layer.

Subhedral monazite in both areas are commonly aligned subparallel with the dominant regional foliation. Monazite from the central Santa Fe range show three distinct compositional domains. Relatively high-Y cores yield monazite-xenotime temperatures from ~750 °C to ~640 °C. Low-Y rims yield temperatures between ~500 °C and ~600 °C. An average U-Th-Pb chemical age for the high-Y domain is 1.47 Ga ± 0.46 Ga. The low-Y rims yield an average U-Th-Pb chemical age of 1.36 Ga ± 0.27 Ga. The rim may represent monazite growth either during slow cooling or a reheating event. A monazite inclusion within garnet is relatively unzoned and yields an average age of 1.41 Ga ± 0.14 Ga. Partial melting and crystallization occurred between ~1.47 Ga and ~1.42 Ga, at P-T conditions of ~750 °C and 5-6 kbar based upon phase equilibria and thermobarometry.

Monazite in the Rincon range show considerable compositional zoning and yield temperatures of 600 °C to 650 °C. U-Th-Pb chemical ages fall between ~1.35 Ga and ~1.40 Ga. One grain yields an age of 1.47 Ga ± 0.66 Ga. Monazite inclusions within garnet are interpreted to have formed pre-melt and yield slightly older ages than matrix monazite. P-T conditions for melting are estimated at ~700 °C and 5-7 kbar, between ~1.42 Ga and ~1.38 Ga.

Leucosomes in both areas are aligned with the dominant regional foliation and are folded at the cm- to m-scale with a subhorizontal enveloping surface. Significant regional deformation is synchronous with and postdates partial melting.