INTEGRATING MONAZITE THERMOCHRONOMETRY INTO THE P-T-t-D EVOLUTION OF MESOPROTEROZOIC ROCKS, NORTH-CENTRAL NEW MEXICO

The integration of monazite (Mnz)/xenotime (Xnt) textures, thermometry, and Mnz U-Th-Pb microprobe age determinations with major phase petrologic and structural analyses provides a powerful new tool to better document the P-T-t-D evolution of orogenic belts. Our application of these techniques shows that regional metamorphism and deformation in the Picuris and Rincon uplifts of north-central New Mexico occurred at ~1400 Ma.

In the northern Picuris range, coexisting Mnz and Xnt are common in aluminous quartzites and yield temperatures of 420-460 °C. Mnz and Xnt are texturally early and occur as inclusions within kyanite, sillimanite and andalusite and in the matrix; temperatures are interpreted to reflect pre-peak metamorphic growth of monazite. Independent Grt-Bt temperature estimates in nearby schists yield peak metamorphic temperatures of 520-530 °C at ~ 4 kbar (Holdaway & Goodge, 1990), consistent with Sil+Ctd assemblages. Monazite rims yield U-Th-Pb ages of ~1400 Ma. Texturally, Mnz contains inclusions of Fe-Ti oxides aligned parallel to the dominant S2 foliation, and many Mnz grains are themselves aligned parallel to S2, suggesting syndeformational growth.

Similar bulk compositions are exposed further east in the Rincon uplift; however, metamorphic temperatures and pressures are higher (600 to 700 °C and ~5 kbar). Mineral assemblages are characterized by Grt+St+Bt at higher elevations in the northern half of the uplift, and, by Sil-bearing migmatites at lower elevations in the southern half of the study area. Mnz-Xnt thermometry yields maximum temperatures of ~620 °C for the staurolite schist and ~660 °C for the migmatites, significantly higher than conventional Grt-Bt thermometry, that yields ~525 °C and 420-620 °C, respectively. The low Grt-Bt temperatures reflect diffusional modification and dissolution of the garnet rim. In the staurolite schist, Mnz and Xnt are found adjacent to garnet, aligned in the dominant S2 foliation and are interpreted to have grown during garnet breakdown thru the reaction Grt+Chl=St+Bt. In the migmatite, Mnz is aligned parallel to the dominant regional foliation and also occurs with Qtz and Fe-oxide inclusion trails within garnet. The calculated temperatures are consistent with the onset of melting. Mnz inclusions within garnet and in the matrix yield ages near 1380 Ma.