TESTING MODELS OF POLYMETAMORPHISM ACROSS NORTHERN NEW MEXICO: U-PB MONAZITE AGES SUGGEST TWO POSSIBLE MESOPROTEROZOIC METAMORPHIC EVENTS AT 1420 MA AND 1380 MA

Preliminary LA-ICPMS U-Pb monazite ages from aluminous quartzite and sillimanite-kspar bearing gneisses in Cedro Canyon, northern Taos Range, yield near concordant monazite ages of ca. 1380 Ma. Subhedral to anhedral monazite range in size between 10 µm and 50 µm, are typically aligned parallel/sub-parallel to the foliation, and generally occur along mineral grain boundaries; a few grains occur as inclusions within Fe-Ti oxides. Backscatter electron imaging revealed little to no compositional zoning. Monazite do not appear to preserve any record of ca. 1650 Ma near-granulite facies metamorphism as previously proposed for this region.

Monazite in the Picuris Mountains, northern New Mexico, yield near-concordant LA-ICPMS U-Pb growth ages from a variety of bulk compositions: a metarhyolite clast from the Marqueñas Fm. (1386 ± 11 Ma), a micaceous quartzite from the underlying Vadito Fm. (1362 ± 3 Ma), a garnet-biotite-staurolite schist from the Piedra Lumbre Fm. (1357 ± 6 Ma), and a metatuff layer from the Pilar Fm. (1359 ± 19 Ma). The majority of monazite grains are euhedral though some rounding or irregular boundaries are apparent due to the partial inclusion of matrix grains. Backscatter electron imaging showed little to no compositional zoning within the grains. Monazite in all samples are generally aligned parallel to the dominant regional foliation, but some clearly overgrow the foliation at a high angle and are interpreted as syn- to post-tectonic. Older monazite ca. 1434 Ma to 1400 Ma are preserved within kyanite and andalusite porphyroblasts of the Ortega Formation.

Older, ca. 1430 Ma - 1400 Ma monazite in the Picuris range are interpreted to record prograde and peak metamorphic monazite growth. Younger ca. 1380 Ma monazite across northern New Mexico may reflect a second regional metamorphic event or pervasive mid-crustal fluid flow event. Both monazite growth events are associated with the Picuris Orogeny.