Monazite–Xenotime Thermochronometry and Al2SiO5 Reaction Textures in the Picuris Range, Northern New Mexico, USA: New Evidence for a 1450–1400 Ma Orogenic Event

Al2SiO5 reaction textures in aluminous schist and quartzite of the northern Picuris range, north-central New Mexico, record a paragenetic sequence of kyanite to sillimanite to andalusite, consistent with a clockwise P–T loop, with minor decompression near the Al2SiO5 triple-point. Peak metamorphic temperatures are estimated at 510–525°C, at 4·0–4·2 kbar. Kyanite and fibrolite are strongly deformed; some prismatic sillimanite, and all andalusite are relatively undeformed. Monazite occurs as inclusions within kyanite, mats of sillimanite and centimetre-scale porphyroblasts of andalusite, and is typically aligned subparallel to the dominant regional foliation (S0/S1 or S2) and extension lineation (L1). Back-scatter electron images and X-ray maps of monazite reveal distinct core, intermediate and rim compositional domains. Monazite–xenotime thermometry from the intermediate and rim domains yields temperatures of 405–470°C (±50°C) and 500–520°C (±50°C), respectively, consistent with the prograde to peak metamorphic growth of monazite. In situ, ion microprobe analyses from five monazites yield an upper intercept age of 1417 ± 9 Ma. Near-concordant to concordant analyses yield 207Pb–206Pb ages from 1434 ± 12 Ma (core) to 1390 ± 20 Ma (rim). We find no evidence of older regional metamorphism related to the ∼1650 Ma Mazatzal Orogeny.