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Journal of Petrology Advance Access published online on August 18, 2005
Journal of Petrology, doi:10.1093/petrology/egi069
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received November 2, 2004
Accepted June 30, 2005

Article

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

CHRISTOPHER G. DANIEL 1* and JOSEPH M. PYLE 2

1 DEPARTMENT OF GEOLOGY, BUCKNELL UNIVERSITY, LEWISBURG, PA 17837, USA
2 DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, RENSSELAER POLYTECHNIC INSTITUTE, TROY, NY 12180-3590, USA

* To whom correspondence should be addressed.
CHRISTOPHER G. DANIEL, E-mail: cdaniel{at}bucknell.edu


  Abstract

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.

Keywords: Al2SiO5; metamorphism; monazite; thermochronometry; triple-point.
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