Journal of Petrology Advance Access originally published online on August 18, 2005
Journal of Petrology 2006 47(1):97-118; doi:10.1093/petrology/egi069
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MonaziteXenotime Thermochronometry and Al2SiO5 Reaction Textures in the Picuris Range, Northern New Mexico, USA: New Evidence for a 14501400 Ma Orogenic Event
1 DEPARTMENT OF GEOLOGY, BUCKNELL UNIVERSITY, LEWISBURG, PA 17837, USA
2 DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, RENSSELAER POLYTECHNIC INSTITUTE, TROY, NY 121803590, USA
RECEIVED NOVEMBER 2, 2004; ACCEPTED JUNE 30, 2005
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 PT loop, with minor decompression near the Al2SiO5 triple-point. Peak metamorphic temperatures are estimated at 510525°C, at 4·04·2 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. Monazitexenotime thermometry from the intermediate and rim domains yields temperatures of 405470°C (±50°C) and 500520°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 207Pb206Pb 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.
KEY WORDS: Al2SiO5; metamorphism; monazite; thermochronometry; triple-point
* Corresponding author. Telephone: (570) 577-1133. Fax: (570) 577-3031. E-mail: cdaniel{at}bucknell.edu