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Research ArticleArticles

Origin of an unusual monazite-xenotime gneiss, Hudson Highlands, New York: SHRIMP U-Pb geochronology and trace element geochemistry

John N. Aleinikoff, Richard I. Grauch, Frank K. Mazdab, Loretta Kwak, C. Mark Fanning and Sandra L. Kamo
American Journal of Science September 2012, 312 (7) 723-765; DOI: https://doi.org/10.2475/07.2012.02
John N. Aleinikoff
*U.S. Geological Survey, MS 963, Denver Federal Center, Denver, Colorado 80225
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  • For correspondence: jaleinikoff@usgs.gov
Richard I. Grauch
**U.S. Geological Survey, MS 973, Denver Federal Center, Denver, Colorado 80225
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Frank K. Mazdab
***Department of Geosciences, 1040 E. 4th Street, Gould-Simpson Building, University of Arizona, Tucson, Arizona 85721
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Loretta Kwak
*U.S. Geological Survey, MS 963, Denver Federal Center, Denver, Colorado 80225
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C. Mark Fanning
§Research School of Earth Sciences, Australian National University, Mills Road, Canberra ACT 0200, Australia
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Sandra L. Kamo
§§Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario, Canada, M5S 3B1
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Abstract

A pod of monazite-xenotime gneiss (MXG) occurs within Mesoproterozoic paragneiss, Hudson Highlands, New York. This outcrop also contains granite of the Crystal Lake pluton, which migmatized the paragneiss. Previously, monazite, xenotime, and zircon from MXG, plus detrital zircon from the paragneiss, and igneous zircon from the granite, were dated using multi-grain thermal ionization mass spectrometry (TIMS). New SEM imagery of dated samples reveals that all minerals contain cores and rims. Thus TIMS analyses comprise mixtures of age components and are geologically meaningless. New spot analyses by sensitive high resolution ion microprobe (SHRIMP) of small homogeneous areas on individual grains allows deconvolution of ages within complexly zoned grains.

Xenotime cores from MXG formed during two episodes (1034 ± 10 and 1014 ± 3 Ma), whereas three episodes of rim formation are recorded (999 ± 7, 961 ± 11, and 874 ± 11 Ma). Monazite cores from MXG mostly formed at 1004 ± 4 Ma; rims formed at 994 ± 4, 913 ± 7, and 890 ± 7 Ma. Zircon from MXG is composed of oscillatory-zoned detrital cores (2000-1170 Ma), plus metamorphic rims (1008 ± 7, 985 ± 5, and ∼950 Ma). In addition, MXG contains an unusual zircon population composed of irregularly-zoned elongate cores dated at 1036 ± 5 Ma, considered to be the time of formation of MXG. The time of granite emplacement is dated by oscillatory-zoned igneous cores at 1058 ± 4 Ma, which provides a minimum age constraint for the time of deposition of the paragneiss.

Selected trace elements, including all REE plus U and Th, provide geochemical evidence for the origin of MXG. MREE-enriched xenotime from MXG are dissimilar from typical HREE-enriched patterns of igneous xenotime. The presence of large negative Eu anomalies and high U and Th in monazite and xenotime are uncharacteristic of typical ore-forming hydrothermal processes. We conclude that MXG is the result of unusual metasomatic processes during high grade metamorphism that was initiated at about 1035 Ma. This rock was then subjected to repeated episodes of dissolution/reprecipitation for about 150 m.y. during regional cooling of the Hudson Highlands.

  • Monazite
  • xenotime
  • zircon
  • U-Pb
  • Hudson Highlands
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American Journal of Science: 312 (7)
American Journal of Science
Vol. 312, Issue 7
1 Sep 2012
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Origin of an unusual monazite-xenotime gneiss, Hudson Highlands, New York: SHRIMP U-Pb geochronology and trace element geochemistry
John N. Aleinikoff, Richard I. Grauch, Frank K. Mazdab, Loretta Kwak, C. Mark Fanning, Sandra L. Kamo
American Journal of Science Sep 2012, 312 (7) 723-765; DOI: 10.2475/07.2012.02

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Origin of an unusual monazite-xenotime gneiss, Hudson Highlands, New York: SHRIMP U-Pb geochronology and trace element geochemistry
John N. Aleinikoff, Richard I. Grauch, Frank K. Mazdab, Loretta Kwak, C. Mark Fanning, Sandra L. Kamo
American Journal of Science Sep 2012, 312 (7) 723-765; DOI: 10.2475/07.2012.02
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Keywords

  • monazite
  • xenotime
  • zircon
  • U-Pb
  • Hudson Highlands

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