CHIME ages of xenotime, monazite and zircon from beryllium pegmatites in the Napier Complex, Khmara Bay, Enderby Land, East Antarctica

• Grew Edward S.

  Department of Geological Sciences, University of Maine

• Suzuki Kazuhiro

  Nagoya University Center for Chronological Research

• Asami Masao

  Department of Earth Sciences, Faculty of Science, Okayama University

Abstract

Dating by the chemical Th-U-total Pb isochron method (CHIME) was carried out on xenotime, monazite and zircon from beryllium pegmatites intruded at about 2500 Ma in the ultrahigh-temperature metamorphic Napier Complex. No meaningful results were obtained on zircon from an associated Be-free pegmatite. All analyzed mineral grains are chronologically heterogeneous, and apparent ages range from -2460 Ma to 550 Ma with conspicuous concentration at 2200, 1700 and 700 Ma. The 2168±36 Ma age for the chronologically homogeneous, non-metamict core of a large xenotime grain dates intense post-emplacement deformation and metamorphism, possibly the granulite-facies event M-2 proposed by E.S. Grew et al. (Polar Geosci., 13, 1, 2000) to explain the breakdown of sapphirine and quartz to sillimanite, surinamite, orthopyroxene and/or garnet in the pegmatites and their host rocks. CHIME ages near 〜1700 Ma and 〜700 Ma could result from incomplete resetting by amphibolite-facies events (M-3, -4) between 〜1100 and 〜500 Ma. In general, the minerals appeared to have experienced no significant new growth during the later events, so that the effect of these events is expressed in the U-Th-Pb system in the original mineral. Because most of the ages were obtained on non-metamict portions of the analyzed grains, we doubt that metamictization can be the cause of most of the observed chronological heterogeneity. The lack of evidence for the 〜2500 Ma emplacement age in the CHIME data could be due to the reliance on total concentrations of Th, U and Pb, which may be more affected by subsequent deformation and metamorphism than the isotope ratios measured in conventional U-Pb dating.

Journal

• Polar geoscience

  Polar geoscience 14 99-118, 2001-10

  National Institute of Polar Research

Keywords

• pegmatite
• beryllium
• Antarctica
• Archean
• CHIME dating
• Napier Complex

Details

• CRID: 1573668926596649728
• NII Article ID: 110000035105
• NII Book ID: AA1130866X
• ISSN: 13443194
• Text Lang: en
• Data Source:
  • CiNii Articles