Masao Asami

Chemical Th-U-total Pb dating by electron microprobe analysis of monazite. Xenotime and zircon from the Archean Napier Complex, East Antarctica : evidence for ultra-high-temperature metamorphism at 2400 Ma

Abstract The chemical Th–U–total Pb isochron method (CHIME) of dating was carried out on monazite, xenotime, zircon, and polycrase in six samples of ultra-high-temperature (UHT) granulites and one of pegmatite from five localities in the Napier Complex, Enderby Land, East Antarctica. Despite the chronological heterogeneity in many grains, the CHIME ages overall show strong evidence for a major event near 2420 Ma. Analyzed monazite and xenotime grains, except monazite from Mt. Riiser-Larsen, have a narrow range of CHIME ages: 2412±42 Ma from Beaver Island and 2429±36, 2421±18 and 2431±22 Ma from Reference Peak. These ages are indistinguishable from CHIME ages on zircon (2436±17 and 2420±20 Ma) and on central older domains of monazite (2404±54 Ma) from Mt. Riiser-Larsen, and on a zircon rim (2430±15 Ma) from Mt. Cronus. We conclude that the ca. 2420 Ma ages relate to the UHT metamorphism, but we do not exclude the possibility that lead loss has lowered the ages by some 50 Ma. This interpretation is consistent with a preponderance of ages obtained by other methods on the Napier Complex particularly the ca. 2475±25 Ma age for a generation of pegmatites coeval with the UHT event and containing the UHT mineral assemblage beryllian sapphirine–khmaralite+quartz. Events prior to 2500 Ma and after 2420 Ma have also left their imprint on the CHIME ages, for example, a 3646 Ma CHIME age suggests that a zircon core is inherited, whereas ca. 2200 and 2000 Ma CHIME ages in zoned monazite could be due to further lead loss during post-UHT activity. The much younger CHIME age of 1094±67 Ma for monazite from a pegmatite at ‘Zircon Point’, Casey Bay, confirms 1000–1100 Ma lead loss ages reported for this region, and could result from fluid activity in an outlier of Rayner activity near the boundary between the Rayner and Napier Complexes. Approximately 2500 Ma ages have been reported from granulite-facies (but not UHT) rocks in the Madras and Nilgiri blocks in South India; these blocks and the Napier Complex could have constituted a single structural unit by late Archean time.

Monazite and Zircon Dating by the Chemical Th‐U‐Total Pb Isochron Method (CHIME) from Alasheyev Bight to the Sør Rondane Mountains, East Antarctica: A Reconnaissance Study of the Mozambique Suture in Eastern Queen Maud Land

Ages based on electron microprobe analysis using the chemical Th‐U‐total Pb isochron method (CHIME) were determined for monazite and zircon in 12 granulite‐facies paragneiss samples from four exposures on or near the coast of East Antarctica: Mt. Vechernyaya (Alasheyev Bight; 46° E, Rayner Complex), East Ongul Island (39°35′ E, Lützow‐Holm Complex), the southernmost Yamato Mountains (36° E, Yamato‐Belgica Complex), and the Sør Rondane Mountains (23°–28° E). Monazite occurs in textural equilibrium with silicate minerals in all samples and as inclusions in garnet in three of these and is presumed to have crystallized under granulite‐facies conditions. Most of the analyzed monazite grains appear to be chronologically homogeneous, but many others are markedly zoned. The CHIME ages on rims of the latter range from \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Dissakisite-(Ce), a new member of the epidote group and the Mg analogue of allanite-(Ce), from Antarctica

514\pm 27

Metamorphic conditions of staurolite schists of the Ryoke metamorphic belt in the Hazu-Hongusan area, central Japan

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PRELIMINARY PETROLOGICAL STUDIES OF THE METAMORPHIC ROCKS OF THE EASTERN SOR RONDANE MOUNTAINS

553\pm 11

The Ryoke Belt of the southwestern part of Shiaku-shoto area in the Seto-naikai Inland Sea

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