[S-MP26] Supercontinents and Crustal Evolution

[SMP26-P04] Zircon geochronology and the interpretation of metamorphism using Th/U ratios in Mt. McMaster, Napier Complex, East Antarctica

*竹原 真美¹、堀江 憲路^1,2、外田 智千^1,2、アラン ナットマン³ (1.国立極地研究所、2.総合研究大学院大学、3.ウーロンゴン大学)

キーワード：ジルコン ウラン-鉛年代学　、トリウム-ウラン比

Th and U are trace elements that have been commonly used as a criterion for the evaluation of the chemical environment during zircon crystallization as the Th/U ratio. In principle, the zircon crystal structure prefers U rather than Th for the suitable ionic radius of U for substitution with Zr, which should result in higher U versus Th in zircon (Ewing, 1999; Hoskin & Schaltegger, 2003). Reflecting this property, zircon crystals indicate Th/U ratios lower than 1. Most igneous zircons from typical crustal rocks indicate Th/U ratios greater than 0.1 and less than 1; however, substitution mechanisms into zircon structures observed in natural conditions are commonly more complicated (Rubatto, 2017 and references therein). In the case of metamorphic zircons, it is a generally accepted idea that the Th/U ratios lower than 0.1 are controlled by the coexistence of Th-rich accessory phases, such as monazite and allanite (Yakymchuk et al., 2018 and references therein). However, in several previous studies, the UHT metamorphic zircons (> 900℃) were reported to have a tendency for the Th/U ratios to be > 0.1 and highly variable, as summarized in Harley et al. (2007), Rubatto (2017) and references therein. In this study, U-Th-Pb dating of zircon using a sensitive high-resolution ion microprobe (SHRIMP-IIe) at the National Institute of Polar Research was performed on a felsic-gneiss of Mt. McMaster to examine the thermal history of the nunataks outside of the ultra-high temperature metamorphism region in the Napier Complex, East Antarctica. In the Napier Complex, geochronological methods, particularly U-Th-Pb zircon age determination using microbeam analysis, have been a powerful tool for unraveling the early earth’s crustal history of the Napier Complex, Antarctica, since the 1980s (e.g., Black et al., 1986; Williams et al., 1984). The zircons from the Mt. McMaster sample have apparent ages of 2462-2985 Ma, with the youngest peak of approximately 2484 Ma. Zircons with high Th/U ratios (>1) show a younger age peak centered at about 2475 Ma, whereas the zircons with the lower Th/U ratios (< 0.1) show an older peak centered at about 2486 Ma, which suggests two-stage zircon growth during ca. 2.5 Ga regional metamorphism. Pre-metamorphic zircon yielded older ages of up to 2981 Ma, which is inconsistent with protolith ages of other felsic orthogneisses, matching that of a granitic gneiss from Mt. Riiser-Larsen.

References
Black et al. (1986) Contributions to Mineralogy and Petrology, 94, 427-437.; Ewing (1999) Proceedings of the National Academy of Sciences, 96, 3432-3439.; Harley et al. (2007) Elements, 3, 25-30.; Hoskin & Schaltegger (2003) Reviews in mineralogy and geochemistry, 53, 27-62.; Rubatto (2017) Reviews in mineralogy and geochemistry, 83, 261-295.; Williams et al. (1984) Contributions to Mineralogy and Petrology, 88, 322-327.; Yakymchuk et al. (2018) Journal of Metamorphic Geology, 36, 715-737.