14:30 〜 14:45
[SMP24-04] Geochemical characterization of zircon for U-Pb age determination in Fyfe Hills of the Napier Complex, East Antarctica
キーワード：ジルコン、U-Pb年代、ナピア岩体
Ultra-high temperature (UHT) metamorphism is critical to understanding the large-scale tectonic processes affecting the deep crust and lithosphere throughout Earth’s history. The Napier Complex in East Antarctica is the location where the regional UHT metamorphism was first recognized (Dallwitz, 1968) and experienced extremely high temperatures (>1100 °C) based on the mineral assemblage of sapphirine + quartz (Harley, 2016 and reference therein). The thermal history of the Napier Complex is essential for unraveling the Earth’s crustal evolution, including deep crust; however, geochronological constraints, such as the timing and duration of the metamorphic events, are still debated. Two hypotheses for the timing are proposed in previous studies: (i) the UHT metamorphism occurred no earlier than 2840 Ma and possibly from 2590 to 2550 Ma (e.g., Harley et al., 2001), (ii) it occurred from around 2500 to 2450 Ma (e.g., Carson et al., 2002).
In this study, U–Pb zircon geochronology integrated with rare earth element (REE) and oxygen isotope was applied to a garnet-bearing quartzo-feldspathic gneiss to confirm the timing of UHT metamorphism in Fyfe Hills in the western part of the Napier Complex. The quartzo-feldspathic gneiss is mainly composed of garnet + mesoperthite (ternary feldspar) + quartz, with small amounts of zircon and opaque minerals (Horie et al., 2012).
The zircons collected from the quartzo-feldspathic gneiss are observed in backscattered electron (BSE) and cathodoluminescence (CL) images obtained using the low-vacuum mode of a scanning electron microscope (JEOL JSM-5900LV) with a Gatan mini CL detector at National Institute of Polar Research, Japan (NIPR). The zircons were analyzed using a sensitive high-resolution ion microprobe (SHRIMP) at National Institute of Polar Research. After the all SHRIMP analysis, true color CL images of the grain mount were obtained using a Gatan ChromaCL2 system installed with a field emission SEM (FE-SEM; JEOL JSM-7100F) at the NIPR. The CL observation and U–Pb ages allowed us to classify the analytical domains into three types: inherited domains, metamorphic domains, and U–Pb system disturbed domains. The REE patterns of metamorphic domains are characterized by a weak fractionation between the middle REE and heavy REE, which reinforces the classification based on the CL observation and the U–Pb ages. The 207Pb/206Pb ages of metamorphic domains have an age peak at 2501 Ma, therefore, the gneiss experienced high-temperature metamorphism at 2501 Ma. The δ18O of zircons are homogeneous among the three groups (5.53 ± 0.11‰, 5.51 ± 0.14‰, and 5.53 ± 0.23‰), which suggests the oxygen isotope compositions in zircon were re-equilibrated after the metamorphism at ca. 2501Ma under dry UHT conditions.
In this study, U–Pb zircon geochronology integrated with rare earth element (REE) and oxygen isotope was applied to a garnet-bearing quartzo-feldspathic gneiss to confirm the timing of UHT metamorphism in Fyfe Hills in the western part of the Napier Complex. The quartzo-feldspathic gneiss is mainly composed of garnet + mesoperthite (ternary feldspar) + quartz, with small amounts of zircon and opaque minerals (Horie et al., 2012).
The zircons collected from the quartzo-feldspathic gneiss are observed in backscattered electron (BSE) and cathodoluminescence (CL) images obtained using the low-vacuum mode of a scanning electron microscope (JEOL JSM-5900LV) with a Gatan mini CL detector at National Institute of Polar Research, Japan (NIPR). The zircons were analyzed using a sensitive high-resolution ion microprobe (SHRIMP) at National Institute of Polar Research. After the all SHRIMP analysis, true color CL images of the grain mount were obtained using a Gatan ChromaCL2 system installed with a field emission SEM (FE-SEM; JEOL JSM-7100F) at the NIPR. The CL observation and U–Pb ages allowed us to classify the analytical domains into three types: inherited domains, metamorphic domains, and U–Pb system disturbed domains. The REE patterns of metamorphic domains are characterized by a weak fractionation between the middle REE and heavy REE, which reinforces the classification based on the CL observation and the U–Pb ages. The 207Pb/206Pb ages of metamorphic domains have an age peak at 2501 Ma, therefore, the gneiss experienced high-temperature metamorphism at 2501 Ma. The δ18O of zircons are homogeneous among the three groups (5.53 ± 0.11‰, 5.51 ± 0.14‰, and 5.53 ± 0.23‰), which suggests the oxygen isotope compositions in zircon were re-equilibrated after the metamorphism at ca. 2501Ma under dry UHT conditions.