Sm-Nd garnet ages of retrograde garnet bearing granulites from Tonagh Island in the Napier Complex, East Antarctica: A preliminary study

• Main Authors: Masaaki Owada, Yasuhito Osanai, Toshiaki Tsunogae, Takuji Hamamoto, Hiroo Kagami, Tsuyoshi Toyoshima, Tomokazu Hokada
• Format: Report
• Language: English
• Published: Department of Earth Sciences, Yamaguchi University/Department of Earth Sciences, Okayama University/Institute of Geoscience, the University of Tsukuba/Graduate School of Science and Technology, Niigata University/Graduate School of Science and Technology, Niigata University/Graduate School of Science and Technology, Niigata University/National Institute of Polar Research 2001
• Subjects: Napier Complex; Sm-Nd garnet age; Proterozoic; retrograde metamorphism; East Antarctica; Napier; Tonagh Island; Antarc*; Antarctica; Polar geoscience
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=3086; http://id.nii.ac.jp/1291/00003086/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=3086&item_no=1&attribute_id=18&file_no=1

Proterozoic Sm-Nd garnet ages were dated from a garnet-bearing quartzo-feldspathic gneiss (Grt-felsic gneiss) and an iron rich garnet-pyroxene gneiss (Fe-rich Grt-Py gneiss) in the northern part of Tonagh Island, the Napier Complex. The Grtfelsic gneiss occurs as a member of layered gneiss. The Fe-rich Grt-Py gneiss appears as a block in the felsic gneiss. Since the layered gneiss contains a critical mineral assemblage of ultrahigh-temperature (UHT) metamorphism such as sapphirine+quartz, the Grt-felsic and the Fe-rich Grt-Py gneisses have undergone UHT metamorphism whose age is believed to be Late Archaean. The inclination line connecting with garnet and whole rock in the Grt-felsic gneiss yields an age of 1897 Ma in the Sm-Nd system. The internal isochron using garnet, pyroxenes and whole rock for the Fe-rich Grt-Py gneiss gives an age of 1557±35 Ma. Both Sm-Nd internal isochron ages correspond to Early Proterozoic. Garnet-quartz symplectite and quartz occur between orthopyroxene and plagioclase in the Grt-felsic gneiss. This texture indicates the following reaction: orthopyroxene+plagioclase=garnet+quartz. The garnet in the Fe-rich Grt-Py gneiss occurs as interstitial mineral with small grained clinopyroxene and quartz. Taking reaction textures into account, the garnet forming reactions in the Grt-felsic and the Fe-rich Grt-Py gneisses would occur during retrograde events after UHT peak metamorphism. The ages of c. 1900 and c. 1550 Ma would therefore reflect tectonothermal events in the Napier Complex.