Geology of ultrahigh-temperature metamorphic rocks from Tonagh Island in the Napier Complex, East Antarctica

• Main Authors: Yasuhito Osanai, Tsuyoshi Toyoshima, Masaaki Owada, Toshiaki Tsunogae, Tomokazu Hokada, Warwick A. Crowe
• Format: Report
• Language: English
• Published: Department of Earth Sciences, Okayama University/Graduate School of Science and Technology, Niigata University/Department of Earth Sciences, Yamaguchi University/Department of Earth Sciences, Shimane University/Department of Polar Science, School of Mathematical and Physical Sciences, The Graduate University for Advanced Studies/Department of Geology and Geophysics, University of Western Australia 1999
• Subjects: Archaean; Napier Complex; Tonagh Island; regional geology; UHT metamorphic rocks; Antarctic; East Antarctica; Napier; Amundsen Bay; Antarc*; Antarctica; Enderby Land; Polar geoscience
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=3040; http://id.nii.ac.jp/1291/00003040/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=3040&item_no=1&attribute_id=18&file_no=1

The summer party of the 39th Japanese Antarctic Research Expedition (1997-1998) carried out geological field work on Tonagh Island, located at the southern-end of Amundsen Bay, northern Enderby Land, which belongs to the central Napier Complex and shows part of the highest-grade metamorphic region in the Napier Complex. The island is underlain by various kinds of ultrahigh-temperature (UHT) metamorphic rocks and subordinate amounts of two types of unmetamorphosed intrusive rocks (alkali-dolerite and granitic pegmatite). UHT-metamorphic rocks are subdivided into five lithologic units (Units I to V) owing to their lithologies and geological structures from north to south, bounded by thrust-shear zones, accompanied with remarkable anhydrous mylonite and later pseudotachylite-cataclasite. The distinctive metamorphosed mafic dikes with tholeiitic composition are also found along the shear zone, characteristically at the boundary between Units II and III. The unmetamorphosed alkali-dolerite dikes, which are designated Amundsen dikes, cut across not only the sequence of metamorphic rocks but also the unit boundary shear zone and metamorphosed mafic dike. A geological perspective of the metamorphic rocks from Tonagh Island is generally classified into eight types on the regional map scale such as 1) orthopyroxene-bearing quartzofeldspathic gneiss, 2) garnet-bearing quartzofeldspathic gneiss, 3) two pyroxene-bearing mafic granulite, 4) garnet-orthopyroxene gneiss and granulite, 5) magnetite-quartz gneiss, 6) metamorphosed ultramafic rocks, 7) layered gneiss 1 (composed mainly of mafic gneiss and orthopyroxene-bearing quartzofeldspathic gneiss), 8) layered gneiss 2 (composed mainly of mafic gneiss and garnet-bearing quartzofeldspathic gneiss) with subordinate meta-impure quartzite, aluminous granulite, and calc-silicate granulite. The orthopyroxene-bearing quartzofeldspathic charnockitic gneiss and garnet-bearing quartzofeldspathic gneiss are the main constituents of Tonagh Island. Unit I has a peculiarity of predominance ...