SHRIMP geochronology of a mylonite zone on Tonagh Island: characterisation of the last high-grade tectonothermal event in the Napier Complex, East Antarctica

• Main Authors: Crowe,WarwickA., Osanai,Yasuhito, Toyoshima,Tsuyoshi, Owada,Masaaki, Tsunogae,Toshiaki, Hokada ,Tomokazu
• Format: Report
• Language: English
• Published: Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia/Department of Earth Sciences, Okayama University/Graduate School of Science and Technology, Niigata University/Department of Earth Sciences, Yamaguchi University/Institute of Geoscience, the University of Tsukuba/Department of Geology, National Science Museum 2002
• Subjects: Napier Complex; Tonagh Island; SHRIMP geochronology; mylonite; East Antarctica; Napier; Antarc*; Antarctica; Polar geoscience
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=3106; http://id.nii.ac.jp/1291/00003106/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=3106&item_no=1&attribute_id=18&file_no=1

Tonagh Island within the highest-grade zone of the Napier Complex, East Antarctica, consists of a strongly layered sequence of quartzofeldspathic gneiss, mafic granulite and ultramafic gneiss subdivided into five lithostratigraphic units by major NE-SW to E-W trending steeply-dipping D_6 shear zones. D_6 shear zones cut the principal foliation defined by D_1 and D_2 structures associated with the ultrahigh-temperature metamorphism. Zircons from two mylonite samples with moderate NE pitching mineral lineations within a D_6 shear zone and a D_6 mylonite and ultramylonite sample from within high strain zones that have an overprinting sub-horizontal mineral lineation were analyzed. All samples have anhydrous mineral assemblages with a late textural association of biotite, which formed pre- to syn- a secondary overprinting high temperature mylonitic fabric. Zircons in all samples contain a structureless zone that constitutes most of the grain area and commonly have a relict discordant, oscillatory-zoned core. The core may be obscured to the extent where only a structureless or an indistinct remnant remains These internal patterns are consistent with recrystallisation and annealing processes resulting in resetting of the U-Pb system during mylonitsation involving possibly three events between ca. 2550 Ma and ca. 2470 Ma. The timing of D_6 mylonitisation between ca. 2470-2550 Ma on Tonagh Island requires an earlier timing for UHT metamorphism. This is consistent with regional models that attribute UHT metamorphism to pre- ca. 2550 Ma tectonothermal events in the Napier Complex.