Fluorine-rich calcic amphiboles in ultrahigh-temperature mafic granulite from Tonagh Island in the Napier Complex, East Antarctica: Preliminary report

• Main Authors: Toshiaki Tsunogae, Yasuhito Osanai, Tsuyoshi Toyoshima, Masaaki Owada, Tomokazu Hokada, Warwick A. Crowe
• Format: Report
• Language: English
• Published: Faculty of Education, Shimane University/Department of Earth Sciences, Okayama University/Graduate School of Science and Technology, Niigata University/Department of Earth Sciences, Yamaguchi University/National Institute of Polar Research/Department of Geology and Geophysics, University of Western Australia 2000
• Subjects: fluorine-rich amphibole; mafic granulite; Tonagh Island; Archean Napier Complex; East Antarctica; Napier; Holloway; Antarc*; Antarctica; Polar geoscience
• Online Access: https://nipr.repo.nii.ac.jp/?action=repository_uri&item_id=3068; http://id.nii.ac.jp/1291/00003068/; https://nipr.repo.nii.ac.jp/?action=repository_action_common_download&item_id=3068&item_no=1&attribute_id=18&file_no=1

Fluorine-rich calcic amphiboles occur in ultrahigh-temperature mafic granulite from Tonagh Island in the Napier Complex, East Antarctica The amphiboles are subdivided into two types high-grade brownish to pale brownish amphibole and retrograde greenish amphibole The brownish amphibole is pargasitic occurring as a subhedral grain in two-pyroxene mafic granulite It contains up to 2 2 wt% fluorine, which corresponds to an F/(F + Cl + OH) ratio of 0 54 (sample B98020801) In sample B98020802B, brownish amphibole is rimmed by an orthopyroxene-plagioclase-quartz corona, probably formed by a devolatilization reaction It has a lower fluorine content of 0 21-0 52 wt% (F/(F +Cl + OH) = 004-0 13) Pale brownish amphibole is a minor phase in metapyroxenite It has the highest fluorine content (2 6 wt%, F/(F + C1 + OH) = 0 60, sample B98012802F) Greenish amphibole is present as fine-grained aggregates with quartz, rimming ortho- and clinopyroxenes Fluorine is almost absent in this amphibole Holloway and Ford (1975) experimentally determined that pargasite with a F/(F +Cl + OH) ratio of 0 48 is stable up to 1100℃ at 5 kbar, and that F/(F + C1 + OH) increases with increasing temperature The fluorine-rich calcic amphiboles in mafic granulites are therefore stable under UHT metamorphic conditions The formation of an orthopyroxene-plagioclase-quartz corona in sample B98020802B can therefore be explained by breakdown of the amphibole due to its low-fluorine content The origin of the fluorine is not known, but it is probably derived from basic magma at the stage of protolith formation