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

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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 of layered gneisses showing thin alternation (centimeters to several meters in thickness) of orthopyroxene- and garnet-bearing quartzofeldspathic gneisses, two-pyroxene mafic granulite, garnet-orthopyroxene gneiss and granulite, sapphirine-bearing aluminous gneiss, garnet-sillimanite gneiss, leucocratic quartzofeld-spathic gneiss, magnetite-quartz gneiss, meta-quartzite and metamorphosed ultramafic rocks (pyroxenite and Iherzolite). Units II and III are characterized by widespread distributions of two pyroxene-bearing mafic granulite (gneiss) and garnet-orthopyroxene gneiss at the upper structural level, although layered gneisses dominate at the lower structural level. In field appearance Unit II and Unit III have nearly the same lithology and may be considered repetition due to thrusting. The magnetite-quartz gneiss occurs only in Units I, II and III characteristically. Unit IV is underlain by garnet- and orthopyroxene-bearing quartzofeldspathic gneisses and layered gneisses, which look very close to the lower structural level of Units II and III. On the other hand, constituents of Unit V are mainly orthopyroxene- and garnet-bearing quartzofeldspathic gneisses with subordinate layered gneiss 1 and traces of aluminous, mafic and ultramafic granulites. Therefore the most effective tectonic boundary may be the shear zone among Units IV and V. In this paper the regional geology of Tonagh Island is described in detail, with a brief description of modes of occurrence and petrographic features of UHT metamorphic rocks and unmetamorphosed intrusive rocks.
国立極地研究所の論文

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

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