Metamorphic Age and Pressure-Temperature Conditions of a Sillimanite–Garnet-Bearing Pelitic Gneiss from Niban–nishi Rock on the Prince Olav Coast - Evidence for Tonian Metamorphism

Sep 18, 2026

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Determining the metamorphic record in a sillimanite–garnet–bearing pelitic gneiss from Niban–nishi Rock (Prince Olav Coast, East Antarctica) clarifies regional tectonics and age architecture. Using electron microprobe U–Th–Pb monazite dating, the rock is shown to differ from typical Lützow–Holm Complex exposures: it records Tonian metamorphism at 940.1 ± 9.8 Ma. Phase-equilibrium modeling and geothermobarometry constrain peak and retrograde conditions to 690–730 °C/0.38–0.68 GPa and 620–670 °C/0.42–0.60 GPa, with additional younger monazite ages indicating later overprints.

LanguageEnglish

CategoryResearch & Report

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Pages17

Reading Time43 min

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Introduction

Regional setting of the Lützow–Holm Complex

Metamorphic age and facies discrepancies among exposures

Importance of amphibolite–facies zone exposures

Metamorphic age and pressure–temperature conditions recorded in a sillimanite–garnet–bearing pelitic gneiss from Niban–nishi Rock of Niban Rock on the Prince Olav Coast, eastern Dronning Maud Land, East Antarctica: Evidence for Tonian metamorphism Yuki MORI *, Tomokazu HOKADA**,***, Tomoharu MIYAMOTO† and Takeshi IKEDA† *Diﬀraction and Scattering Division, Japan Synchrotron Radiation Research Institute (JASRI/SPring–8), Hyogo 679–5198, Japan **Geoscience Group, National Institute of Polar Research, Tokyo 190–8518, Japan ***The Graduate Institute for Advanced Studies, SOKENDAI, Tokyo 190–8518, Japan †Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, Fukuoka 819–0395, Japan We determined the metamorphic age and pressure–temperature conditions recorded in a sillimanite–garnet– bearing pelitic gneiss from Niban–nishi Rock, which is part of Niban Rock, on the Prince Olav Coast, eastern Dronning Maud Land, East Antarctica. Niban–nishi Rock is recognized as a component of the Lützow–Holm Complex (LHC), which is characterized by metamorphic age of 600–520 Ma and metamorphic grade of amphibolite to granulite facies. Electron microprobe U–Th–Pb monazite dating of the examined gneiss revealed that, unlike the typical exposures ofthe LHC, Niban–nishi Rock experienced Tonian metamorphism at 940.1 ± 9.8 Ma (2σ level), and is thus more similar to the neighboring exposures of Cape Hinode and Akebono Rock. Small numbers of younger monazite ages of 827–531 Ma were also detected, and some of which might relate to the metamorphism of the LHC. Phase equilibrium modeling and geothermobarometry indicate metamorphic conditions of 690–730 °C/0.38–0.68 GPa and 620–670 °C/0.42–0.60 GPa for peak and retrograde stages, respectively. The obtained peak temperature is lower than that of typical exposures in transitional– and granulite– facies zones ofthe LHC, such as Akarui Point and Skallen. Metamorphic features of Niban–nishi Rock, such asthe upper amphibolite–facies condition and occurrences of garnet with retrograde zoning and sillimanite in the matrix, diﬀer from those of Cape Hinode and Akebono Rock, with the former belonging to granulite facies and the latter showing kyanite in the matrix and garnet with growth zoning. Investigating Niban–nishi Rock is key in revealing the tectonic relation between the LHC and Cape Hinode (the Hinode Block) . Further ﬁeld surveys are required to reveal the metamorphic variations and the relations among exposures on the Prince Olav Coast. Keywords: Lützow–Holm Complex, East Antarctica, Tonian metamorphism, Pressure–temperature condition, U–Th–Pb monazite dating INTRODUCTION An area that extends over approximately 400 km from the Prince Olav Coast to Prince Harald Coast in eastern Dronning Maud Land, East Antarctica was previously considered to be occupied by the Lützow–Holm Complex (LHC), which is characterized by a metamorphic age of 600–520 Ma (Dunkley et al., 2020 and reference therein) with the metamorphic grade of amphibolite to granulite doi:10.2465/jmps.221124 Y. Mori, [yuki.mori](yuki.mori@spring8.or.jp)[@](yuki.mori@spring8.or.jp)[spring8.or.jp](yuki.mori@spring8.or.jp) Corresponding author © 2023 Japan Association of Mineralogical Sciences facies (e.g., Hiroi et al., 1991) (Fig. 1a) . Hiroi et al.(1991), who found a continuous increase in the metamorphic grade from the Prince Olav Coast to Prince Harald Coast, divided the area into three metamorphic zones referred to as the amphibolite–facies zone, transitional zone, and granulite–facies zone from northeast to southwest (Fig. 1a) . Suzuki and Kawakami (2019) recently revealed that the metamorphic grade of the transitional zone is similar to that of the granulite–facies zone. In contrast, the distinct discrepancies in metamorphic age and lithology observed in other exposures ofthe LHC has been conﬁrmed in Cape Hinode, an exposure located 2 Y. Mori, T. Hokada, T. Miyamoto and T. Ikeda Figure 1.

FAQs

What metamorphic age was determined for Niban–nishi Rock, and how was it measured?

Electron microprobe U–Th–Pb monazite dating indicates Tonian metamorphism at 940.1 ± 9.8 Ma (2σ level). This age contrasts with typical Lützow–Holm Complex exposures.

What pressure–temperature conditions were inferred for the peak and retrograde stages?

Phase equilibrium modeling and geothermobarometry give peak conditions of 690–730 °C / 0.38–0.68 GPa and retrograde conditions of 620–670 °C / 0.42–0.60 GPa.

How do metamorphic features of Niban–nishi Rock compare with Cape Hinode and Akebono Rock?

Niban–nishi Rock shows an upper amphibolite–facies condition and garnet with retrograde zoning plus sillimanite in the matrix. In contrast, Cape Hinode is granulite facies and Akebono Rock shows kyanite in the matrix with garnet growth zoning.

Sep 18, 2026

Uploaded by Himbo