13:45 〜 14:00
[SMP25-01] Long-lived high-temperature metamorphism followed by clockwise P-T path from Menipa, Sør Rondane Mountains, East Antarctica
キーワード：大陸衝突帯、グラニュライト、変成作用、U-Pbジルコン年代、U-Pbザクロ石年代
The Sør Rondane Mountains (SRM) in East Antarctica is located at the crossing point of the East African Orogen (EAO) and the Kuunga Orogen [1]. It is therefore a paleo-geographically important area to understand the formation process of Gondwana. Several tectonic models are proposed to explain the formation process of the SRM. The Main Tectonic Boundary (MTB) divides the SRM into the NE and SW terranes: the granulite-facies rocks in the NE terrane records clockwise P-T-t path, while those in the SW terrane records counter-clockwise P-T-t path [2]. Timing of peak metamorphism is regarded as ca. 650-600 Ma in both terranes, whereas ca. 590-530 Ma is considered as a timing of retrograde metamorphism in the andalusite field. Based on these, [2] concluded that the NE terrane overthrusted onto the SW terrane at ca. 650-600 Ma. On the other hand, the eastern SRM is proposed to be a part of the hanging wall of a mega-nappe bounding the Namuno Terrane (part of EAO) and Nampula Terrane [3]. Overthrusting of the Namuno Terrane onto the Nampla Terrane is proposed to have taken place at ca. 580-540 Ma [3]. Collage-style tectonics is also proposed for the formation of the SRM [4].
In order to understand the tectonic evolution of the SRM, we constructed a P-T-t path using a Sil-Bt-Grt gneiss from southern Menipa (central SRM). The garnet in the Sil-Bt-Grt gneiss consists of three parts: core with calcic plagioclase inclusions, inner rim with rare kyanite and sillimanite inclusions, and outer rim with sillimanite inclusions. Rutile inclusions are ubiquitous in garnet. The stability of Sil+Ky combined with the temperature estimate by the Zr-in-rutile geothermometer [5] applied to rutile enclosed in the inner rim of garnet gave P-T condition of ~1.0 GPa, ~800 oC. The early retrograde P-T condition of ~0.38 GPa, ~610 oC was constrained by applying a GASP geobarometer and a Grt-Bt geothermometer [6, 7] to biotite and plagioclase in contact with the garnet rim. Later retrograde metamorphism in the kyanite field is evidenced by a fine-grained Ky+Bt intergrowth locally replacing the garnet rim. These three P-T constrains support a clockwise P-T evolution. The in situ LA-ICP-MS U-Pb zircon dating revealed that metamorphic dates characterized by low Th/U (from ca. 615 Ma to ca. 525 Ma) are preserved in the matrix zircon and zircon enclosed in garnet. The ca. 560 Ma zircon with Ybn/Gdn~0.5-1.2 is included in the outer-rim of garnet, suggesting equilibrium growth of the zircon with garnet. Kyanite-grade metamorphism took place before ca. 560 Ma because kyanite is included in the inner-rim of the garnet. This was followed by Sil-grade metamorphism started at ca. 560 Ma. The 602 ± 7 Ma zircon domain (weighted mean age) also showed Ybn/Gdn ~1, suggesting equilibrium growth of zircon and garnet. We interpret that the prograde garnet growth in the Sil-Bt-Grt gneiss started at ca. 602 Ma and continued until ca. 560 Ma.
A V-bearing, green grossular garnet surrounded by kelyphite occurs in a calcareous metapelite in Menipa [8]. This garnet was dated by in situ LA-ICP-MS U-Pb method [9], and the rim yielded U-Pb garnet age of 593 ± 8 Ma and 586 ± 9 Ma [10]. Titanite in the kelyphite gave concordant U-Pb age ranging from ca. 550 to ca. 500 Ma. Apatite in kelyphite gave U-Pb age of 496 ± 9 Ma [9]. From these data, green garnet growth took place at ca. 593-586 Ma, and breakdown of the green garnet to form kelyphite likely started before ca. 550 Ma.
Coincidence of ca. 602 Ma metamorphic age and green-garnet growth age (ca. 593-586 Ma) within analytical uncertainties suggests that the ca. 602 Ma was the timing of prograde metamorphism possibly in the kyanite field in southern Menipa. On the other hand, ca. 560 Ma is the final timing of garnet growth at ~800 oC under the sillimanite field. Retrograde metamorphism was already ongoing at ca. 550 Ma, suggested by ca. 550-500 Ma U-Pb titanite age from kelyphite that records cooling age at 700-500 oC [e.g.,11]. Therefore, high-T duration of about 40 Myr from ca. 602 Ma to ca. 560 Ma is likely, followed by the decompression cooling in the sillimanite field at ca. 550 Ma. Taking the Ky-grade retrograde metamorphism into account, the clockwise P-T-t path is constructed for southern Menipa rocks in this study. This is not consistent with the tectonic model by [2], while the EAO-aged metamorphic rock recording the clockwise decompression starting from ca. 560-550 Ma is consistent with the model by [3]. The model by [3] may be also applicable to the central SRM.
[1] Satish-Kumar et al. 2013 [2] Osanai et al. 2013 [3] Grantham et al. 2013 [4] Ruppel et al. 2020 [5] Tomkins et al. 2007 [6] Holdaway et al. 1998 [7] Holdaway 2001 [8] Osanai et al. 1990 [9] Niki et al. 2021 [10] Niki et al. 2022 [11] Cherniak 2010.
In order to understand the tectonic evolution of the SRM, we constructed a P-T-t path using a Sil-Bt-Grt gneiss from southern Menipa (central SRM). The garnet in the Sil-Bt-Grt gneiss consists of three parts: core with calcic plagioclase inclusions, inner rim with rare kyanite and sillimanite inclusions, and outer rim with sillimanite inclusions. Rutile inclusions are ubiquitous in garnet. The stability of Sil+Ky combined with the temperature estimate by the Zr-in-rutile geothermometer [5] applied to rutile enclosed in the inner rim of garnet gave P-T condition of ~1.0 GPa, ~800 oC. The early retrograde P-T condition of ~0.38 GPa, ~610 oC was constrained by applying a GASP geobarometer and a Grt-Bt geothermometer [6, 7] to biotite and plagioclase in contact with the garnet rim. Later retrograde metamorphism in the kyanite field is evidenced by a fine-grained Ky+Bt intergrowth locally replacing the garnet rim. These three P-T constrains support a clockwise P-T evolution. The in situ LA-ICP-MS U-Pb zircon dating revealed that metamorphic dates characterized by low Th/U (from ca. 615 Ma to ca. 525 Ma) are preserved in the matrix zircon and zircon enclosed in garnet. The ca. 560 Ma zircon with Ybn/Gdn~0.5-1.2 is included in the outer-rim of garnet, suggesting equilibrium growth of the zircon with garnet. Kyanite-grade metamorphism took place before ca. 560 Ma because kyanite is included in the inner-rim of the garnet. This was followed by Sil-grade metamorphism started at ca. 560 Ma. The 602 ± 7 Ma zircon domain (weighted mean age) also showed Ybn/Gdn ~1, suggesting equilibrium growth of zircon and garnet. We interpret that the prograde garnet growth in the Sil-Bt-Grt gneiss started at ca. 602 Ma and continued until ca. 560 Ma.
A V-bearing, green grossular garnet surrounded by kelyphite occurs in a calcareous metapelite in Menipa [8]. This garnet was dated by in situ LA-ICP-MS U-Pb method [9], and the rim yielded U-Pb garnet age of 593 ± 8 Ma and 586 ± 9 Ma [10]. Titanite in the kelyphite gave concordant U-Pb age ranging from ca. 550 to ca. 500 Ma. Apatite in kelyphite gave U-Pb age of 496 ± 9 Ma [9]. From these data, green garnet growth took place at ca. 593-586 Ma, and breakdown of the green garnet to form kelyphite likely started before ca. 550 Ma.
Coincidence of ca. 602 Ma metamorphic age and green-garnet growth age (ca. 593-586 Ma) within analytical uncertainties suggests that the ca. 602 Ma was the timing of prograde metamorphism possibly in the kyanite field in southern Menipa. On the other hand, ca. 560 Ma is the final timing of garnet growth at ~800 oC under the sillimanite field. Retrograde metamorphism was already ongoing at ca. 550 Ma, suggested by ca. 550-500 Ma U-Pb titanite age from kelyphite that records cooling age at 700-500 oC [e.g.,11]. Therefore, high-T duration of about 40 Myr from ca. 602 Ma to ca. 560 Ma is likely, followed by the decompression cooling in the sillimanite field at ca. 550 Ma. Taking the Ky-grade retrograde metamorphism into account, the clockwise P-T-t path is constructed for southern Menipa rocks in this study. This is not consistent with the tectonic model by [2], while the EAO-aged metamorphic rock recording the clockwise decompression starting from ca. 560-550 Ma is consistent with the model by [3]. The model by [3] may be also applicable to the central SRM.
[1] Satish-Kumar et al. 2013 [2] Osanai et al. 2013 [3] Grantham et al. 2013 [4] Ruppel et al. 2020 [5] Tomkins et al. 2007 [6] Holdaway et al. 1998 [7] Holdaway 2001 [8] Osanai et al. 1990 [9] Niki et al. 2021 [10] Niki et al. 2022 [11] Cherniak 2010.