14:00 〜 14:15
[SMP25-02] Two distinct timings of metamorphism detected from Menipa, Sør Rondane Mountains, East Antarctica.
キーワード：LA-ICP-MS ジルコン年代測定、セール・ロンダーネ山地、南極
The Sør Rondane Mountains (SRM) comprise medium to high-grade metamorphic rocks with granitic and syenitic intrusions and minor mafic dikes [1]. The SRM are divided into the NE terrane and the SW terrane by the Main Tectonic Boundary (MTB), and each terrane is defined by shape of P-T path and age distribution of detrital zircon [2]. The SRM are considered to be formed by collision of both terranes at 650-600 Ma during which the NE terrane have thrusted up over the SW terrane [2]. This model also suggests that the whole SW terrane behaved as a single geological body during and after the 650-600 Ma collision event. However, recently we found a boundary where rocks with different timings of metamorphism are in direct contact with each other at Brattnipene in the SW terrane [3]. In this study, we report results of LA-ICP-MS zircon U-Pb dating of rocks from Menipa in the SW terrane, and tested whether the similar relationship as found in Brattnipene can be observed in Menipa.
At Menipa, pelitic gneisses and marbles are distributed at high elevation and quartzofeldspathic gneisses at low elevation. The dip of the foliation in the metamorphic rocks is commonly low angle in this area, indicating the rocks at higher elevation are located at structurally upper part. These metamorphic rocks are intruded by massive fluorite-bearing granite.
Sillimanite-garnet-biotite gneiss (sample No. TA19121302B), located at structurally upper part, preserves prograde textures indicating pressure increase before the peak condition [4]. Zircon grains in this sample show oscillatory-zoned cores truncated by vaguely-zoned rims. The dates from the core range ca. 1000-800 Ma, and those from the rim cluster at 618 ± 7 Ma (Th/U: 0.05-0.00; Ybn/Gdn: 66-1.3). The several rim domains show Ybn/Gdn ~ 1 indicating that the zircon grew in equilibrium with garnet. It means that garnet probably started to grow at ca. 618 Ma at latest. This age is interpreted as the timing of prograde metamorphism. This result is consistent with the timing of prograde metamorphism under kyanite stability field reported by [5] from the same area.
In contrast, the quartzofeldspathic gneisses, located at structurally lower part, yield younger metamorphic age of ca. 560-550 Ma. Garnet-cummingtonite gneiss (TA19121902A) contains orthopyroxene partially replaced by cummingtonite suggesting retrograde hydration. The U-Pb zircon dates from the oscillatory-zoned core range ca. 950-800 Ma and those from the vaguely-zoned rim cluster at 566 ± 5 Ma (Th/U: 0.10-0.00; Ybn/Gdn: 320-4.4). Garnet-biotite gneiss (TA19121903A) contains garnet partially replaced by biotite + plagioclase suggesting retrograde hydration. The U-Pb zircon dates from the oscillatory-zoned core range ca. 1000-800 Ma and those from the vaguely-zoned rim cluster at 547 ± 6 Ma (Th/U: 1.16-0.02; Ybn/Gdn: 80-1.0). These ages of ca. 570-550 Ma are interpreted as the timing of metamorphism of the quartzofeldspathic gneisses.
Fluorite-bearing granite (TA19121901H) contains euhedral and oscillatory-zoned/unzoned zircons yielding 504 ± 7 Ma (Th/U: 0.59-0.02), interpreted as the timing of intrusion of this granite.
As above, the sillimanite-garnet-biotite gneiss at structurally upper part records ca. 618 Ma metamorphism whereas the quartzofeldspathic gneisses at lower part record ca. 570-550 Ma a metamorphism at Menipa. This suggests that the two geological units with different timing of metamorphism are probably in contact with each other at Menipa, which is the same geological relationship as found in Brattnipene [3]. This clearly means the whole SW terrane did not behave as a single geological body during and after the 650-600 Ma collision event as proposed by [2]. The timing of metamorphism of the lower unit coincides with that of retrograde metamorphism of the upper unit at Menipa [5, 7]. These different P-T changes recorded at ca. 570-550 Ma suggest that the upper unit thrusted onto the lower unit at this time. This tectonic model is consistent with that by [6]. Magmatic activity of fluorite-bearing granite at ca. 504 Ma is coincident with the youngest age obtained from apatite in the kelyphite texture developed around the green V-bearing grossular in calcareous metapelite from Menipa [7], suggesting the final stage of the tectonic event in the SRM.
References
[1] Shiraishi et al. (1997) Antarctic Geol. Map Ser. [2] Osanai et al. (2013) Precambrian Res. [3] Adachi et al (2021) JpGU meeting abstract. [4] Adachi et al. (2021) GSJ meeting abstract. [5] Kawakami et al. (2022) JpGU meeting Abstract. [6] Grantham et al. (2013) Precambrian Res. [7] Niki et al. (2022) JpGU meeting abstract.
At Menipa, pelitic gneisses and marbles are distributed at high elevation and quartzofeldspathic gneisses at low elevation. The dip of the foliation in the metamorphic rocks is commonly low angle in this area, indicating the rocks at higher elevation are located at structurally upper part. These metamorphic rocks are intruded by massive fluorite-bearing granite.
Sillimanite-garnet-biotite gneiss (sample No. TA19121302B), located at structurally upper part, preserves prograde textures indicating pressure increase before the peak condition [4]. Zircon grains in this sample show oscillatory-zoned cores truncated by vaguely-zoned rims. The dates from the core range ca. 1000-800 Ma, and those from the rim cluster at 618 ± 7 Ma (Th/U: 0.05-0.00; Ybn/Gdn: 66-1.3). The several rim domains show Ybn/Gdn ~ 1 indicating that the zircon grew in equilibrium with garnet. It means that garnet probably started to grow at ca. 618 Ma at latest. This age is interpreted as the timing of prograde metamorphism. This result is consistent with the timing of prograde metamorphism under kyanite stability field reported by [5] from the same area.
In contrast, the quartzofeldspathic gneisses, located at structurally lower part, yield younger metamorphic age of ca. 560-550 Ma. Garnet-cummingtonite gneiss (TA19121902A) contains orthopyroxene partially replaced by cummingtonite suggesting retrograde hydration. The U-Pb zircon dates from the oscillatory-zoned core range ca. 950-800 Ma and those from the vaguely-zoned rim cluster at 566 ± 5 Ma (Th/U: 0.10-0.00; Ybn/Gdn: 320-4.4). Garnet-biotite gneiss (TA19121903A) contains garnet partially replaced by biotite + plagioclase suggesting retrograde hydration. The U-Pb zircon dates from the oscillatory-zoned core range ca. 1000-800 Ma and those from the vaguely-zoned rim cluster at 547 ± 6 Ma (Th/U: 1.16-0.02; Ybn/Gdn: 80-1.0). These ages of ca. 570-550 Ma are interpreted as the timing of metamorphism of the quartzofeldspathic gneisses.
Fluorite-bearing granite (TA19121901H) contains euhedral and oscillatory-zoned/unzoned zircons yielding 504 ± 7 Ma (Th/U: 0.59-0.02), interpreted as the timing of intrusion of this granite.
As above, the sillimanite-garnet-biotite gneiss at structurally upper part records ca. 618 Ma metamorphism whereas the quartzofeldspathic gneisses at lower part record ca. 570-550 Ma a metamorphism at Menipa. This suggests that the two geological units with different timing of metamorphism are probably in contact with each other at Menipa, which is the same geological relationship as found in Brattnipene [3]. This clearly means the whole SW terrane did not behave as a single geological body during and after the 650-600 Ma collision event as proposed by [2]. The timing of metamorphism of the lower unit coincides with that of retrograde metamorphism of the upper unit at Menipa [5, 7]. These different P-T changes recorded at ca. 570-550 Ma suggest that the upper unit thrusted onto the lower unit at this time. This tectonic model is consistent with that by [6]. Magmatic activity of fluorite-bearing granite at ca. 504 Ma is coincident with the youngest age obtained from apatite in the kelyphite texture developed around the green V-bearing grossular in calcareous metapelite from Menipa [7], suggesting the final stage of the tectonic event in the SRM.
References
[1] Shiraishi et al. (1997) Antarctic Geol. Map Ser. [2] Osanai et al. (2013) Precambrian Res. [3] Adachi et al (2021) JpGU meeting abstract. [4] Adachi et al. (2021) GSJ meeting abstract. [5] Kawakami et al. (2022) JpGU meeting Abstract. [6] Grantham et al. (2013) Precambrian Res. [7] Niki et al. (2022) JpGU meeting abstract.