11:00 〜 13:00
[SMP27-P06] 東南極セール・ロンダーネ山地メーニパに産する泥質変成岩の柘榴石，チタン石および燐灰石ウラン–鉛年代系に基づく三種年代分析
キーワード：柘榴石、U-Pb年代、レーザーアブレーションICP質量分析法、チタン石、燐灰石
Age analysis from multiple minerals reflecting each metamorphic stage is important to unveil complicated metamorphic histories (e.g. [1]). To do this, discussion based on the direct linkage between in situ age determinations and petrological observations is highly desired. Despite the importance, practical applications of in situ dating methods have been retarded mainly due to lack in the elemental sensitivity of analytical techniques, and thus, the U-Pb dating was mainly focused on specific minerals, especially U-bearing minerals such as zircon or monazite (e.g. [2]).
Among the metamorphic minerals, garnet is important for constraining both metamorphic environments and the timing of metamorphic events (e.g. [3]). Despite the significance of garnet, the limited number of in situ age determinations were reported on garnet, mainly due to the low U concentration. However, owing to the developments of a multiple-spot femtosecond laser ablation (msfsLA) system [4] and a multiple-collector ICP-MS (MC-ICP-MS) [5, 6] at our research group, in situ U-Pb dating of metamorphic grossular (Grs) garnet can be conducted [7].
In this study, for determining the timing of prograde and subsequent retrograde metamorphic events, garnet and minerals in its decomposition texture such as titanite and apatite were measured by the U-Pb dating method using msfsLA-MC-ICP-MS. The samples analyzed here were two thin sections (TK2019121301F03 and TK2019121301T01; renamed MNP1 and MNP2 hereafter for simplicity) of metapelites containing vanadium-bearing green Grs collected from Menipa, Sør Rondane Mountains (SRM), East Antarctica, which is a high-temperature metamorphic terrain. The green Grs with the lower V2O3 content (~0.5-1.0 wt%; Type 1) is surrounded by a kelyphitic rim, which is the decomposition product of the garnet with quartz, plagioclase, clinopyroxene, and graphite [8]. The kelyphitic rim contains V2O3-enriched garnet (up to 21.2 wt%; Type 2 and Type 3) [8].
The Type 1 Grs is a coarse-grained porphyroblast with the U concentration of 1-3 µg g-1. The resulting U-Pb ages were 593 ± 8 Ma and 586 ± 9 Ma for MNP1 and MNP2, respectively. In the kelyphitic rim of the Type 1 Grs, the Type 2 Grs occurred as fine-grained crystals constituting the kelyphytic rim. The Type 2 Grs hardly contained U while U was concentrated in coexisting titanite (U: 100-1000 µg g-1) in the kelyphite. The U-Pb age of the titanite ranged from 550 to 500 Ma for the MNP1 and was 548 ± 7 Ma for the MNP2. The Type 3 garnet (goldmanite) is found in the outermost kelyphitic rim of the MNP1. The Type 3 garnet also barely contained U and a coexisting apatite was a U-bearing mineral (U: 50-90 µg g-1). The U-Pb age of the apatite was 496 ± 9 Ma.
The obtained U-Pb ages of the Type 1 Grs (ca. 590 Ma) correspond to the reported timing of the prograde metamorphism in the SRM [9]. The titanite U-Pb age in the MNP2 (ca. 550 Ma) records the timing of the subsequent retrograde metamorphism. For the MNP1 with the outermost rim, the dispersion of the titanite U-Pb ages (550-500 Ma) and the coincidence of U-Pb ages between apatite and the youngest titanite (ca. 500 Ma) can be attributed to the resetting of the U-Pb system by a possible fluid activity driven by a magmatic activity [10]. Based on the data presented here, the important conclusion is that the timings of the prograde metamorphism and two retrograde metamorphic events in the SRM were constrained, and thus, in situ dating of garnet and its decomposition products can be a powerful tool for elucidating multiple metamorphic events.
References
[1] Kirkland, C. L. et al., 2017, Precambrian Res. [2] Kohn, M. J. et al., 2015, Am. Min. [3] Wallis, S. et al., 2009, J. Metamorph. Geol. [4] Makino, Y. et al., 2019, JAAS. [5] Hattori, K. et al., 2017, JAAS. [6] O-bayashi, H. et al., 2017, JAAS. [7] Niki, S. et al., 2022, under review. [8] Osanai, Y. et al., 1999, Nankyoku Siryo. [9] Kawakami, T. et al., 2022, JpGU. [10] Adachi, T. et al., 2022, JpGU.
Among the metamorphic minerals, garnet is important for constraining both metamorphic environments and the timing of metamorphic events (e.g. [3]). Despite the significance of garnet, the limited number of in situ age determinations were reported on garnet, mainly due to the low U concentration. However, owing to the developments of a multiple-spot femtosecond laser ablation (msfsLA) system [4] and a multiple-collector ICP-MS (MC-ICP-MS) [5, 6] at our research group, in situ U-Pb dating of metamorphic grossular (Grs) garnet can be conducted [7].
In this study, for determining the timing of prograde and subsequent retrograde metamorphic events, garnet and minerals in its decomposition texture such as titanite and apatite were measured by the U-Pb dating method using msfsLA-MC-ICP-MS. The samples analyzed here were two thin sections (TK2019121301F03 and TK2019121301T01; renamed MNP1 and MNP2 hereafter for simplicity) of metapelites containing vanadium-bearing green Grs collected from Menipa, Sør Rondane Mountains (SRM), East Antarctica, which is a high-temperature metamorphic terrain. The green Grs with the lower V2O3 content (~0.5-1.0 wt%; Type 1) is surrounded by a kelyphitic rim, which is the decomposition product of the garnet with quartz, plagioclase, clinopyroxene, and graphite [8]. The kelyphitic rim contains V2O3-enriched garnet (up to 21.2 wt%; Type 2 and Type 3) [8].
The Type 1 Grs is a coarse-grained porphyroblast with the U concentration of 1-3 µg g-1. The resulting U-Pb ages were 593 ± 8 Ma and 586 ± 9 Ma for MNP1 and MNP2, respectively. In the kelyphitic rim of the Type 1 Grs, the Type 2 Grs occurred as fine-grained crystals constituting the kelyphytic rim. The Type 2 Grs hardly contained U while U was concentrated in coexisting titanite (U: 100-1000 µg g-1) in the kelyphite. The U-Pb age of the titanite ranged from 550 to 500 Ma for the MNP1 and was 548 ± 7 Ma for the MNP2. The Type 3 garnet (goldmanite) is found in the outermost kelyphitic rim of the MNP1. The Type 3 garnet also barely contained U and a coexisting apatite was a U-bearing mineral (U: 50-90 µg g-1). The U-Pb age of the apatite was 496 ± 9 Ma.
The obtained U-Pb ages of the Type 1 Grs (ca. 590 Ma) correspond to the reported timing of the prograde metamorphism in the SRM [9]. The titanite U-Pb age in the MNP2 (ca. 550 Ma) records the timing of the subsequent retrograde metamorphism. For the MNP1 with the outermost rim, the dispersion of the titanite U-Pb ages (550-500 Ma) and the coincidence of U-Pb ages between apatite and the youngest titanite (ca. 500 Ma) can be attributed to the resetting of the U-Pb system by a possible fluid activity driven by a magmatic activity [10]. Based on the data presented here, the important conclusion is that the timings of the prograde metamorphism and two retrograde metamorphic events in the SRM were constrained, and thus, in situ dating of garnet and its decomposition products can be a powerful tool for elucidating multiple metamorphic events.
References
[1] Kirkland, C. L. et al., 2017, Precambrian Res. [2] Kohn, M. J. et al., 2015, Am. Min. [3] Wallis, S. et al., 2009, J. Metamorph. Geol. [4] Makino, Y. et al., 2019, JAAS. [5] Hattori, K. et al., 2017, JAAS. [6] O-bayashi, H. et al., 2017, JAAS. [7] Niki, S. et al., 2022, under review. [8] Osanai, Y. et al., 1999, Nankyoku Siryo. [9] Kawakami, T. et al., 2022, JpGU. [10] Adachi, T. et al., 2022, JpGU.