14:45 〜 15:00
[SMP26-05] Hairpin-shaped P-T path of the Higher Himalayan Crystalline nappe in Dhankuta, eastern Nepal
キーワード：大陸衝突帯、ヒマラヤ、変成作用、高ヒマラヤ変成岩、温度圧力履歴
In the Himalaya, middle- to lower-crustal high-grade metamorphic rocks, so-called Higher Himalayan Crystallines (HHC), are exposed over an area of about 2000 km from east to west and 100 km from north to south. The mechanism of exhumation of these highly metamorphosed rocks to the surface is important for understanding the tectonics of the active middle to lower crust of the collision zones. One of the most effective methods for constraining the exhumation process is to construct the temperature-pressure-time path (P-T-t path) recorded in rocks. Several P-T-t paths of the metamorphic rocks have been reported so far from eastern Nepal [e.g. 1, 2, 3], but prograde P-T paths are poorly constrained to validate forward models such as the channel flow model [4] and the critical taper model [5]. The former model is characterized by clockwise P-T path with isobaric heating followed by isothermal decompression, whereas the latter model is marked by hairpin-shaped P-T path. In this study, we used microstructural observations and state-of-the-art geothermobarometer to construct a more detailed P-T path including prograde metamorphism and compared them with P-T paths from previous studies to investigate the exhumation mechanism.
For P-T path estimation, we selected two garnet-kyanite-biotite gneisses (18111304 and 19111202) collected from the root part and middle part of the HHC nappe in eastern Nepal, respectively [6]. The main matrix mineral assemblage of the garnet-kyanite-biotite gneiss (19111202) is garnet, biotite, kyanite, sillimanite, plagioclase, K-feldspar, and quartz. The garnet can be divided into two stages: the core which is rich in inclusions, and the rim which characteristically encloses nanogranitoids. The garnet is partly replaced by coarse-grained Bt+Ky+Qtz±Pl and fine-grained Bt+Sil+Qtz±Pl, which are considered to be retrograde microstructures [7]. The matrix kyanite can be distinguished by blue-colored core and uncolored rim, which is partly replaced by sillimanite. The Zr-in-Rt geothermometer [8] and GASP geobarometer [9] applied to rutile, kyanite and plagioclase included in the garnet core gave the P-T conditions of 700-720 °C, 7.8-8.2 kbar. The Grt-Bt geothermometer [10] and GASP geobarometer [9] applied to the garnet replacement microstructures of Bt+Ky+Qtz±Pl and Bt+Sil+Qtz±Pl gave P-T conditions of 630-670 °C, 6.8-7.2 kbar and 600-620 °C, 4.2-5.8 kbar, respectively. The Zr-in-Rt geothermometer [8] applied to the rutile included in the kyanite rim gave P-T conditions of 710-730°C, >8 kbar. Since this P-T conditions are consistent with those for muscovite dehydration melting reaction (Ms + Ab + Qtz = Ky + Kfs + melt) [7], the kyanite rim is considered to have been formed by this reaction. Nanogranitoids included in the garnet rim and the increase in grossular content from garnet core to rim indicates that the garnet rim grew while incorporating the melt during temperature and pressure increase. From these pieces of observation, it is inferred that this sample experienced a hairpin-like P-T path starting from the garnet core formation to the formation of kyanite rim and garnet rim, followed by the formation of replacement microstructures during regrade metamorphism. Similar hairpin-shaped P-T path was also obtained from sample 18111304 by applying Zr-in-Rt geothermometer [8], Grt-Bt geothermometer [10], GASP geobarometer [9], and Qtz-in-Grt geobarometer [11, 12, 13] to several metamorphic stages recognized from microstructural observation.
As a result, the estimates of P-T conditions based on observation of garnet inclusions allowed this study to extract metamorphic processes during temperature increase that were not estimated by [2]. Furthermore, the P-T paths obtained in this study can explain the mineral descriptions and mineral reaction processes of the published HHC sample from eastern Nepal for which clockwise P-T paths were proposed [2]. Therefore, we believe that hairpin-shaped P-T paths are the common characteristics of HHC rocks in eastern Nepal. The constructed P-T paths in this study agrees with that inferred from the critical taper model [5] in that it is hairpin-shaped, suggesting that the HHC nappe was exposed at the surface by an exhumation mechanism similar to the critical taper model.
References
[1] Imayama et al., 2010. [2] Imayama et al., 2012. [3] Imayama et al., 2019. [4] Jamieson et al., 2004. [5] Kohn, 2008. [6] Nakajima et al., 2022 [7] Spear et al., 1999. [8] Tomkins et al., 2007. [9] Holdaway, 2001. [10] Holdaway, 2000. [11] Angel et al., 2017a. [12] Angel et al., 2017b. [13] Schmidt & Ziemann, 2000.
For P-T path estimation, we selected two garnet-kyanite-biotite gneisses (18111304 and 19111202) collected from the root part and middle part of the HHC nappe in eastern Nepal, respectively [6]. The main matrix mineral assemblage of the garnet-kyanite-biotite gneiss (19111202) is garnet, biotite, kyanite, sillimanite, plagioclase, K-feldspar, and quartz. The garnet can be divided into two stages: the core which is rich in inclusions, and the rim which characteristically encloses nanogranitoids. The garnet is partly replaced by coarse-grained Bt+Ky+Qtz±Pl and fine-grained Bt+Sil+Qtz±Pl, which are considered to be retrograde microstructures [7]. The matrix kyanite can be distinguished by blue-colored core and uncolored rim, which is partly replaced by sillimanite. The Zr-in-Rt geothermometer [8] and GASP geobarometer [9] applied to rutile, kyanite and plagioclase included in the garnet core gave the P-T conditions of 700-720 °C, 7.8-8.2 kbar. The Grt-Bt geothermometer [10] and GASP geobarometer [9] applied to the garnet replacement microstructures of Bt+Ky+Qtz±Pl and Bt+Sil+Qtz±Pl gave P-T conditions of 630-670 °C, 6.8-7.2 kbar and 600-620 °C, 4.2-5.8 kbar, respectively. The Zr-in-Rt geothermometer [8] applied to the rutile included in the kyanite rim gave P-T conditions of 710-730°C, >8 kbar. Since this P-T conditions are consistent with those for muscovite dehydration melting reaction (Ms + Ab + Qtz = Ky + Kfs + melt) [7], the kyanite rim is considered to have been formed by this reaction. Nanogranitoids included in the garnet rim and the increase in grossular content from garnet core to rim indicates that the garnet rim grew while incorporating the melt during temperature and pressure increase. From these pieces of observation, it is inferred that this sample experienced a hairpin-like P-T path starting from the garnet core formation to the formation of kyanite rim and garnet rim, followed by the formation of replacement microstructures during regrade metamorphism. Similar hairpin-shaped P-T path was also obtained from sample 18111304 by applying Zr-in-Rt geothermometer [8], Grt-Bt geothermometer [10], GASP geobarometer [9], and Qtz-in-Grt geobarometer [11, 12, 13] to several metamorphic stages recognized from microstructural observation.
As a result, the estimates of P-T conditions based on observation of garnet inclusions allowed this study to extract metamorphic processes during temperature increase that were not estimated by [2]. Furthermore, the P-T paths obtained in this study can explain the mineral descriptions and mineral reaction processes of the published HHC sample from eastern Nepal for which clockwise P-T paths were proposed [2]. Therefore, we believe that hairpin-shaped P-T paths are the common characteristics of HHC rocks in eastern Nepal. The constructed P-T paths in this study agrees with that inferred from the critical taper model [5] in that it is hairpin-shaped, suggesting that the HHC nappe was exposed at the surface by an exhumation mechanism similar to the critical taper model.
References
[1] Imayama et al., 2010. [2] Imayama et al., 2012. [3] Imayama et al., 2019. [4] Jamieson et al., 2004. [5] Kohn, 2008. [6] Nakajima et al., 2022 [7] Spear et al., 1999. [8] Tomkins et al., 2007. [9] Holdaway, 2001. [10] Holdaway, 2000. [11] Angel et al., 2017a. [12] Angel et al., 2017b. [13] Schmidt & Ziemann, 2000.