12:00 〜 12:15
[SMP27-06] Metamorphic zone mapping and P-T path of the Higher Himalayan Crystalline nappe in Dhankuta, Eastern Nepal
キーワード：ヒマラヤ、変成岩、高ヒマラヤ変成岩ナップ
In the Himalaya, how high-grade metamorphic rocks, so-called Higher Himalayan Crystallines (HHC), exhumed up to the surface has long been studied and various models for the exhumation mechanism have been proposed [e.g., 1]. Estimating pressure-temperature-time (P-T-t) paths of the metamorphic rocks is indispensable to discuss the exhumation mechanism. In the Tamor-Ghuna section, far eastern Nepal Himalaya, [2,3,4] reported several number of P-T-t paths from the HHC and revealed that a discontinuity within the HHC [cf. 5], the High Himal Thrust (HHT), also played an important role in the exhumation in addition to the activity of the Main Central Thrust (MCT) as a southern boundary and the South Tibetan Detachment System as a northern boundary of the HHC. [3,4] pointed out that the HHC in eastern Nepal is divided into the upper and the lower HHC and the two parts of the HHC experienced different exhumation processes. However, P-T-t paths have not been reported from the HHC nappe in eastern Nepal, which refers to the part of the HHC overlying extensive area of the Lesser Himalaya [cf. 6]. Therefore, revealing the P-T-t path of the HHC nappe is required in order to understand the exhumation process of the entire HHC sequences that represent the active middle to lower crust of the collision zone.
In this study, we performed metamorphic zone mapping using metapelites collected from the north of Dhankuta, eastern Nepal and estimated a P-T path of a garnet-biotite-kyanite gneiss. Based on the location of the HHT described in [4] and the MCT defined in [7] and [8], our study area belongs to the lower HHC in the HHC nappe. Based on the field survey and microscopic observation, we newly defined the “kyanite-in” isograd in the study area. We also confirmed “sillimanite-in” and “muscovite-out” isograds consistent with those reported in [9]. The studied sample was collected from the north of the muscovite-out isograd where Ms+Qtz is unstable. The main matrix mineral assemblage is Grt+Bt+Ky+Sil+Kfs+Pl+Qtz. The garnet is about 3 mm in diameter and consists of the core with abundant inclusions and the rim with a few inclusions. The core encloses kyanite, plagioclase, quartz, rutile, ilmenite and zircon, whereas the rim encloses plagioclase, rutile, ilmenite, zircon and nanogranitoid inclusions. Zr-in-rutile geothermometer [10] applied to the rutile grains in the kyanite-bearing garnet core gave ca. 700 ℃ at ca. >8 kbar. The garnet rim is partly replaced by the mineral assemblage of Bt+Ky+Qtz±Pl and Bt+Sil+Qtz±Pl. We consider these retrograde microstructures are produced by the back reaction with melt [e.g., 3]. Garnet-biotite geothermometer [11] applied to the garnet replacement structures of Bt+Ky+Qtz±Pl and Bt+Sil+Qtz±Pl gave ca. 600 ℃ at ca. >6 kbar and ca. 600 ℃ at ca. 4-6 kbar, respectively. From these pieces of observation, this sample is supposed to have experienced the P-T path starting from the kyanite stability field (ca. >8 kbar, ca. 700℃: garnet core stage) to the sillimanite stability field (ca. 4-6 kbar, ca. 600℃: garnet rim stage), and the replacement structure of Bt+Ky+Qtz±Pl was formed in between these P-T conditions. The studied path is similar to that of lower HHC (sample H2101) reported in [3] in that H2101 have experienced the P-T evolution starting from the kyanite stability field (ca. 8-14 kbar, ca. 750 ℃) to the sillimanite stability field (ca. 4-6 kbar, ca. 650-700 ℃). Therefore, we confirmed that lower HHC rocks in the HHC nappe have experienced similar P-T path with those from the root zone.
References
[1] Jamieson et al., 2004. Journal of Geophysical Research, 109, B06407. [2] Imayama, et al., 2010. Journal of Metamorphic Geology, 28, 527-549. [3] Imayama et al., 2012. Lithos, 134-135, 1-22. [4] Imayama et al., 2018. The Geological Society of London, Special Publications, 481, 147-173. [5] Goscombe et al., 2006. Gondwana Research, 10, 232-255. [6] Sakai et al., 2013. Island Arc, 22, 338-360. [7] Kawakami et al., 2019. Lithos, 348–349, 105175. [8] Sato et al., 2020. Island Arc, 29(1), 1–9. [9] Groppo et al., 2009. Journal of Petrology, 50(6), 1149–1180. [10] Tomkins et al., 2007. Journal of metamorphic Geology, 25(6), 703-713. [11] Holdaway, 2001. American Mineralogist, 86(10), 1117-1129.
In this study, we performed metamorphic zone mapping using metapelites collected from the north of Dhankuta, eastern Nepal and estimated a P-T path of a garnet-biotite-kyanite gneiss. Based on the location of the HHT described in [4] and the MCT defined in [7] and [8], our study area belongs to the lower HHC in the HHC nappe. Based on the field survey and microscopic observation, we newly defined the “kyanite-in” isograd in the study area. We also confirmed “sillimanite-in” and “muscovite-out” isograds consistent with those reported in [9]. The studied sample was collected from the north of the muscovite-out isograd where Ms+Qtz is unstable. The main matrix mineral assemblage is Grt+Bt+Ky+Sil+Kfs+Pl+Qtz. The garnet is about 3 mm in diameter and consists of the core with abundant inclusions and the rim with a few inclusions. The core encloses kyanite, plagioclase, quartz, rutile, ilmenite and zircon, whereas the rim encloses plagioclase, rutile, ilmenite, zircon and nanogranitoid inclusions. Zr-in-rutile geothermometer [10] applied to the rutile grains in the kyanite-bearing garnet core gave ca. 700 ℃ at ca. >8 kbar. The garnet rim is partly replaced by the mineral assemblage of Bt+Ky+Qtz±Pl and Bt+Sil+Qtz±Pl. We consider these retrograde microstructures are produced by the back reaction with melt [e.g., 3]. Garnet-biotite geothermometer [11] applied to the garnet replacement structures of Bt+Ky+Qtz±Pl and Bt+Sil+Qtz±Pl gave ca. 600 ℃ at ca. >6 kbar and ca. 600 ℃ at ca. 4-6 kbar, respectively. From these pieces of observation, this sample is supposed to have experienced the P-T path starting from the kyanite stability field (ca. >8 kbar, ca. 700℃: garnet core stage) to the sillimanite stability field (ca. 4-6 kbar, ca. 600℃: garnet rim stage), and the replacement structure of Bt+Ky+Qtz±Pl was formed in between these P-T conditions. The studied path is similar to that of lower HHC (sample H2101) reported in [3] in that H2101 have experienced the P-T evolution starting from the kyanite stability field (ca. 8-14 kbar, ca. 750 ℃) to the sillimanite stability field (ca. 4-6 kbar, ca. 650-700 ℃). Therefore, we confirmed that lower HHC rocks in the HHC nappe have experienced similar P-T path with those from the root zone.
References
[1] Jamieson et al., 2004. Journal of Geophysical Research, 109, B06407. [2] Imayama, et al., 2010. Journal of Metamorphic Geology, 28, 527-549. [3] Imayama et al., 2012. Lithos, 134-135, 1-22. [4] Imayama et al., 2018. The Geological Society of London, Special Publications, 481, 147-173. [5] Goscombe et al., 2006. Gondwana Research, 10, 232-255. [6] Sakai et al., 2013. Island Arc, 22, 338-360. [7] Kawakami et al., 2019. Lithos, 348–349, 105175. [8] Sato et al., 2020. Island Arc, 29(1), 1–9. [9] Groppo et al., 2009. Journal of Petrology, 50(6), 1149–1180. [10] Tomkins et al., 2007. Journal of metamorphic Geology, 25(6), 703-713. [11] Holdaway, 2001. American Mineralogist, 86(10), 1117-1129.