11:00 〜 11:15
[SMP25-02] Multiple post-peak fluid infiltrations in southern Perlebandet, Sør Rondane Mountains, East Antarctica
キーワード：変成流体、グラニュライト、閃緑岩、ラマン分光分析、Cl-CO2-H2O 流体
Low water activity fluids have been recognized as important fluid species present in the crust [e.g., 1]. The NaCl-CO2-H2O system is often assumed as fluid composition in the crust [2]. From the petrochronological point of view, mineral compositions have an advantage to understand P-T-t-fluid evolution comparing with fluid inclusion study. In the Sør Rondane Mountains (SRM), East Antarctica, where are located at the key area to understand the Gondwana amalgamation [3], saline fluid activities have been studied extensively [4-5]. However, CO2-bearing fluid is not reported in the SRM so far. Perlebandet is ~10 km long nunataks located at the western SRM. [4] reports counter-clockwise P-T-t path and Cl-rich fluid infiltration during prograde metamorphism based on petrochronological constraint from northern part of Perlebandet. This study reports multiple post-peak fluid infiltrations from southern Perlebandet, considering possibility of coexistence of saline fluid with CO2-bearing fluid.
The studied sample is Grt-Sil-Bt gneisses whose gneissose structure is cut by ~ 1 mm-thick Bt-bearing selvage. The selvage is mainly composed of Cl-rich Bt (~ 0.7 wt% Cl) and And is exclusively present within the selvage. The selvage-forming temperature is estimated to be ~ 490 °C under the pressure of And stability field, using two-feldspar thermometry [6-7]. This suggests that Cl-bearing aqueous fluid infiltrated through a thin crack at ~ 490 °C.
In the wall rock, Grt breakdown to Bt + Crd intergrowth is observed. The intergrowth is cut by Bt-bearing selvage. Within the intergrowth, Bt has ~ 0.2-0.3 wt% Cl. In addition, CO2 and H2O peaks were detected in Crd by Raman spectroscopy. This suggests that the Cl- and CO2-bearing aqueous fluid infiltration triggered the Grt breakdown reaction. The P-T conditions of Grt breakdown were estimated to be 730-770 °C, 0.2-0.4 GPa [cf. 8]. This is higher-T and lower-P conditions, comparing with Grt breakdown near the P-T conditions of the Al2SiO5 triple point reported in [4]. Using the equation of [9], CO2 concentration in Crd was estimated to be ~1.3-1.7 wt%. Chlorine concentration of fluid coexisting with Bt within the intergrowth was calculated to be ~ 30 wt% Cl and ~ 12 wt% Cl respectively in the case of melt-present and melt-absent conditions [10-11]. Assuming the cation coupled with Cl in the fluid is Na, these values are considered to be upper limits for NaCl concentration in the fluid. The NaCl-CO2-H2O diagram indicates that the NaCl- and CO2-bearing aqueous fluid is present as a single phase at ~ 750 ºC and ~ 0.3 GPa [12]. In addition, peak P-T conditions are estimated to be 0.9-1.1 GPa and ~ 800-900 °C, using the GASP geobarometer [13] and the ternary feldspar thermometry using the solvus of [6]. This is consistent with the peak P-T conditions reported in [4].
These observations suggest multiple post-peak fluid infiltrations and decompression-cooling path in southern Perlebandet; peak metamorphism followed by Grt breakdown reaction triggered by Cl-CO2-H2O fluid infiltration and Cl-bearing aqueous fluid infiltration under And stability field. The inconsistence with P-T path reported from northern Perlebandet is probably due to the granodiorite body beneath the studied metamorphic rocks. This is supported by the solidification P-T conditions of the granodiorite estimated to be ~ 670 °C and ~ 0.50 GPa using Al-in-Hbl geobarometer and Hbl-Pl geothermometer [14-15]. This implies that the fluid infiltration causing the Grt breakdown reaction is originated from the granodiorite.
[1] Touret & Huizenga 2011 GSA [2] Liebscher & Heinrich 2007 RMG [3] Satish-Kumar+ 2013 PR [4] Kawakami+ 2017 Lithos [5] Higashino+ 2019 JPet [6] Benisek+ 2004 AM [7] Benisek+ 2010 CMP [8] Spear+ 1999 CMP [9] Kaindl+ 2006 EJM [10] Aranovich 2017 Petrology [11] Chevychelov+ 2008 GeoInt [12] Shmulovich & Graham 2004 CMP [13] Holdaway 2001 AM [14] Anderson & Smith 1995 AM [15] Holland & Blundy 1994 CMP
The studied sample is Grt-Sil-Bt gneisses whose gneissose structure is cut by ~ 1 mm-thick Bt-bearing selvage. The selvage is mainly composed of Cl-rich Bt (~ 0.7 wt% Cl) and And is exclusively present within the selvage. The selvage-forming temperature is estimated to be ~ 490 °C under the pressure of And stability field, using two-feldspar thermometry [6-7]. This suggests that Cl-bearing aqueous fluid infiltrated through a thin crack at ~ 490 °C.
In the wall rock, Grt breakdown to Bt + Crd intergrowth is observed. The intergrowth is cut by Bt-bearing selvage. Within the intergrowth, Bt has ~ 0.2-0.3 wt% Cl. In addition, CO2 and H2O peaks were detected in Crd by Raman spectroscopy. This suggests that the Cl- and CO2-bearing aqueous fluid infiltration triggered the Grt breakdown reaction. The P-T conditions of Grt breakdown were estimated to be 730-770 °C, 0.2-0.4 GPa [cf. 8]. This is higher-T and lower-P conditions, comparing with Grt breakdown near the P-T conditions of the Al2SiO5 triple point reported in [4]. Using the equation of [9], CO2 concentration in Crd was estimated to be ~1.3-1.7 wt%. Chlorine concentration of fluid coexisting with Bt within the intergrowth was calculated to be ~ 30 wt% Cl and ~ 12 wt% Cl respectively in the case of melt-present and melt-absent conditions [10-11]. Assuming the cation coupled with Cl in the fluid is Na, these values are considered to be upper limits for NaCl concentration in the fluid. The NaCl-CO2-H2O diagram indicates that the NaCl- and CO2-bearing aqueous fluid is present as a single phase at ~ 750 ºC and ~ 0.3 GPa [12]. In addition, peak P-T conditions are estimated to be 0.9-1.1 GPa and ~ 800-900 °C, using the GASP geobarometer [13] and the ternary feldspar thermometry using the solvus of [6]. This is consistent with the peak P-T conditions reported in [4].
These observations suggest multiple post-peak fluid infiltrations and decompression-cooling path in southern Perlebandet; peak metamorphism followed by Grt breakdown reaction triggered by Cl-CO2-H2O fluid infiltration and Cl-bearing aqueous fluid infiltration under And stability field. The inconsistence with P-T path reported from northern Perlebandet is probably due to the granodiorite body beneath the studied metamorphic rocks. This is supported by the solidification P-T conditions of the granodiorite estimated to be ~ 670 °C and ~ 0.50 GPa using Al-in-Hbl geobarometer and Hbl-Pl geothermometer [14-15]. This implies that the fluid infiltration causing the Grt breakdown reaction is originated from the granodiorite.
[1] Touret & Huizenga 2011 GSA [2] Liebscher & Heinrich 2007 RMG [3] Satish-Kumar+ 2013 PR [4] Kawakami+ 2017 Lithos [5] Higashino+ 2019 JPet [6] Benisek+ 2004 AM [7] Benisek+ 2010 CMP [8] Spear+ 1999 CMP [9] Kaindl+ 2006 EJM [10] Aranovich 2017 Petrology [11] Chevychelov+ 2008 GeoInt [12] Shmulovich & Graham 2004 CMP [13] Holdaway 2001 AM [14] Anderson & Smith 1995 AM [15] Holland & Blundy 1994 CMP