[S-MP46_28PM2] Deformed rocks, Metamorphic rocks and Tectonics

[SMP46-12] Significance of multi-stage chloride brine activity- An example from Sor Rondane Mountains, East Antarctica

*Fumiko HIGASHINO¹, Tetsuo KAWAKAMI¹, Noriyoshi TSUCHIYA², Satish-kumar MADHUSOODHAN³, Masahiro ISHIKAWA⁴, Geoffrey H. GRANTHAM⁵ (1.Kyoto University, 2.Tohoku University, 3.Niigata University, 4.Yokohama National University, 5.Council for Groscience, South Africa)

Keywords:NaCl-KCl brine, fluid infiltration, continental collision zone, Sor Rondane Mountains

It has been gradually recognized chloride brine potentially plays an important role in large-scale mass transfer during high-grade metamorphism without partial melting. This is because brine is a powerful solvent, can coexist with CO₂-rich fluid under the granulite facies conditions and has low-H₂O activity (Newton & Manning, 2010; Heinrich et al., 2004). In natural observation, evidence for the presence of brine is often found as fluid inclusions. In metamorphic rocks under granulite facies conditions, however, brine inclusions are only rarely found (Markl & Bucher, 1998). This is partly because brines have a high mobility due to their low viscosity and low wetting angle (Watson & Brenan, 1987; Holness, 1997). On the other hand, hydrous minerals such as biotite (Bt), hornblende (Hb) and apatite can record the f_H2O/f_HCl of the last equilibrated Cl-rich fluid as their mineral compositions. In order to understand the metamorphic fluid activity using these minerals, P-T condition under which these minerals equilibrated with a fluid as well as the crystallographic requirements for these minerals to record the fluid composition should be known (e.g., Makino, 2000). However, there still are a lot of unsolved issues about brines, for example, cations transported in the fluid, P-T condition and areal scale of brine activity, and its origin (Newton et al., 1998). In order to understand the multiple brine activities and the cation composition in brines, two meta-mafic gneisses are studied in detail in Brattnipene, Sør Rondane Mountains (SRM), East Antarctica. In a Grt-Bt-Hb gneiss, Cl-rich Bt is exclusively included in garnet (Grt). Bt, Hb, and cummingtonite (Cum) in the matrix are Cl-poor. These compositional differences imply that Bt included in the Grt formed under the presence of chloride brine and Cl-poor fluid infiltrated after Grt formation. Grt is enveloped by the gneissosity defined by the arrangement of Cum overgrown by Hb, and Bt. Therefore, chloride brine activity predated or was simultaneous with the penetrative gneissosity formation in this area. After considering the effect of Mg-Cl avoidance rule and compositional change during retrogression, the geothermobarometry (Holdaway, 2000; Wu et al., 2004) gave 650 < T < 800 ℃ and 0.96 GPa for the peak P-T condition of this sample. The Cl-rich Bt entrapment was probably predated or simultaneous with the attainment of this P-T condition.In a Grt-Opx-Hb gneiss, ca. 1cm-thick Grt-Hb vein cut the penetrative gneissosity in this area. Cl-content of Hb and Bt, and K-content of Hb decrease with the distance from the vein center and become constant at ca. 1.6 cm from the vein center. Plagioclase present next to the vein has a core (An₅₅) and mantle (An₆₈) which is sharply overgrown by Na-richer rim (An₅₁). Plagioclase in the vein is An₅₁ without zoning, and development of Na-richer rim gets thinner with a distance from the vein. Therefore, the Grt-Hb vein was probably formed by NaCl-KCl brine infiltration. This is also supported by the isocon analysis of whole-rock chemistry determined for wall rock of the vein. P-T condition of the vein formation is estimated as 720 ℃ and 0.70 GPa (Holdaway, 2000; Wu et al., 2004).P-T conditions of multi-stage chloride brine activity, cation composition of the brine, and outcrop-scale pathways of the brine are constrained in SRM from these two gneisses. From the pelitic gneisses in SRM, Cl-rich fluid or melt activity with a linear distribution over 200 km has been reported (Higashino et al., 2013). Additionally, this study revealed that the brine activity in SRM is not controlled by the lithology or specific deformational stages. It is clear that chloride brine in SRM was not a result of in situ fractionation through the selective consumption of H₂O in the fluid (Kullerud, 1995), but substantial amount of brine was actually mo