11:30 〜 11:45
[SMP22-10] Metamorphic P-T conditions of sillimanite-biotite-garnet gneiss from Berckmanskampen, Sør Rondane Mountains, East Antarctica
キーワード：高温変成作用、岩石学、十字石、大陸衝突帯
Gondwana supercontinent is widely believed to have formed through amalgamation of East and West Gondwana continents [e.g., 1]. In contrast, the collision between Northern and Southern Gondwana is proposed along another orogenic belt between Southern Africa and Antarctica [e.g., 2]. Since the Sør Rondane Mountains (SRM), East Antarctica is located around the crossing point of these orogenic belts, it is a key area to reveal whether the latter collision occurred or not. Berckmanskampen is a mountain located between Menipa and Mefjell in the central SRM. All of them belong to the Southwestern terrane which is composed of granulites and lower-grade metamorphic rocks with counter-clockwise P-T-t path [3]. However, clockwise and hairpin-type P-T paths are recently reported from Menipa and Mefjell, respectively [4-5]. This means that reconstruction of the tectonic model of the SRM is necessary based on petrochronological studies. Therefore, this study focuses on the Berckmanskampen where the metamorphic history has not been revealed.
The studied samples are Sil-Bt-Grt gneisses, collected from northwestern part of Berckmanskampen. Main matrix mineral assemblage is Grt + Sil + Bt + Pl + Qz. Garnet porphyroblast is up to 20 mm in diameter, wrapped by the foliation defined by the arrangement of biotite and sillimanite. The garnet porphyroblast has P-poor inner core, P-rich outer core, P-poor mantle and P-rich rim defined by discontinuous zoning in P concentration. The inner core of garnet is high in Ca and Mn, and low in Mg, whereas the gradual rimward decreases of Ca and Mn and increase of Mg are observed. Iron content is homogeneous from the inner core to the rim. Increase of Fe and Mn and decrease of Mg are observed at the margin of garnet rim in contact with matrix biotite.
Garnet inner core includes staurolite, sillimanite and quartz, whereas garnet outer core includes staurolite, kyanite, sillimanite, plagioclase (An37-40), biotite and quartz. Staurolite in both cores has a round shape, including kyanite, K-feldspar, apatite, quartz and plagioclase with various composition (An35-84). All sillimanite inclusions in both cores have a serrated boundary with garnet, accompanied by spinel and secondary muscovite. Tiny staurolite inclusion next to the sillimanite and each mineral composition satisfy following reaction
St --> Alm + Sil + Hc + H2O.
A reaction line with large dP/dT is shown by Spear [6] at ~600-700ºC for the following reaction
St --> Alm + Sil + H2O
in KFASH system. This suggests that sillimanite with serrated outline can be formed through the breakdown of staurolite by temperature increase, and the P-T conditions are inferred to be ~600-700ºC, ~0.3-0.8 GPa [6]. This probably follows staurolite and garnet outer core formation under kyanite stability field.
Garnet mantle and rim include sillimanite, biotite, plagioclase (An30-34) and quartz. Sillimanite in garnet mantle and rim is included as a single phase and has a prismatic and euhedral shape, suggesting that the garnet mantle and rim were formed in the sillimanite stability field. P-T conditions for retrograde reequilibrium between garnet and matrix biotite were estimated using mineral assemblage of garnet rim, plagioclase and biotite in contact with the garnet rim. The Grt-Bt geothermometer [7] and GASP geobarometer [8] gave the P-T conditions of ~600 ºC and ~0.4 GPa. In the matrix, in addition to sillimanite porphyroblast, fibrous kyanite-quartz aggregate is locally present at grain boundaries between plagioclase (~An30) in the matrix. This suggests that final retrograde metamorphism occurred in the kyanite stability field.
In summary, this study indicates clockwise or hairpin-type P-T path from Berckmanskampen. Berckmanskampen, Menipa and Mefjell in the central SRM are the exceptional areas in the Southwestern terrane from the perspective of P-T paths, being a key area for reconstructing the collision model in the SRM.
References
[1] Jacobs et al. 2003 PR [2] Meert 2003 Tectonophysics [3] Osanai et al. 2013 PR [4] Tsubokawa et al. 2017 JMPS [5] Kawakami et al. 2022 IAGR Abst. [6] Spear 1995 MSA [7] Holdaway 2000 Am. Min. [8] Holdaway 2001 Am. Min.
The studied samples are Sil-Bt-Grt gneisses, collected from northwestern part of Berckmanskampen. Main matrix mineral assemblage is Grt + Sil + Bt + Pl + Qz. Garnet porphyroblast is up to 20 mm in diameter, wrapped by the foliation defined by the arrangement of biotite and sillimanite. The garnet porphyroblast has P-poor inner core, P-rich outer core, P-poor mantle and P-rich rim defined by discontinuous zoning in P concentration. The inner core of garnet is high in Ca and Mn, and low in Mg, whereas the gradual rimward decreases of Ca and Mn and increase of Mg are observed. Iron content is homogeneous from the inner core to the rim. Increase of Fe and Mn and decrease of Mg are observed at the margin of garnet rim in contact with matrix biotite.
Garnet inner core includes staurolite, sillimanite and quartz, whereas garnet outer core includes staurolite, kyanite, sillimanite, plagioclase (An37-40), biotite and quartz. Staurolite in both cores has a round shape, including kyanite, K-feldspar, apatite, quartz and plagioclase with various composition (An35-84). All sillimanite inclusions in both cores have a serrated boundary with garnet, accompanied by spinel and secondary muscovite. Tiny staurolite inclusion next to the sillimanite and each mineral composition satisfy following reaction
St --> Alm + Sil + Hc + H2O.
A reaction line with large dP/dT is shown by Spear [6] at ~600-700ºC for the following reaction
St --> Alm + Sil + H2O
in KFASH system. This suggests that sillimanite with serrated outline can be formed through the breakdown of staurolite by temperature increase, and the P-T conditions are inferred to be ~600-700ºC, ~0.3-0.8 GPa [6]. This probably follows staurolite and garnet outer core formation under kyanite stability field.
Garnet mantle and rim include sillimanite, biotite, plagioclase (An30-34) and quartz. Sillimanite in garnet mantle and rim is included as a single phase and has a prismatic and euhedral shape, suggesting that the garnet mantle and rim were formed in the sillimanite stability field. P-T conditions for retrograde reequilibrium between garnet and matrix biotite were estimated using mineral assemblage of garnet rim, plagioclase and biotite in contact with the garnet rim. The Grt-Bt geothermometer [7] and GASP geobarometer [8] gave the P-T conditions of ~600 ºC and ~0.4 GPa. In the matrix, in addition to sillimanite porphyroblast, fibrous kyanite-quartz aggregate is locally present at grain boundaries between plagioclase (~An30) in the matrix. This suggests that final retrograde metamorphism occurred in the kyanite stability field.
In summary, this study indicates clockwise or hairpin-type P-T path from Berckmanskampen. Berckmanskampen, Menipa and Mefjell in the central SRM are the exceptional areas in the Southwestern terrane from the perspective of P-T paths, being a key area for reconstructing the collision model in the SRM.
References
[1] Jacobs et al. 2003 PR [2] Meert 2003 Tectonophysics [3] Osanai et al. 2013 PR [4] Tsubokawa et al. 2017 JMPS [5] Kawakami et al. 2022 IAGR Abst. [6] Spear 1995 MSA [7] Holdaway 2000 Am. Min. [8] Holdaway 2001 Am. Min.