1:45 PM - 3:15 PM
[O11-P87] Magma Differentiation of Bingi Bingi Complex, Southeast NSW, Australia (Part 2)
Keywords:Hydrothermal Residual Solution, Oscillatory Zoned Structure, Subsolidus, Progressive Oxidation
Magma does not differentiate within a closed system, and elucidating the magma differentiation process is not straightforward. Kawakatsu and Yamaguchi discovered the oscillatory zoned structures in the hornblende of the Daito-Yokota granodiorite, suggesting that already crystallized minerals are secondarily replaced by hydrothermal residual solution, which have much lower energy than metamorphic processes1). Inspired by this, we discovered the oscillatory zoned structures in the hornblende of the Ibo-River granodiorite and clarified its formed mechanism2). While it is difficult to trace the process of replacement if there is a drastic change that replaces the entire mineral, the oscillatory zoned structures leave evidence of the secondary influences that once crystallized minerals experienced, allowing us to estimate the environment at the late magma differentiation process. The oscillatory zoned structures seen in the hornblende of plutonic rocks are universally observed in commercially available plutonic hornblende as well.
We discovered the oscillatory zoned structures of the hornblende in the diorite and tonalite collected from the Bingi Bingi Point composite granitoid in southeastern New South Wales, Australia3). With the cooperation of the Faculty of Science at Kyoto University, we conducted EPMA analysis of the oscillatory zoned structures of hornblende. The results suggest that during the late magma differentiation, circulation of the hydrothemal residual solution occurred due to the foaming and dehydration of magma in an oxidizing environment, leading to the development of the oscillatory zoned structures in the rim of the already crystallized hornblende. In this study, I aimed to determine the specific temperature and pressure at the late magma differentiation by the EPMA analysis of feldspars and opaque minerals coexisting with hornblende, which has a developed the oscillatory zoned structures (Fig.1).
The rim of the plagioclase that coexists with hornblende exhibiting the oscillatory zoned structures shows a reaction boundary with microcline. Using a hornblend-plagioclase thermometer4), we calculated the reequilibration temperature of the oscillatory zoned structure region is calculated to be 545°C at less than 2 atmospheres. Additionally, the plagioclase-K-feldspar thermometer5) that coexists with the oscillatory zoned structures of hornblende indicates a temperature of 500°C under 1 atmosphere conditions. Furthermore, the pressure estimate calculated based on the total Al of amphiboles6) are around 2 atmospheres. These results indicate that the oscillatory zoned structures of the amphibole formed under an oxidizing environment of subsolidus conditions. The oscillatory zoned structures of hornblende in intrusive rocks may serve as an indicator of subsolidus conditions. Almost all opaque minerals are high-purity pyrite, indicating that progressive oxidation continued even after the magma solidified.
1) Kawakatsu,K. and Yamaguchi, Y. 1987. Successive zoning of amphiboles during progressive oxidation oin the Daito-Yokota granitic complex, San-in belt, southwest Japan. Geochim.Cosmocim.Acta,51, 535-540.
2) Himejihigashi Senior High School Earth Science Club. 2023. The circulation of hydrothermal residual solution at the late magma differentiation in the southwest Japan, Sanyo and San'in belts - based on the oscillatory zoned structures found in hornblende of plutonic rocks. Abstract of the 130th Annual Meeting of the Geological Society of Japan. (in Japanese)
3) Himejihigashi Senior High School Earth Science Club. 2024. The environment at the late magma differentiation of the composite plutonic body at Bingi Bingi Point on the southeastern coast of New South Wales, southeastern Australia - based on the oscillatory zoned structure of hornblende. Abstract of the 131st Annual Meeting of the Geological Society of Japan. (in Japanese)
4) Offten,M.T. 1984. The origin of brown hornblende in the Artfjallet gabbro and dolerites. Contrib.Mineral.Petrol,86, 189-199.
5) Whitney,J.A. and Stormer,J.C. 1977. The distribution of NaAlSi3O8 between coexisting microcline and plagioclase and its effect on geothermometric calculations. Amer.Mineral.62, 687-691.
6) Mutch,E.J.F.,Blundy,J.D.,Tattitch,B.C.,Cooper,F.J. and Brooker,R.A. 2016. An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer. Contrib.Mineral.Petrol, 171:85.
We discovered the oscillatory zoned structures of the hornblende in the diorite and tonalite collected from the Bingi Bingi Point composite granitoid in southeastern New South Wales, Australia3). With the cooperation of the Faculty of Science at Kyoto University, we conducted EPMA analysis of the oscillatory zoned structures of hornblende. The results suggest that during the late magma differentiation, circulation of the hydrothemal residual solution occurred due to the foaming and dehydration of magma in an oxidizing environment, leading to the development of the oscillatory zoned structures in the rim of the already crystallized hornblende. In this study, I aimed to determine the specific temperature and pressure at the late magma differentiation by the EPMA analysis of feldspars and opaque minerals coexisting with hornblende, which has a developed the oscillatory zoned structures (Fig.1).
The rim of the plagioclase that coexists with hornblende exhibiting the oscillatory zoned structures shows a reaction boundary with microcline. Using a hornblend-plagioclase thermometer4), we calculated the reequilibration temperature of the oscillatory zoned structure region is calculated to be 545°C at less than 2 atmospheres. Additionally, the plagioclase-K-feldspar thermometer5) that coexists with the oscillatory zoned structures of hornblende indicates a temperature of 500°C under 1 atmosphere conditions. Furthermore, the pressure estimate calculated based on the total Al of amphiboles6) are around 2 atmospheres. These results indicate that the oscillatory zoned structures of the amphibole formed under an oxidizing environment of subsolidus conditions. The oscillatory zoned structures of hornblende in intrusive rocks may serve as an indicator of subsolidus conditions. Almost all opaque minerals are high-purity pyrite, indicating that progressive oxidation continued even after the magma solidified.
1) Kawakatsu,K. and Yamaguchi, Y. 1987. Successive zoning of amphiboles during progressive oxidation oin the Daito-Yokota granitic complex, San-in belt, southwest Japan. Geochim.Cosmocim.Acta,51, 535-540.
2) Himejihigashi Senior High School Earth Science Club. 2023. The circulation of hydrothermal residual solution at the late magma differentiation in the southwest Japan, Sanyo and San'in belts - based on the oscillatory zoned structures found in hornblende of plutonic rocks. Abstract of the 130th Annual Meeting of the Geological Society of Japan. (in Japanese)
3) Himejihigashi Senior High School Earth Science Club. 2024. The environment at the late magma differentiation of the composite plutonic body at Bingi Bingi Point on the southeastern coast of New South Wales, southeastern Australia - based on the oscillatory zoned structure of hornblende. Abstract of the 131st Annual Meeting of the Geological Society of Japan. (in Japanese)
4) Offten,M.T. 1984. The origin of brown hornblende in the Artfjallet gabbro and dolerites. Contrib.Mineral.Petrol,86, 189-199.
5) Whitney,J.A. and Stormer,J.C. 1977. The distribution of NaAlSi3O8 between coexisting microcline and plagioclase and its effect on geothermometric calculations. Amer.Mineral.62, 687-691.
6) Mutch,E.J.F.,Blundy,J.D.,Tattitch,B.C.,Cooper,F.J. and Brooker,R.A. 2016. An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer. Contrib.Mineral.Petrol, 171:85.