17:15 〜 19:15
[SCG51-P04] Microstructural and petrological analyses of the gabbroic rocks along the Vulcan Transform in the Southern Ocean
キーワード：Mineral fabric、Mineral composition、Gabbroic rock、Vulcan transform、America-Antarctic Ridge
The Vulcan Transform is located at 59ºS, 16-18ºW in the America-Antarctic Ridge, which separates the Antarctic and South American plates (LaBrecque et al., 1981; Lawrence and Dick, 1983), and consists of a 120 km long with east-west trending strike-slip transform fault. The geological and geophysical surveys in the Vulcan Transform were conducted during the R/V Hakuho-maru KH-19-6 cruise in 2019, and preliminary report and the detail geophysical study were presented at the JpGU 2021 and 2024 annual meetings (Okino et al., 2021, 2024). In this cruise, the geophysical mapping and systematic rocks sampling (at five dredge sites) were conducted along 130 km long E-W trending survey lines in the southern segment along the Vulcan Transform (Okino et al., 2021, 2024).
Geophysical study combined with geochronological result by Okino et al. (2024) revealed that the spreading rate is much faster than the predicted spreading rate, which is classified as ultra-slow spreading rate (15-16 mm/yr in the full rate) based on the global plate model (MORVEL, DeMets et al., 2010). In addition, they proposed that the southern segment also shows a large temporal variation of crustal formation.
Although there are several petrological studies on mantle peridotites in this area, microstructural and petrological studies of lower oceanic crustal rocks remain scarce. To understand magmatic and subsequent deformation-metamorphic processes on lower oceanic crustal rocks in the Vulcan Transform, we investigate microstructural and geochemical analyses of 20 gabbroic rocks recovered from one dredge site (KH-19-6 D17 dredge site) in the southern segment of Vulcan Transform.
The gabbroic rocks in this study were classified as oxide gabbro, composed of plagioclase, clinopyroxene, amphibole (brown and green hornblendes), ilmenite, magnetite, apatite and chlorite. Most of the oxide gabbros undergo ductile deformation and retrograde metamorphism, however, undeformed microstructure, such as the granular textures of plagioclase and clinopyroxene are also preserved. The degree of ductile deformation varies from weak to moderate but some oxide gabbros exhibit locally strong ductile deformation related with shearing. Additionally, felsic veins were observed intruding into the oxide gabbros. These felsic veins consist of mainly plagioclase and quartz. Occasionally, we observed a quartz distributed in spot- and diffusive vein-like.
The mineral compositions of the constituted minerals in these oxide gabbros were analyzed using EPMA by GSJ-Lab. The plagioclase ranged from An6 to An44. Clinopyroxene Mg# values (Mg/(Mg+Fe)) ranged from Mg#56 to Mg#77. The composition of amphibole varies from pargasite, magnesio-hornblende (i.e., brown hornblende) to actinolite (i.e., green hornblende).
The mineral fabrics of the plagioclase, clinopyroxene and amphibole in the oxide gabbros were also analyzed using SEM-EBSD-EDS system by GSJ-Lab. Plagioclase and clinopyroxene exhibited weak to moderate preferred orientation because of their shape formed during magmatic processes and subsequent ductile deformation. The amphibole also showed weak or no characteristic preferred orientations in undeformed and weakly deformed oxide gabbros, whereas it showed strong preferred orientations in deformed oxide gabbros.
In this presentation, we would like to present the results of detailed microstructural observations, mineral chemical compositions, and crystallographic orientation analyses of these oxide gabbros. Furthermore, the temperature estimation and formation processes of the oxide gabbros will be discussed.
Geophysical study combined with geochronological result by Okino et al. (2024) revealed that the spreading rate is much faster than the predicted spreading rate, which is classified as ultra-slow spreading rate (15-16 mm/yr in the full rate) based on the global plate model (MORVEL, DeMets et al., 2010). In addition, they proposed that the southern segment also shows a large temporal variation of crustal formation.
Although there are several petrological studies on mantle peridotites in this area, microstructural and petrological studies of lower oceanic crustal rocks remain scarce. To understand magmatic and subsequent deformation-metamorphic processes on lower oceanic crustal rocks in the Vulcan Transform, we investigate microstructural and geochemical analyses of 20 gabbroic rocks recovered from one dredge site (KH-19-6 D17 dredge site) in the southern segment of Vulcan Transform.
The gabbroic rocks in this study were classified as oxide gabbro, composed of plagioclase, clinopyroxene, amphibole (brown and green hornblendes), ilmenite, magnetite, apatite and chlorite. Most of the oxide gabbros undergo ductile deformation and retrograde metamorphism, however, undeformed microstructure, such as the granular textures of plagioclase and clinopyroxene are also preserved. The degree of ductile deformation varies from weak to moderate but some oxide gabbros exhibit locally strong ductile deformation related with shearing. Additionally, felsic veins were observed intruding into the oxide gabbros. These felsic veins consist of mainly plagioclase and quartz. Occasionally, we observed a quartz distributed in spot- and diffusive vein-like.
The mineral compositions of the constituted minerals in these oxide gabbros were analyzed using EPMA by GSJ-Lab. The plagioclase ranged from An6 to An44. Clinopyroxene Mg# values (Mg/(Mg+Fe)) ranged from Mg#56 to Mg#77. The composition of amphibole varies from pargasite, magnesio-hornblende (i.e., brown hornblende) to actinolite (i.e., green hornblende).
The mineral fabrics of the plagioclase, clinopyroxene and amphibole in the oxide gabbros were also analyzed using SEM-EBSD-EDS system by GSJ-Lab. Plagioclase and clinopyroxene exhibited weak to moderate preferred orientation because of their shape formed during magmatic processes and subsequent ductile deformation. The amphibole also showed weak or no characteristic preferred orientations in undeformed and weakly deformed oxide gabbros, whereas it showed strong preferred orientations in deformed oxide gabbros.
In this presentation, we would like to present the results of detailed microstructural observations, mineral chemical compositions, and crystallographic orientation analyses of these oxide gabbros. Furthermore, the temperature estimation and formation processes of the oxide gabbros will be discussed.