09:00 〜 09:15
[SCG46-06] Lower crustal construction of Oman ophiolite: Insight from petrology and geochemistry of gabbroic lithologies (Oman DP)
キーワード：Lower crustal gabbros, mid oceanic ridge, Oman ophiolite, Oman Drilling Project
Ophiolites are the fragments of the ancient oceanic lithosphere that are exposed (mostly as dismembered) on the land surface. Magmatic processes responsible for accretion of the lower oceanic crust remain one of the least-restrained components of the global seafloor spreading system. Lack of sufficient exposures regarding the systematic representation of the oceanic lithosphere is another major hindrance in deciphering crust-mantle differentiation.
In order to achieve this goal lower oceanic crust thought to be a crucial litho-member which turns out to be pretty rare in a global context to observe spatial variations in magmatic flow within in situ lower crust. In this contribution, we report recovered lower crust from Oman drilling Project aiming on the Phase 1 drilling, is to document the whole section from foliated gabbro to layered gabbro in the lower crustal section of Oman ophiolite. For this study we focused on the samples that can cover the total range of diversity of gabbroic lithologies. These consist of (1) layered gabbros, with modal variations in olivine, clinopyroxene, and plagioclase on a centimeter to meter scale defining layering in GT1A (recovered 401.52 m of total cores) and (2) overlying foliated gabbros in GT2A with preferred mineral orientations defining foliations (total depth of 406.77m). Petrographic observation of representative samples from both phases, we found two dominant rock types: Gabbbro and Olivine gabbro, where later dominates the lower part of the drill hole. Interlayered Troctolites in association with gabbroic rock are also noted in Phase 1 samples. Gabbroic lithologies represented by anhedral olivine with tabular to elongate shaped. Clinopyroxenes are tabular to subequant in habit. Some thin film of orthopyroxene is present as a corona around olivine. Most of the samples have undergone alteration and obscured the igneous origin. Gabbros contain plagioclase and clinopyroxene (showing poikilitic texture) as primary minerals associated with few amounts of oxides. Clinopyroxenes are often replaced by brown amphibole along cleavages or at the rim. In olivine gabbros, olivine grains are present as subhedral to skeletal in nature. Magmatic foliation is often demarked by laths of plagioclase followed by elongated clinopyroxenes. Also, symplectitic intergrowth is present between spinel and orthopyroxene. All lithologies from different holes show the different textural relationships, which will be included in this discussion. Downhole variations of major mineral compositions and trace element compositions in both phases indicate several cycles of crystallization history. Detailed petrographic, microstructural and geochemical studies of all the representative samples will be presented. In this study we will attempt to address a number of questions: 1) How the parent melts are different in GT1A and GT2A, 2) how are melt transport and melt-rock reaction affect the two stratigraphic litho units, and 3) How the gabbroic architectures is define by melt transportation. To do that here we present a study of the igneous petrology and geochemistry of systematically chosen samples from GT1A and GT2A holes. We present new mineral and whole rock major and trace element data, plagioclase-clinopyroxene thermometry, Fe-Ti oxybarometry and olivine oxidation signature. Our data will be used to ascertain the hypotheses on accretion process of lower crust formation by ‘The gabbro glacier’ model (Nicolas et al., 1988), ‘Sheeted sill’ model (Kelemen et al., 1997) and maybe a hybrid model (Lissenberg et al., 2013) of both and its role to modification of composition of MORB. Moreover, assimilation-fractional crystallization and the percentage of trapped melt estimation will give the total scenario of the formation of lower crustal gabbros.
In order to achieve this goal lower oceanic crust thought to be a crucial litho-member which turns out to be pretty rare in a global context to observe spatial variations in magmatic flow within in situ lower crust. In this contribution, we report recovered lower crust from Oman drilling Project aiming on the Phase 1 drilling, is to document the whole section from foliated gabbro to layered gabbro in the lower crustal section of Oman ophiolite. For this study we focused on the samples that can cover the total range of diversity of gabbroic lithologies. These consist of (1) layered gabbros, with modal variations in olivine, clinopyroxene, and plagioclase on a centimeter to meter scale defining layering in GT1A (recovered 401.52 m of total cores) and (2) overlying foliated gabbros in GT2A with preferred mineral orientations defining foliations (total depth of 406.77m). Petrographic observation of representative samples from both phases, we found two dominant rock types: Gabbbro and Olivine gabbro, where later dominates the lower part of the drill hole. Interlayered Troctolites in association with gabbroic rock are also noted in Phase 1 samples. Gabbroic lithologies represented by anhedral olivine with tabular to elongate shaped. Clinopyroxenes are tabular to subequant in habit. Some thin film of orthopyroxene is present as a corona around olivine. Most of the samples have undergone alteration and obscured the igneous origin. Gabbros contain plagioclase and clinopyroxene (showing poikilitic texture) as primary minerals associated with few amounts of oxides. Clinopyroxenes are often replaced by brown amphibole along cleavages or at the rim. In olivine gabbros, olivine grains are present as subhedral to skeletal in nature. Magmatic foliation is often demarked by laths of plagioclase followed by elongated clinopyroxenes. Also, symplectitic intergrowth is present between spinel and orthopyroxene. All lithologies from different holes show the different textural relationships, which will be included in this discussion. Downhole variations of major mineral compositions and trace element compositions in both phases indicate several cycles of crystallization history. Detailed petrographic, microstructural and geochemical studies of all the representative samples will be presented. In this study we will attempt to address a number of questions: 1) How the parent melts are different in GT1A and GT2A, 2) how are melt transport and melt-rock reaction affect the two stratigraphic litho units, and 3) How the gabbroic architectures is define by melt transportation. To do that here we present a study of the igneous petrology and geochemistry of systematically chosen samples from GT1A and GT2A holes. We present new mineral and whole rock major and trace element data, plagioclase-clinopyroxene thermometry, Fe-Ti oxybarometry and olivine oxidation signature. Our data will be used to ascertain the hypotheses on accretion process of lower crust formation by ‘The gabbro glacier’ model (Nicolas et al., 1988), ‘Sheeted sill’ model (Kelemen et al., 1997) and maybe a hybrid model (Lissenberg et al., 2013) of both and its role to modification of composition of MORB. Moreover, assimilation-fractional crystallization and the percentage of trapped melt estimation will give the total scenario of the formation of lower crustal gabbros.