09:45 〜 10:00
[SCG46-09] Petrological characteristics of serpentinized peridotites from Fizh block, Oman ophiolite
キーワード：オマーンオフィオライト、蛇紋岩化作用、変質鉱物
The ophiolite in Fizh region, which is located in northern Oman, is known as one of the best well-preserved ophiolites in the world, as the ophiolite crops out from basal thrust in the west to mantle-crust transition in the east in the region (in the scale of 25km W-E and 18km N-S). The N-S fast-spreading ridge axis is inferred to be located along the crust section in the east of the Fizh block, and the center and end of segment are to be in the south and north respectively (Miyashita et al., 2003; Adachi & Miyashita, 2003), which could record the formation of oceanic lithosphere (e.g., Umino et al., 2003; LeMee et al., 2004). Thus, the contiguous mantle melting history or alteration process as well, such as serpentinization, could have been marked in Fizh ophiolite from basal thrust to Moho while it was generated at mid-ocean ridge, and during obduction process toward the Arabian continent in the Cretaceous period.
At the basal thrust in northern Fizh, according to Yoshida (master thesis, 2018), it is inferred that hydrothermal alteration process could have been caused due to the dehydration of metamorphic sole with the relatively high-temperature fluids (>700℃), which was to be provided into the mantle section (e.g., Ishikawa et al., 2005; Kanke and Takazawa, 2014). The detailed description of metamorphic and hydrous mineral assemblage revealed rough range of alteration temperature at the base and indicated that the hydrothermal fluid intrusion through intensive local faults at basal thrust or Moho could serpentinize the surrounding rocks. However, whether the fluids could succeed in penetration into whole mantle section to promote ubiquitous serpentinization and in formation of altered products via the fluids on the mantle rocks have not been poorly discussed.
In this study, thus we will try to unravel the spatial fluid infiltration that could lead to serpentinization in the mantle section both in northern and southern Fizh block, by targeting the serpentinized peridotites in the whole mantle section, relating with plenty of petrological and geochemical data from previous studies, giving a detailed description of the reaction process of primary and altered secondary mineral assemblages, and finally constraining the reaction temperature, pressure and fluid composition to form the altered products.
Petrological description using microscope and Raman spectroscopy are conducted for the thin section samples so far. The available peridotite samples, harzburgite, dunite and quite minor lherzolite, taken from both northern and southern Fizh block, suffered moderate to highly serpentinized. Olivines in sub- to mylonitic textures or deformed orthopyroxene were found in local shear zones, commonly occurred over the mantle section. Several species of altered products after the primary minerals in the peridotites were detected;
・Lizardite mainly formed mesh textures and veins, and chrysotile and very minor antigorite in veins. Their mixtures also exist. Relict of olivine is still remained in most of the samples.
・Bastite textures in orthopyroxene. Exsolved olivine or clinopyroxene are visible in incompletely altered orthopyroxene. Talc forming in the rim when the relict of orthopyroxene is remained, or completely replacing of orthopyroxene. Quite minor tremolite in vicinity of orthopyroxene in needle shape.
The serpentinization degree or occurrence of altered mineral assemblages are up to now heterogeneous and apparently not associated with the lithology, textures (such as porphyroclastic or granular) or the intensity of deformation at locally formed shear zones over basal thrust to Moho.
More careful and detailed observation is needed increasing number of samples and widening range of sample location in north and south (the edge and center of segmentation), from west to east (basal thrust to Moho), to discuss formation process of altered products distributed in the mantle section.
At the basal thrust in northern Fizh, according to Yoshida (master thesis, 2018), it is inferred that hydrothermal alteration process could have been caused due to the dehydration of metamorphic sole with the relatively high-temperature fluids (>700℃), which was to be provided into the mantle section (e.g., Ishikawa et al., 2005; Kanke and Takazawa, 2014). The detailed description of metamorphic and hydrous mineral assemblage revealed rough range of alteration temperature at the base and indicated that the hydrothermal fluid intrusion through intensive local faults at basal thrust or Moho could serpentinize the surrounding rocks. However, whether the fluids could succeed in penetration into whole mantle section to promote ubiquitous serpentinization and in formation of altered products via the fluids on the mantle rocks have not been poorly discussed.
In this study, thus we will try to unravel the spatial fluid infiltration that could lead to serpentinization in the mantle section both in northern and southern Fizh block, by targeting the serpentinized peridotites in the whole mantle section, relating with plenty of petrological and geochemical data from previous studies, giving a detailed description of the reaction process of primary and altered secondary mineral assemblages, and finally constraining the reaction temperature, pressure and fluid composition to form the altered products.
Petrological description using microscope and Raman spectroscopy are conducted for the thin section samples so far. The available peridotite samples, harzburgite, dunite and quite minor lherzolite, taken from both northern and southern Fizh block, suffered moderate to highly serpentinized. Olivines in sub- to mylonitic textures or deformed orthopyroxene were found in local shear zones, commonly occurred over the mantle section. Several species of altered products after the primary minerals in the peridotites were detected;
・Lizardite mainly formed mesh textures and veins, and chrysotile and very minor antigorite in veins. Their mixtures also exist. Relict of olivine is still remained in most of the samples.
・Bastite textures in orthopyroxene. Exsolved olivine or clinopyroxene are visible in incompletely altered orthopyroxene. Talc forming in the rim when the relict of orthopyroxene is remained, or completely replacing of orthopyroxene. Quite minor tremolite in vicinity of orthopyroxene in needle shape.
The serpentinization degree or occurrence of altered mineral assemblages are up to now heterogeneous and apparently not associated with the lithology, textures (such as porphyroclastic or granular) or the intensity of deformation at locally formed shear zones over basal thrust to Moho.
More careful and detailed observation is needed increasing number of samples and widening range of sample location in north and south (the edge and center of segmentation), from west to east (basal thrust to Moho), to discuss formation process of altered products distributed in the mantle section.