10:45 〜 11:00
[SCG41-06] Heterogeneity in the mantle section of the Oman ophiolite in terms of spinel and clinopyroxene compositions
キーワード：オマーンオフィオライト、上部マントル、拡大海嶺、衝上、マントルーメルト反応
The tectonic environment during the formation of Oman ophiolite is still debated, but it becomes consensus that ridge spreading worked in the early stage [1, 2]. The age of the volcanic and plutonic rocks in the crustal section formed by the spreading ridge is 96-95 Ma, which is close to the oldest age (~ 95-94 Ma) of the metamorphic sole, indicating that the thrusting of ophiolite had started during or immediately after the formation of the crustal section [3, 4]. It is important to clarify the compositional modification that the residual mantle peridotite have received during thrusting/subduction following the partial melting in the spreading ridge. In this talk, I will summarize what has been clarified at this time.
Spinel's Cr# (= Cr/[Cr+Al] atomic ratio) and abundance of heavy rare earth elements in clinopyroxene (e.g., Yb) are used as an indicator to estimate the depletion of peridotites with upper mantle origin [5, 6, 7]. Research team at Niigata University has examined spatial compositional heterogeneity in the mantle section of the Oman ophiolite. The study area has been focused in the Fizh and Salahi massifs from the northern part and the Wadi Tayin massif from the southern part of the Oman ophiolite.
In the Fizh and Salahi massifs, the Cr# of spinels in harzburgite falls within the composition range of abyssal peridotites except for some areas. Areas where Cr# is higher than the composition range of abyssal peridotites exist locally as the "high refractory harzburgite zone" [8] in the northern part of the Fizh mantle section and as the ultramafic complex of the Salahi mantle section [9]. In the Wadi Tayin mantle section in the southern part of Oman ophiolite, Cr# of harzbergite spinel is mostly within the composition range of abyssal peridotite.
On the other hand, the Cr# of dunite spinel has a wide range from abyssal peridotite to forearc peridotite, and spinel with a high value of 0.7 or more widely distributed in the Fizh and Sarahi mantle section. The high Cr# region of the dunite spinel spreads finger-like in the mantle section, suggesting a reaction with melts (island arc tholeiite and boninite) derived from highly refractory peridotite. Even in the Wadi Tayin mantle section in the southern part of Oman ophiolite, some of the dunite spinel have high Cr#, and their distribution is limited to the shear zone and the vicinity of the basal thrust.
The rare earth element pattern of Cpx in the peridotites in the mantle section of the Oman ophiolite tends to change with the coexisting spinel Cr#. That is, as the Cr# of spinel increases, the Ce/Yb ratio of Cpx increases. There is a difference in the correlation between the northern part and the southern part of Oman ophiolite, and when comparing the peridotites with the same Cr# of spinel, the Ce/Yb ratio of Cpx is higher in the southern part. This is thought to represent a difference in the composition of the melt that reacted with harzburgite to form dunite, and while boninite melt was involved in the north, it is possible that MORB may be an island arc tholeiite in the south. The former is characterized by a higher amount of Cr and a lower abundance of rare earth elements than the latter.
References: [1] Nicolas et al., 1988, Tectonophysics, 151, 87–105; [2] Umino, 1995, Journal of Geography (Chigaku Zasshi), 104, 321–349; [3] Rioux et al., 2012, JGR, 117, B07201, doi:10.1029/2012JB009273; [4] Rioux et al., 2013, JGR, 118, 2085-2101; [5] Dick and Bullen, 1984, CMP, 86, 54-76; [6] Arai, 1994, Chemical Geology, 113, 191-204; [7] Hellebrand et al., 2001, Nature, 410, 677-681; [8] Kanke and Takazawa, 2014, Geological Society, London, Special Publications, 392, 229–246; [9] Nomoto and Takazawa, 2013, JMMPS, 42, 123-135.
Spinel's Cr# (= Cr/[Cr+Al] atomic ratio) and abundance of heavy rare earth elements in clinopyroxene (e.g., Yb) are used as an indicator to estimate the depletion of peridotites with upper mantle origin [5, 6, 7]. Research team at Niigata University has examined spatial compositional heterogeneity in the mantle section of the Oman ophiolite. The study area has been focused in the Fizh and Salahi massifs from the northern part and the Wadi Tayin massif from the southern part of the Oman ophiolite.
In the Fizh and Salahi massifs, the Cr# of spinels in harzburgite falls within the composition range of abyssal peridotites except for some areas. Areas where Cr# is higher than the composition range of abyssal peridotites exist locally as the "high refractory harzburgite zone" [8] in the northern part of the Fizh mantle section and as the ultramafic complex of the Salahi mantle section [9]. In the Wadi Tayin mantle section in the southern part of Oman ophiolite, Cr# of harzbergite spinel is mostly within the composition range of abyssal peridotite.
On the other hand, the Cr# of dunite spinel has a wide range from abyssal peridotite to forearc peridotite, and spinel with a high value of 0.7 or more widely distributed in the Fizh and Sarahi mantle section. The high Cr# region of the dunite spinel spreads finger-like in the mantle section, suggesting a reaction with melts (island arc tholeiite and boninite) derived from highly refractory peridotite. Even in the Wadi Tayin mantle section in the southern part of Oman ophiolite, some of the dunite spinel have high Cr#, and their distribution is limited to the shear zone and the vicinity of the basal thrust.
The rare earth element pattern of Cpx in the peridotites in the mantle section of the Oman ophiolite tends to change with the coexisting spinel Cr#. That is, as the Cr# of spinel increases, the Ce/Yb ratio of Cpx increases. There is a difference in the correlation between the northern part and the southern part of Oman ophiolite, and when comparing the peridotites with the same Cr# of spinel, the Ce/Yb ratio of Cpx is higher in the southern part. This is thought to represent a difference in the composition of the melt that reacted with harzburgite to form dunite, and while boninite melt was involved in the north, it is possible that MORB may be an island arc tholeiite in the south. The former is characterized by a higher amount of Cr and a lower abundance of rare earth elements than the latter.
References: [1] Nicolas et al., 1988, Tectonophysics, 151, 87–105; [2] Umino, 1995, Journal of Geography (Chigaku Zasshi), 104, 321–349; [3] Rioux et al., 2012, JGR, 117, B07201, doi:10.1029/2012JB009273; [4] Rioux et al., 2013, JGR, 118, 2085-2101; [5] Dick and Bullen, 1984, CMP, 86, 54-76; [6] Arai, 1994, Chemical Geology, 113, 191-204; [7] Hellebrand et al., 2001, Nature, 410, 677-681; [8] Kanke and Takazawa, 2014, Geological Society, London, Special Publications, 392, 229–246; [9] Nomoto and Takazawa, 2013, JMMPS, 42, 123-135.