17:15 〜 18:30
[SCG41-P05] Peridotites beneath the ophiolite: Petrology and chemistry of peridotite bodies located below the metamorphic sole of the Oman ophiolite
キーワード：オマーンオフィオライト、Taww 橄欖岩、部分溶融度
The Oman ophiolite is a large slab of Tethyan ocean floor obducted onto the Arabic peninsula, comprising one of the largest piece of exposed lithospheric peridotites. It is the last of several nappes thrusted onto the autochtonous units of the Arabian margin (Saih Hatat and Jebel Akhdar), and then the highest in the general stratigraphic column of Oman. The emplacement of the ophiolite resulted from intraoceanic subduction in North-northeast direction followed by a period of continental subduction leading to the Oman margin burying.
The Taww region is located at the northernmost border of the Jebel Akhdar, south to the Kahwad ophiolitic massif. It is characterised by the presence of small peridotite occurrences that are in direct contact with the Jebel Akhdar autochthonous units, and is located below the Hawasina nappes and the metamorphic sole (in this study, we call them the “Taww peridotites”). Similar peridotite bodies are found in various small regions around the Jebel Akhdar and the Saih Hatat. They crop out as a few meter to a few kilometre wide massifs, most of the time highly serpentinised or carbonated and deformed. They are characterized by the absence of magmatic veins and dykes, their stratigraphic location below the metamorphic sole of the ophiolite, and when found as ophiolitic melange, by the inclusion of limestone blocks. In these 3 points, they differ from the ophiolite mantle main body. In order to understand the petrogenesis of the Taww peridotites, we sampled several peridotite bodies from both location below and above the metamorphic sole and Hawasina units. Only one of our samples taken near Ibra had an unclear stratigraphic position (this sample is called “Ibra” in the present study).
The ophiolitic peridotites are all from the lowermost ophiolite, taken a few meter to a few tens of meter above the metamorphic sole. Their petrological characteristics are similar to what was described for basal peridotite by other researchers: they are mostly high depleted harzburgites containing 1 to 7% of Cpx (mostly from refertilised sources) with granular and porphyroclastic textures. The Taww peridotites seem to contain slightly less Cpx and more Opx but their texture are basically very similar to the usual ophiolite peridotites. The mineral chemistry shows that the Taww peridotites as well as the ophiolite peridotites are residual harzburgites. However, the Taww peridotites, instead more depleted in Cpx than the ophiolite peridotites, show lower Cr-spinel Cr# and higher Al content in Opx, suggesting a possible lower melting degree than in the ophiolite but also a relatively Al-rich chemical environment. In our study, Cpx in ophiolite lherzolites and Cpx-rich harzburgites (2 to 5% Cpx) show a relatively Na and Ti-rich chemistry showing that they are mainly from trapped melts and refertilisation processes. By contrast, ophiolite harzburgites and the Taww peridotites, independently of their lithology, show a very depleted chemistry suggesting that they are mostly melting residue. Cpx REE composition and calculation of the peridotites melting degree (Fmax) show that The Taww peridotites and the ophiolite peridotites had distinct melting history. Taww peridotites are richer in HREE and Al and less melted than the ophiolite harzburgites, (Fmax from 9 to 15% in the Taww peridotites vs 14-22% in the ophiolite), suggesting a possible higher melting pressure. They do botshow any traces of refertilisation or metasomatism and are unlikely to have undergone any ridge-related processes (high degree of decompression melting, melt-rock reaction, refertilisation at low pressure, etc.). All these data, in addition to the Taww peridotites structural general context (Breton et al., 2004) led us to think that the Taww peridotites were not a part of the Tethys ocean mantle but they are more probably a piece of the mantle that have underlied the Arabian thinned margin.
The Taww region is located at the northernmost border of the Jebel Akhdar, south to the Kahwad ophiolitic massif. It is characterised by the presence of small peridotite occurrences that are in direct contact with the Jebel Akhdar autochthonous units, and is located below the Hawasina nappes and the metamorphic sole (in this study, we call them the “Taww peridotites”). Similar peridotite bodies are found in various small regions around the Jebel Akhdar and the Saih Hatat. They crop out as a few meter to a few kilometre wide massifs, most of the time highly serpentinised or carbonated and deformed. They are characterized by the absence of magmatic veins and dykes, their stratigraphic location below the metamorphic sole of the ophiolite, and when found as ophiolitic melange, by the inclusion of limestone blocks. In these 3 points, they differ from the ophiolite mantle main body. In order to understand the petrogenesis of the Taww peridotites, we sampled several peridotite bodies from both location below and above the metamorphic sole and Hawasina units. Only one of our samples taken near Ibra had an unclear stratigraphic position (this sample is called “Ibra” in the present study).
The ophiolitic peridotites are all from the lowermost ophiolite, taken a few meter to a few tens of meter above the metamorphic sole. Their petrological characteristics are similar to what was described for basal peridotite by other researchers: they are mostly high depleted harzburgites containing 1 to 7% of Cpx (mostly from refertilised sources) with granular and porphyroclastic textures. The Taww peridotites seem to contain slightly less Cpx and more Opx but their texture are basically very similar to the usual ophiolite peridotites. The mineral chemistry shows that the Taww peridotites as well as the ophiolite peridotites are residual harzburgites. However, the Taww peridotites, instead more depleted in Cpx than the ophiolite peridotites, show lower Cr-spinel Cr# and higher Al content in Opx, suggesting a possible lower melting degree than in the ophiolite but also a relatively Al-rich chemical environment. In our study, Cpx in ophiolite lherzolites and Cpx-rich harzburgites (2 to 5% Cpx) show a relatively Na and Ti-rich chemistry showing that they are mainly from trapped melts and refertilisation processes. By contrast, ophiolite harzburgites and the Taww peridotites, independently of their lithology, show a very depleted chemistry suggesting that they are mostly melting residue. Cpx REE composition and calculation of the peridotites melting degree (Fmax) show that The Taww peridotites and the ophiolite peridotites had distinct melting history. Taww peridotites are richer in HREE and Al and less melted than the ophiolite harzburgites, (Fmax from 9 to 15% in the Taww peridotites vs 14-22% in the ophiolite), suggesting a possible higher melting pressure. They do botshow any traces of refertilisation or metasomatism and are unlikely to have undergone any ridge-related processes (high degree of decompression melting, melt-rock reaction, refertilisation at low pressure, etc.). All these data, in addition to the Taww peridotites structural general context (Breton et al., 2004) led us to think that the Taww peridotites were not a part of the Tethys ocean mantle but they are more probably a piece of the mantle that have underlied the Arabian thinned margin.