17:15 〜 18:45
[SCG42-P03] Petrological characteristic of a slice of the primitive section of oceanic crust in the Oman ophiolite (Wadi Mahram, Maqsad massif)
キーワード：オマーンオフィオライト、トロクトライト、海洋地殻、海嶺軸
The Oman ophiolite consists of several massifs cropping out along the coast of Oman. It is one of the best-exposed sections of old oceanic crust and mantle in the world. A North-South gradient in igneous processes and composition is observed in the ophiolite: the northern massifs recording a polygenetic igneous history involving a relatively important subduction component, whereas the southern massifs only recorded a single-stage process at a submarine spreading ridge. Previous studies showed a well-defined mantle structural diapir in the Maqsad (or Sumail) massif. This structural diapir is well matched with other petrographical and chemical data, allowing us to locate the axis of the former spreading centre near wadi Mahram and Tuff. In the Maqsad massif, the mantle section was well studied and mantle structure, chemistry as well as mantle dykes petrological and chemical characteristics are well known. However, the crustal section is only locally known in its lowest part located near the mantle/crust boundary. In this study, we conducted a sequential study of the about 1,500m thick Wadi Mahram crustal section (from mantle-crust boundary to sheeted dyke transition) located directly above the mantle diapir, i.e. directly above the Oman spreading axis.
The studied section is, without surprise, layered at the lower level and formed of varitextured and isotropic gabbros at the higher level. The uppermost gabbros are cut by dolerite dykes showing a progressive transition to the sheeted dyke complex, although the complete transition is not visible in the field due to the presence of Wadi at this precise level. The whole Mahram section is exceptionally rich in troctolites as this lithology represents more than three-quarters of the observed facies and is present at any level of the section from bottom to top. Layered troctolites at the lowermost level (above the mantle-crust transition zone) are not uncommon in ophiolites and oceanic crust but, this is quite unexpected lithology at a high level, close to the transition with sheeted dyke complex.
Near the mantle-crust transition, layered troctolites are intercalated with minor olivine gabbros and dunite layers. The layering is locally crosscut by centimeter-scale olivine gabbro dykes. From the middle to the top of the section, vary textured troctolites progressively become isotropic at the boundary with the sheeted dyke complex. Meter-scale olivine gabbros and olivine-free layers are more abundant at the top. Dolerite dyke cutting troctolite and gabbro structures are abundant near the sheeted dyke complex and probably represent its root system.
The chemical composition of the Mahram troctolites is compatible with a MORB melt with no or little hydration signs. Comparison with crustal sections studied in the Wadi Tayin massif (Wadi Gideah, Wadi Khafifah and Wadi Kadir) show that the Wadi Mahram lithologies are significantly more primitive than in Wadi Tayin, which is in accordance with the abundance of troctolites found in Wadi Mahram compared to Wadi Tayin. On the other hand, they show a similar primitive chemistry as troctolite-rich oceanic sections in Hess Deep (EPR), Pito Deep (EPR), Atlantis Massif (Atlantic ridge). However, the effect of the melt-rock reaction as well as the assimilation of olivines from the mantle, which are two processes that strongly influenced the EPR and Atlantic ridge gabbros and troctolites chemical compositions, seem to have had much less influence in Wadi Mahram.
The studied section is, without surprise, layered at the lower level and formed of varitextured and isotropic gabbros at the higher level. The uppermost gabbros are cut by dolerite dykes showing a progressive transition to the sheeted dyke complex, although the complete transition is not visible in the field due to the presence of Wadi at this precise level. The whole Mahram section is exceptionally rich in troctolites as this lithology represents more than three-quarters of the observed facies and is present at any level of the section from bottom to top. Layered troctolites at the lowermost level (above the mantle-crust transition zone) are not uncommon in ophiolites and oceanic crust but, this is quite unexpected lithology at a high level, close to the transition with sheeted dyke complex.
Near the mantle-crust transition, layered troctolites are intercalated with minor olivine gabbros and dunite layers. The layering is locally crosscut by centimeter-scale olivine gabbro dykes. From the middle to the top of the section, vary textured troctolites progressively become isotropic at the boundary with the sheeted dyke complex. Meter-scale olivine gabbros and olivine-free layers are more abundant at the top. Dolerite dyke cutting troctolite and gabbro structures are abundant near the sheeted dyke complex and probably represent its root system.
The chemical composition of the Mahram troctolites is compatible with a MORB melt with no or little hydration signs. Comparison with crustal sections studied in the Wadi Tayin massif (Wadi Gideah, Wadi Khafifah and Wadi Kadir) show that the Wadi Mahram lithologies are significantly more primitive than in Wadi Tayin, which is in accordance with the abundance of troctolites found in Wadi Mahram compared to Wadi Tayin. On the other hand, they show a similar primitive chemistry as troctolite-rich oceanic sections in Hess Deep (EPR), Pito Deep (EPR), Atlantis Massif (Atlantic ridge). However, the effect of the melt-rock reaction as well as the assimilation of olivines from the mantle, which are two processes that strongly influenced the EPR and Atlantic ridge gabbros and troctolites chemical compositions, seem to have had much less influence in Wadi Mahram.