16:15 〜 16:30
[SCG51-09] Formation of Primitive Lower Oceanic Crust at a Paleo-Spreading Center in the Oman Ophiolite
キーワード：海洋地殻、はんれい岩、拡大軸、オマーンオフィオライト
The formation process of the lower oceanic crust remains poorly understood due to the difficulty in direct access. Ophiolites are obducted ancient oceanic lithosphere and although careful attention must be paid to their formation environment and structural changes, they provide an opportunity to study oceanic crust. The Oman Ophiolite is one of the best-exposed and well-studied ophiolites in the world. Previous structural studies have identified a mantle diapir belonging to a paleo-spreading center within the Maqsad (or Sumail) massif in the southern Oman Ophiolite. While the mantle section and the mantle-crust transition zone of the Maqsad massif have been studied in detail, its crustal section remains less explored. Here, we present the geological, petrological, and geochemical profile of the lower oceanic crust (gabbros) of Wadi Mahram in the Maqsad massif, which is located directly above the Oman paleo-spreading center, and discuss its composition and formation.
The studied section shows a part of a typical ophiolite sequence, with layered gabbros at lower levels, varitextured and isotropic gabbros at higher levels, and an uppermost section intruded by dolerite dykes, marking a progressive transition to the sheeted dyke complex. The estimated thickness of the lower crustal section is ca. 1.5 km, thinner than that of modern fast-spreading oceanic crust and other massifs of the Oman Ophiolite. Troctolites are abundant throughout the section, with olivine-rich at all levels and clinopyroxene occurring primarily as poikilitic crystals. Mineral chemistry shows olivine Fo77–90, plagioclase An77–95, and clinopyroxene Mg#=79–91, with upward differentiation and an expanded compositional range in the upper part of the section.
A Comparison with previously studied crustal sections in Wadi Tayin massif (Wadi Gideah [1], Khafifah [2], Kadir [3], and Namarah [4]), Rustaq massif (Wadi Abyad [5, 6]), and the present-day East Pacific Rise (Hess Deep [7]) indicates that Wadi Mahram has a notably more primitive composition. This indicates that the lower oceanic crust in the Oman Ophiolite is compositionally heterogeneous and varies by location. The primitive nature of the Wadi Mahram section could be attributed to the following factors: (i) variations in primary magma composition, (ii) limited differentiation associated with a relatively thin crust (i.e., low melt supply), and (iii) a frozen magma chamber process, as Wadi Mahram locates exactly above the inferred spreading center and the mantle diapir, or a combination of these elements. These may also be linked to mantle heterogeneity, temporal and spatial variations in partial melting, and/or a dying spreading system. Future studies in the Maqsad–Mahram region will enable three-dimensional observations from the mantle to the crust at the spreading center, providing new insights into the magmatism and formation of the oceanic lithosphere.
References:
[1] Koepke et al. (2021) JGR Solid Earth 127, e2021JB022735. [2] VanTongeren et al. (2021) JGR Solid Earth 126, e2021JB021986. [3] Pallister and Hopson (1981) JGR 86, 2593–2644. [4] Coogan (2014) Treatise on Geochemistry (2nd Ed.) 4, 497–541. [5] Browning (1982) Ph.D. thesis, Open Univ, U.K. [6] MacLeod and Yaouancq (2000) EPSL 176, 357–373. [7] Lissenberg et al. (2013) EPSL 361, 436–447.
The studied section shows a part of a typical ophiolite sequence, with layered gabbros at lower levels, varitextured and isotropic gabbros at higher levels, and an uppermost section intruded by dolerite dykes, marking a progressive transition to the sheeted dyke complex. The estimated thickness of the lower crustal section is ca. 1.5 km, thinner than that of modern fast-spreading oceanic crust and other massifs of the Oman Ophiolite. Troctolites are abundant throughout the section, with olivine-rich at all levels and clinopyroxene occurring primarily as poikilitic crystals. Mineral chemistry shows olivine Fo77–90, plagioclase An77–95, and clinopyroxene Mg#=79–91, with upward differentiation and an expanded compositional range in the upper part of the section.
A Comparison with previously studied crustal sections in Wadi Tayin massif (Wadi Gideah [1], Khafifah [2], Kadir [3], and Namarah [4]), Rustaq massif (Wadi Abyad [5, 6]), and the present-day East Pacific Rise (Hess Deep [7]) indicates that Wadi Mahram has a notably more primitive composition. This indicates that the lower oceanic crust in the Oman Ophiolite is compositionally heterogeneous and varies by location. The primitive nature of the Wadi Mahram section could be attributed to the following factors: (i) variations in primary magma composition, (ii) limited differentiation associated with a relatively thin crust (i.e., low melt supply), and (iii) a frozen magma chamber process, as Wadi Mahram locates exactly above the inferred spreading center and the mantle diapir, or a combination of these elements. These may also be linked to mantle heterogeneity, temporal and spatial variations in partial melting, and/or a dying spreading system. Future studies in the Maqsad–Mahram region will enable three-dimensional observations from the mantle to the crust at the spreading center, providing new insights into the magmatism and formation of the oceanic lithosphere.
References:
[1] Koepke et al. (2021) JGR Solid Earth 127, e2021JB022735. [2] VanTongeren et al. (2021) JGR Solid Earth 126, e2021JB021986. [3] Pallister and Hopson (1981) JGR 86, 2593–2644. [4] Coogan (2014) Treatise on Geochemistry (2nd Ed.) 4, 497–541. [5] Browning (1982) Ph.D. thesis, Open Univ, U.K. [6] MacLeod and Yaouancq (2000) EPSL 176, 357–373. [7] Lissenberg et al. (2013) EPSL 361, 436–447.