A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Insights From Crystallographic Preferred Orientations

Abstract

Based on marine geophysics, seafloor geology, and ophiolite studies, the classical picture of a uniformly layered oceanic crust (with basaltic lava flows, basaltic sheeted dikes, and gabbros, from top to bottom) emerged in the early 1970s (Anonymous, 1972). About 50 years later, our vision of the architecture of the ocean crust has considerably evolved, with a continuously growing understanding of its variability at the global scale. This variability results from various modes of accretion that are controlled by magma supply to the ridge, which in turn is related on the first order to spreading rate (e.g.

A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Petrography and Petrology

“…At the boundary between LFG and UFG the modal amount of the late stage minerals increases significantly, exactly (Figures 4c and 4e), where Mock, Ildefonse, et al. (2021) found the changing in the emplacement mechanisms, based on an interpretation of the microstructures. Figure 4f presents the grade of alteration which shows a typical hour glass structure, with stronger alteration in the lower and upper parts of the crust.…”

“…(2017) for varitextured gabbros. For our differentiation scenario by fractional crystallization we used an approach following the conclusions Mock, Ildefonse, et al., 2021, who suggested in‐situ crystallization in the lower part of the plutonic crust (layered gabbro and LFG), and a vertical transport of melt or mush according to a glacier process in the upper part. Accordingly, for the COMAGMAT calculations we divided the whole gabbroic crust in two parts with different modes of fractionation/mixing: For the lower part, we simulated, in accord with the sheeted sill model, upward differentiation, starting at the mantle/crust boundary, and for the upper part we simulated downward differentiation starting in the AML horizon, in accord with the gabbro glacier model.…”

A Reference Section Through Fast‐Spread Lower Oceanic Crust, Wadi Gideah, Samail Ophiolite (Sultanate of Oman): Trace Element Systematics and REE Crystallization Temperatures—Implications for Hybrid Crustal Accretion

“…Recent studies try to synthesize the ideas and observations as mentioned above and develop hybrid models where a gabbro glacier forms the upper crust, while the lower crust is formed by in situ crystallization (Boudier & Nicolas, 2011; Buck, 2000; Hopson, 2007; Jousselin et al., 2012; Mock, Ildefonse, et al., 2021). Among many authors working on oceanic crustal accretion models, there is agreement that the upper‐most part of the crust, the high‐level gabbros (sometimes called isotropic gabbros and in this study referred to as varitextured gabbros), form as a result of crystallization within, or at the level of, the AML.…”

“…While gabbros and mantle rocks are well exposed along the Wadi, basalts, and sheeted dikes are generally only present as small hills within the lower Wadi plain. Based on field observations and structural data (Mock, Ildefonse, et al., 2021), the profile can be divided into the following units (from top to bottom): Axial melt lens, AML, ∼5.0 km h.a.M. (height above the mantle‐crust transition zone, for calculation of h.a.M., see Section 2.1), Varitextured gabbros, VG, ∼4.1–5.0 km h.a.M., Upper foliated gabbros, UFG; ∼3.5–4.1 km h.a.M., Lower foliated gabbros, LFG, ∼2.6–3.5 km h.a.M., Layered gabbros, LG, ∼0.2–2.6 km h.a.M., mantle‐crust transition zone, MTZ, ∼0–0.2 km h.a.M.…”

“…Numbers in brackets represent the respective thickness and percentage in reference to the plutonic section, according to Mock, Ildefonse, et al. (2021). Separation between upper and lower sheeted dikes after Garbe‐Schönberg et al.…”