Petrologic profile of peridotite layers under a possible Moho in the northern Oman ophiolite: an example from Wadi Fizh

Abstract

We examined vertical variations of the petrological characteristics of a 33 -m -thick peridotite section under the layered gabbro section along Wadi Fizh of the northern Oman ophiolite to understand the formation mechanism of the Mohorovicic discontinuity (Moho) beneath a spreading center. Here, we refer to the base of the layered gabbro section as "L -Moho" for the sake of simplicity. Network -like gabbro sills in peridotites increase in frequency upward to the L -Moho. The L -Moho is underlain by a 1 -m -thick wehrlite layer, under which exists a 10 -m -thick dunite layer, overlying a harzburgite layer where total pyroxenes slightly increase downward. Wehrlite is also found as screens between gabbro layers above the L -Moho. The mineral chemistry indicates systematic variations toward the L -Moho within the peridotite section; the Fo content (91 to 85) and NiO (0.4 to 0.2 wt%) of olivine decrease; the TiO 2 content of clinopyroxene (0.1 to 0.6 wt%) and spinel (nil to 1.4 wt%) and atomic ratios of Cr/(Cr + Al) (0.5 to 0.6) and Fe 3+ /(Cr + Al + Fe 3+ ) (0.05 to >0.1) in spinel increase upsection from the base (harzburgite) to the around L -Moho wehrlite via dunite. These variations are essentially similar to those observed in harzburgite/MORB reaction products from Hess Deep, East Pacific Rise, and possibly indicate that the lithological and mineral chemical variations within the examined peridotite layer resulted from the reaction between a harzburgite and a melt that produced the layered gabbros.

The dunitic mantle-crust transition zone in the Oman ophiolite: Residue of melt-rock interaction, cumulates from high-MgO melts, or both?

“…The model of Figure 4 offers a simple framework to explain the large variations in the DTZ thickness in ophiolites: thick DTZ bearing witness of a long-standing interaction history between a diapir and the lithosphere (Rabinowicz et al, 1987) and thin DTZ corresponding to incipient diapirism (Akizawa and Arai, 2009). …”

“…The DTZ is mostly made of pure dunite (olivine + scattered Cr-spinel) with some mafi c intervals; its thickness ranges from a few meters to a few hundred meters. It is considered either as a pile of ultramafi c cumulates from dense picritic melts stagnating at the base of the crust (e.g., Elthon, 1979) or as residual peridotite leached by melts becoming undersaturated with pyroxene as pressure decreases (Dick and Natland, 1996;Koga et al, 2001;Akizawa and Arai, 2009).…”

Compositional variations and tectonic settings of podiform chromitites and associated ultramafic rocks of the Neoproterozoic ophiolite at Wadi Al Hwanet, northwestern Saudi Arabia

“…Primary olivine is still preserved in the massive dunite keeping the primary internal structure of the rock. It can be considered as a Moho transition zone dunite as the gabbroic veins and sills become predominant toward the S-SW parts of the area, before changing into crustal layered gabbros (Akizawa and Arai, 2009). …”

Crustal diopsidites from the northern Oman ophiolite: Evidence for hydrothermal circulation through suboceanic Moho

“…1). We can observe sharp gabbro/peridotite transition along Wadi Fizh in the northern Oman ophiolite (e.g., Uesugi et al, 2003;Akizawa and Arai, 2009), which reveals excellent exposures of crust -mantle rocks formed at a segment margin of a spreading ridge (Miyashita et al, 2003). Dark brown chlorite rocks were formed in isotropic gabbros, which are located just below the sheeted dike complex along Wadi Bani Umar in the Fizh block of the northern Oman ophiolite (Yoshitake et al, 2009).…”

“…We examined an anorthite diopsidite network cutting a layered gabbro in the lowermost crust about 50 m above the layered gabbro/peridotite boundary at Wadi Fizh in the northern Oman ophiolite (see Akizawa and Arai, 2009) (Figs. 1 and 2a).…”