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 TiO2 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 Fe3+/(Cr + Al + Fe3+) (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.
