Cross section through the peridotite in the Samail Ophiolite, southeastern Oman Mountains
Authors: Boudier F. , Coleman R. G. 
Published: February 2008

A continuous cross section through the peridotite of the Samail ophiolite south of Wadi Tayin offers a relatively undisturbed slice of oceanic mantle 9–12 km thick. The whole section consists of a regularly foliated harzburgite banded with orthopyroxenite except for a narrow zone at the base where the primary banding is dominantly dunitic. Websterite dikes, sometimes deformed and branching, are observed throughout the whole sequence, whereas undeformed gabbro dikes appear abruptly about 2–5 km above the base. The top of the foliated harzburgite is in contact with dunite cumulates that grade upward into wehrlite and gabbro cumulates. The websterite and gabbro dikes are formed by crystal accumulation from ascending, off‐axis olivine‐poor tholeiite magma that passes through the tectonized harzburgite. Discordant and deformed dunite within the harzburgite represent fractionated olivine accumulates derived from the deep‐seated primitive picritic tholeiite magmas as they moved through the harzburgite within the spreading axis. The homogeneous chemistry, extreme depletion, and absence of frozen partial melts demonstrate that the harzburgite tectonite is a highly refractory residue. It is suggested that the harzburgite tectonite does not represent the contemporaneous cogenetic residue from which the deep‐seated primitive picritic tholeiite was derived. The discordant dunite, gabbro, and websterite dikes record crystal fractionation products of ascending basaltic liquids derived from partial melting of fertile mantle at greater depths, whereas at least some of these dunites may represent a partial melt residue. A spinel lineation dipping slightly to the southwest, present all through the cross section, parallels isoclinal fold axes of the primary banding in the harzburgite. The tectonic fabric of the harzburgite exhibits clear maxima of olivine crystallographic axes related to the regional foliation and the spinel lineation. The magmatic reaction, the comparison of tectonic fabrics, size of neoblasts and subgrains in olivine with experimental data, suggest that the harzburgite sequence was deformed at or near solidus temperatures and under a deviatoric stress increasing from 300 to 400 bars in the upper part of the sequence and from 600 to 700 bars in the lower part. Disequilibrium between primary phases due to subsolidus transformation precludes establishment of temperatures and pressures of partial melting by using present KD ratios of coexisting phases in the harzburgite. Mylonitic textures in narrow bands at the base of the sequence grade upward into porphyroclastic textures with rapidly increasing neoblast size. These textures are the imprint of a superposed deformation at lower temperatures and under a deviatoric stress reaching 1 to 2 kbar that affected the narrow sole of the sequence. A narrow amphibolite‐facies zone at the base of the harzburgite tectonite nappe exhibits a similar fabric as the overlying mylonitic harzburgite. This deformation is thought to have occurred during thrusting and detachment of the Samail ophiolite nappes shortly after their formation as oceanic lithosphere.