Petrological nature and origin of the ultramafic complex in the basal part of the Salahi mantle section, the Oman ophiolite

Extensive lenticular-shaped ultramafic complexes that are composed of dominant amount of dunite associated with harzburgite and irregular-shaped pyroxenite cumulates, occur in the basal part of the Salahi mantle section in the northern Oman ophiolite. Petrologic characteristics such as rock types, textures and mineral compositions were examined to understand the origin of the ultramafic complex. Peridotites exhibit coarse-granular to very coarse-granular texture with grain size greater than five millimeters. The Cr#[=Cr/(Cr+Al) atomic ratio] of spinels in harzburgites ranges from 0.56 to 0.72, and is most frequent at 0.64-0.66 while spinel Cr# of dunites ranges from 0.61 to 0.84, and is most frequent at 0.76-0.82. The origin of voluminous dunite with high Cr# spinel can be explained by flux melting of residual harzburgite as a result of infiltration of large amount of fluid from the base of the ophiolite during oceanic thrusting. Dunites with very coarse-granular texture also support this hypothesis. The dunites in the central part of the complex tend to have both olivine Fo content and spinel Cr# greater than those in the host harzburgites. We speculate that not only orthopyroxene but also olivine were consumed during flux melting in this region resulted in the formation of voluminous dunites in the core of the complex. On the other hand, the dunites from the periphery of the complex have olivine with Fo content lower than those in the host harzburgites indicating precipitation of olivine from the melt. Moreover, crystallization of pyroxene formed numerous pyroxenites in this region. The southern part of the Salahi mantle section has been considered as a paleo-ridge segment end region. Our results support that the occurrence of highly refractory peridotites is spatially related to the segment end region in the northern Oman ophiolite.