Modern sand from obducted ophiolite belts - (Sultanate of Oman and United Arab Emirates)

Oman and the United Arab Emirates host the largest ophiolites on Earth and are characterized by arid tropical climate and desert conditions. They thus provide an unexcelled natural laboratory in which to study sediments derived from allochthonous oceanic lithosphere and to define an actualistic petrogenetic model for obduction-orogen provenance. Spectacularly exposed in SE Arabia are two distinct obducted ophiolite sequences-Sama'il and Masirah-that contrast widely in formation setting, spreading rate, detachment level, thermal state during emplacement, and relief. Masirah beach sand chiefly consists of basaltic, diabase to metadiabase, plagioclase, gabbroic, and dunite grains from progressively deeper-seated levels of the multilayered oceanic crust. Dense minerals include diopsidic clinopyroxene and hornblende from gabbros, epidote and tremolite/actinolite from altered dikes, and augite from basaltic lavas. In contrast, sand from the Sama'il ophiolite includes cellular serpentinite grains and enstatite from residual mantle harzburgites, with subordinate olivine and minor chrome spinel. Diopside, hornblende, and hypersthene are provided by gabbroic rocks. Supply from upper-crustal levels is subordinate. Mafic to ultramafic detritus shed by obduction orogens virtually lacks quartz, K-feldspar, and metasedimentary rock fragments, thus contrasting radically with detritus from continental-collision orogens. Only when and where erosion bites beneath the oceanic nappe may polycrystalline quartz to metapelite and metafelsite grains be supplied by the metamorphic sole (developed at the base of young ophiolites) or by subducted continental-margin rocks. Outer- to inner-continental-margin successions exposed in tectonic windows provide additional chert, shale to slate, limestone, dolostone, and quartzose sandstone grains. Subducted ophiolites or blueschist melange along the suture zone of Alpine-type collision orogens also shed ultramafic grains, but their pervasively foliated, interpenetrating to interlocking textures are readily distinguished from the dominantly pseudomorphic mesh textures that characterize the serpentinite grains derived from obducted mantle slabs.