Peridotites beneath the ophiolite: Petrology and chemistry of peridotite bodies located below the metamorphic sole of the Oman ophiolite

Abstract:

The Oman ophiolite is a large slab of Tethyan ocean floor comprising one of the largest piece of exposed oceanic lithospheric peridotites. The ophiolite is the last of several nappes thrusted onto the autochtonous units (Saih Hatat and Jebel Akhdar).

The Taww region is located at the northernmost border of the Jebel Akhdar, south to the Kahwad ophiolitic massif. It is characterised by the presence of small peridotite occurrences that are in direct contact with the Jebel Akhdar autochthonous units and located below the Hawasina nappes and the metamorphic sole. Similar peridotite bodies are found in various region around the Jebel Akhdar and the Saih Hatat. They crop out as a few meter to a few kilometre wide massifs, most of the time highly serpentinised or carbonated and deformed. They are characterized by the absence of magmatic veins and dykes, their stratigraphic location below the metamorphic sole of the ophiolite, and when found as ophiolitic melange, by the inclusion of limestone blocks. In these 3 points, they differ from the ophiolite mantle main body, which is located above the Hawasina units and metamorphic sole, contains ubiquitously magmatic dykes and veins, and when observed as ophiolitic melange, contains blocks of amphibolite and/or pelagic sediment from the Hawasina nappes.

The ophiolitic basal peridotites are mostly high depleted harzburgites containing 1 to 7% of Cpx (mostly from refertilised sources) with granular and porphyroclastic textures. The Taww peridotites seem to contain slightly less Cpx and more Opx but their textures are basically very similar to the usual ophiolite peridotites.

The Taww peridotites, like the ophiolite peridotites, are residual harzburgites but their chemical characteristics show lower Cr-spinel Cr# and higher Al content in Opx, suggesting a melting history different from the ophiolite peridotites. The Taww peridotites were not a part of the Tethys ocean mantle but they are more probably a piece of the mantle that have underlied the Arabian thinned margin.