Geologic section through the Samail Ophiolite and associated rocks along a Muscat‐Ibra Transect, southeastern Oman Mountains
Authors: Hopson C. A. , Coleman R. G. , Gregory R. T. , Pallister J. S. , Bailey E. H. 
Published: February 2008

Regional mapping at a 1 : 60,000 scale of a 30‐km strip from the Gulf of Oman (Muscat) across the Oman Mountains, 130 km to the south, provides the geologic setting for the (∼95 m.y.) Ibra section of the Samail ophiolite. Where best preserved, the Ibra ophiolite section is an ∼8 km‐thick section of oceanic consisting of ∼0.5 km of pillow lavas, 1.2–1.6 km of sheeted diabase dike complex, 0.2–1.0 km of high‐level noncumulate gabbro, and 3.0–5.0 km of cumulate gabbro that is underlain by tectonite peridotite 9–12 km thick. The Ibra section is found on the southward dipping limb of the Sayah Hatat antiform. The tectonite peridotite represents uniformly depleted harzburgite and dunite that have been deformed by high‐temperature, low‐stress asthenospheric flow. Discordant dunites within the tectonite peridotite appear to represent either flow crystallization products from primary picritic liquids or reaction products of these liquids with the harzburgite. The structural base of the tectonite peridotite is overprinted by a high‐stress, low‐temperature deformation that can be related to its oceanic detachment. The layered gabbros are predominantly olivine‐clinopyroxene‐plagioclase cumulates, and orthopyroxene does not occur as a cumulus phase. Occurrence of cumulate wehrlites and picrites at high stratigraphic levels within the layered gabbros is evidence that the gabbroic section crystallized predominantly from the bottom upward in a periodically replenished magma chamber. High‐level gabbro represents remnants of crystallization at the roof of the magma chamber and intrudes most overlying diabase dikes. Both the diabase dike complex and pillow lavas are hydrothermally altered, and alteration and metamorphism increase downward (zeolite (?) to epidote‐amphibolite facies). In spite of pervasive alteration, relict primary mineralogy and bulk chemistry suggest that the diabase dikes and pillow lavas are cogenetic with the underlying gabbros. The present‐day Samail thrust surface truncates ophiolite stratigraphy and puts the Samail ophiolite on top of unmetamorphosed Hawasina melange. The last motion on this surface was probably no older than Maestrichtian (70–65 m.y.). Garnet amphibolites exposed as remnants of earlier thrusting (∼90 m.y.) record initial ophiolite detachment at a 14–20‐km depth within the Tethyan oceanic lithosphere. The Hawasina Group, which underlays the Samail ophiolite, is a block melange where exposed near Muscat in the north, and it grades into an imbricated broken formation at the southern limit of the map area. The Hawasina Group is thrust over Permian to Late Cretaceous shelf carbonates that represent autochthonous Arabian continental shelf deposits. Recent (post‐Miocene) collapse and dome structures, such as the Ibra dome, have complicated ophiolite stratigraphy south of Jabal Dimh. Our geologic studies strongly indicate the Samail ophiolite represents a large, coherent slab of transported oceanic lithosphere formed at a Late Cretaceous spreading center in the Tethyan Sea.