Geologic section through the Samail Ophiolite and associated rocks along a Muscat‐Ibra Transect, southeastern Oman Mountains

Abstract

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.

Spatial distribution of melt conduits in the mantle beneath oceanic spreading ridges: Observations from the Ingalls and Oman ophiolites

“…Previous work [e.g., Bishimetal Exploration, 1987;Hopson et al, 1981], combined with the width/frequency data and observed sizes of dunites presented here, shows that tabular dunite veins more than 1 m wide are common in ophiolite mantle sections and thus can satisfy the requirement for chemically isolated transport of melt through the shallow mantle. Our width/frequency data suggest that dunites may have been part of a coalescing channel network.…”

“…Thus the formation of a coales- All widths were determined by Kelemen and Braun via chain and compass mapping of large tabular dunites that are completely surrounded by residual, mantle harzburgite, more than 1 km structurally below the crust/mantle transition zone. The best exposed, large dunites we have mapped are (1) in the area between Muscat and Muttrah, but not in laterally continuous parts of the same mantle massif to the south, between Al Bustayn and Ruwi (our ongoing work), (2) in the eastern Wadi Tayin massif several kilometers structurally below the MTZ, but not in the western half of the massif at the same depth below the MTZ [Hopson et al, 1981], and (3) near the base of the Hilti massif, $5 km structurally below the MTZ, but not at shallower depths below the MTZ [Bishimetal Exploration, 1987].…”

An oxygen isotope profile in a section of Cretaceous oceanic crust, Samail Ophiolite, Oman: Evidence for δ18O buffering of the oceans by deep (>5 km) seawater‐hydrothermal circulation at mid‐ocean ridges

“…The geology of this area is discussed in detail by Hopson et aL [1981], so only those geological relationships necessary for this discussion will be reviewed here.…”

“…Maestrichtian shales from the underlying melange require that last movement of the Samaft nappe is probably no older than 65-71 m.y. [Hopson et al, 1981].…”

The Northern Samail Ophiolite: An Oxygen isotope, microprobe, and field study

“…This is the first of a series of studies by us on the oxygen isotope and mineralogical relationships in the northern half of the Samail ophiolite, arguably the largest, best exposed, and most complete mass of oceanic lithosphere amenable to geologic study anywhere on Earth [e.g., Glennie et Hopson et aL, 1981). Field studies and sample traverses were undertaken beginning in 1981 and continuing in 1983 and 1985 in order to obtain an appropriate suite of rocks that would aUow us to determine the isotopic variability within a number of c5 18 0 versus depth profiles along the strike of this large fragment of oceanic crust.…”