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The Semail Ophiolites

Prof. Dr. Sobhi Nasir

Department of Earth Science, Sultan Qaboos University

Semail ophiolite/(Oman)Geotourism: Geological excursions in the Oman Mountains

Ophiolites are very common in the Middle East They occur in elongate belts that make up an integral part of the Alpine mountain chains. These ophiolites extend eastward and southoward from Cyprus into Syira, the Turkey-Iran boerder fold belt, through Neyriz in Iran , then across the Arabian Gulf into Oman. The Semail ophiolite , Sultanate of Oman, is part of these ophiolits. It provides the best exposure in the world to study oceanic lithosphere. The Semail ophiolite crops out in a belt 600 km long and 150 km wide and between 5 and 10 km thick. The Sultanate of Oman forms the southeastern corner of the Arabian Plate. It can be divided into five structural elements: The Arabian Paltform; the Huqf-Haushi Uplift; the Oman Mountains; the Masirah Ophiolite Uplift and the Gulf of Oman.

The Arabian Peninsula, which was earlier attached to Africa, comprises a Precambrian basements (Proterozoic) that is overlain by a thick Phanerzoic shelf succession (5500 m thick).

The Huqf-Haushi Uplift, southeastern margin of Oman, trending NE-SW, formed due to the subsidence of the Ghaba Salt Basin to the west and the Masirah Trough to the east. Uplifting started in the Infra-Cambrian and continued until the Late Cretaceous.

The Oman Mountains represent the the margin of the Arabian continental paltform. They stretches from the Strait of Hormuz in the northwest to the Arabian Sea in the southeast. The mountains contain a great thickness of authochthonous shelf carbonate rocks. The mountains are geogically distinct from the rest of the Arabian Peninsula because of the presence of extensive nappes of pleagic sedimentary deposites and ophiolites which overly the shelf carbonate rocks tectonically.

The Oman ophiolite is a remnant of the Tethysian oceanic lithosphere obducted on the Arabian continental margin during the Maastrichtian (around 70 Ma). The obduction followed an intra-oceanic detachment which occurred 95-100 Ma ago).

The geology of the Oman mountains has been widely investigated and several reports and papers have been published. Seven major rock units were defined, which from bottom to top are:

1- Pre-Permian basement.

2- Mid-Permain to Mid-Cretaceous Hajar Supergroup (shelf succession).

3- Turonian to Maastrichtian Aruma Group (syn-tectonic sediments related to emplacement of the Oman passive margin).

4- Permian to Late Cretaceous Sumeini Group (carbonate plateform slope sediments).

5- Permian to Late Cretaceous Hawasina Group (tectonically sliced basinal sediments).

6- Cretaceous Semail ophiolite (a slap of oceanic crust and mantle)

7- Late Maastrichtian and Early Tertiary neoautochthonous limestone and basal clastics

The geologists divided the Oman Mountain rock units into four major groups:

1-An authochtonous Group: Pr-Permain crystalline basements are exposed at Sih Hatat and Jabal Akhdar. They contain Hercynian metasediments and metavolcanic The Pre-Permian metamorphic rocks are overlained by about 3700 m of middle Permian to middle Senonian shallow marine shelf carbonates. These rokcs are referred to as the Hajar Supergroup. Exposure of the basment are limited to structural highs within the mountains (e.g. Jabal Ja’alan). The Aruma Group is represented by Tournain to Maastrichtian sediments related to Late Cretaceous obduction of the nappes.

The Hawasina Group: The group is represented by deep sea limestone, radiolarian chert, shale with volcanics. These rocks were deposited parallel with the Hajar Supergroup. The Sumeini Group is represented by Permain to Late Cretaceous carbonate slope deposits.

3-The Semail Ophiolite: A tick allochthonous sequence of peridotite, gabbro, diabase, pillow lava and pelagic Cretaceous sedimentary rocks.

4- Post-ophiolite authochthonous: These comprise the Upper Maastrichtian-Lower Tertiary transgressive shallow marine limestone and basal clastics and an Upper Tertiary-Holocene limestone, terraces and wadi alluvium.

The Masirha Ophiolite was upthrust in the Early Cretaceous during the northward movement of the Gondwanaland away from the Oman coast.

Stratigraphy of the Semail Ophiolite

The Semail Ophiolite can be broadly divided into two major units

1 - Mantle Sequence: The sequence represent the upper sub-oceanic mantle. It is composed of tectonized harzburgites (85-95 %), lherzolites and dunites (5-15%). The sequence may reach a thickness of 10-12 km. The sequnce is being cut by numerous dukes, veins and bods of ultramafic and mafic rock types. The primary silicates are usually altered to lizardite and chrysotile. The dunites locally contain chromatite pods. The contact between the mantle sequence and the overlying crustal layers is marked by a structural and petrological boundary which is taken to represent the sub-oceanic "petrological Moho".

2 - Crustal Sequence: Consisting of a layered series (cumulate peridotites and gabbros) , which is overlain by non-layered plutonic rocks (high-level Intrusives), a sheeted Dyke Complex and an Extrusive sequence of lavas which is interbedded with and overlain by pelagic sediments. The sequence varies in thickness between 4 and 9 km. The layered series of the Semail Ophiolites consists of layered peridotites (dunite and wehrlite 25 %) and gabbros (75 %) The series vary in thickness between 0.5-6 km. They rest disconformably on the petrologic Moho. They are characterized by rhythmic layering on scale of 0.5 cm to 2 m. The cyclic layering points to open system fractionation. The overlying high level gabbros are characterized by the absence of layerig and variable texture. They form a discontineous unit up to 700 m thick. They are characterized by medium-grained hypidiomorphic to ophitic textures. The gabbros grade in turn into diorites to trondhjemites. Late intrusive complexes composed of peridotites, gabbros, diorites and plagiogranites intrude up into the upper crustal units of the ophiolite. These complexes were subdivided into an older series of differentiated gabbro to plagiogranite plutons and a younger group of peridotite-gabbro intrusions. The older group composed mostley of layered and massive gabbros and diorites with few plagiogranite. The younger group consists of a completely differentiated series from wehrlite at the base, overlain by layered and non-layered gabbros and diorites and plagiogranite. The sheeted dyke complex occurs between the plutonic rocks and an overlying volcanic sequence. The complex consists of near vertical dykes that have lava and gabbro in the upper and lower contact zones. The upper part of the Semail Ophiolite, that overlying the Sheeted Dyke Complex, consists of up to 2000 m of pillowed basaltic lavas. The metalliferous and associated pelagic sediments found in the upper parts of the ophiolite sequence, provide strong evidence for a deep-water origin for the lavas.

The base of the Semail Ophiolite is marked by a major thrust zone that separates the ophiolite and its basal metamorphic sheet from the underlying rocks. The metamorphic sheet forms a separate thrust slice attached to the base of the nappe and is composed of about 500 m of amphibolite to green shcist facies rocks which show a marked decrease in temperatre from top (~800oC) to the base (~450oC)

The Oman Tehtys (Neo-Tethys) appears to have formed in the Early Triassic by the rifting off and migration of small continents (e.g., Iran, Tibet, Afghanistan) northward from the remainder Gondwanaland. The Triassic marks the first appearnance of a deep water basin in which the pelagic Hawasina sediments were deposited. The Haypi Volcanics started int eh Late Triassic and shows a trend from alkaline through transitional to later tholeiitic compositon. Quartz-rich turbidites were deposited in the Hawasina basin in the Mid-Jurassic. In the Early Cretaceous there were deep-water conditions across the whole continental margin. In the mid-Cretaceous tectonic instability was reflkected by uplift and erosion and the formation of sedimentary melanges. The Semail Nappe represents a slice of oceanic lithosphere that was formed above a northward-dipping subduction zone in a marginal basin tectonic setting. The nappe was attached by intr-oceanic thrusting int he Turonia and was then obducted southwestward over the Hawasina and Haybi allochthons onto the continental margin in the Santonian to Campanian.

Case studies

- New Carbonatite Occurrences within the Semail Ophilite


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