Volcanic rocks, locally up to 700 m thick, occur within a complex imbricate zone beneath the Upper Cretaceous Semail Ophiolite nappe in the northern Oman mountains. Although the rocks were considerably disrupted during late Cretaceous thrusting and nappe emplacement, intact sequences occur and show that there is a lower unit of alkaline lavas and pyroclastics, including ankaramites, alkali basalts and trachytes, and an upper one of predominantly tholeiitic pillow lavas. The latter contain interbedded cherts and limestones and well as large blocks of Permian and Triassic ‘exotic’ limestones. In places the volcanics are intruded by minor intrusives of alkaline peridotite and gabbro.

Geochemical studies, particularly of the ‘immobile’ elements, show that the lower volcanics and the intrusives are strongly alkaline with high Ti, P, Zr and Nb contents and steep LREE enriched rare earth patterns. They are typical ‘within-plate’ alkaline magmas characteristic of continental rift zones and some ocean islands. Within the tholeiitic basalt lava pile two types are recognized: a predominant relatively trace element enriched ‘transitional’ type which probably formed in a within-plate oceanic setting, and a ‘depleted’ type with the geochemical characteristics of some island arc tholeiites.

K-Ar ages of biotite separates from the alkaline lavas give Triassic ages (230-200 Ma) while one of the alkali periodotite sills gives an Upper Cretaceous age (92 Ma). The bulk of the tholeiites are late Triassic as they are interbedded with or enclose the Upper Triassic ‘exotics’. The Triassic volcanics are interpreted as having formed during continental rifting and then as marginal ocean islands during the early stages of ocean basin development. However, preliminary radiolarian ages from interbedded cherts in the uppermost lavas are Cretaceous. The presence of these island-arc type tholeiites is taken as evidence that subduction off the Oman continental margin began prior to ophiolite emplacement in the late Cretaceous.

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