SummaryK–Ar ages of biotites from a variety of alkaline volcanics and minor intrusive rocks in the nothern Oman mountains allochthon give a range of ages from Triassic (230 Ma) to mid Cretaceous (92 Ma) and represent igneous activity on the Oman continental margin throughout the Mesozoic. This was a passive margin destroyed by the emplacement across it of a pile of nappes in the late Cretaceous, including a largely intact thrust sheet of Upper Cretaceous oceanic lithosphere (the Semail ophiolite). Biotite ankaramite dykes, cutting compositionally similar volcanics, in the thrust complex immediately beneath the ophiolite, give Triassic ages and are related to the rifting and break-up of the northeast Arabian margin at the beginning of formation of the Oman Tethys. Mid Cretaceous (Cenomanian–Turonian) ages are mostly recorded from the northern part of the mountains where there are alkaline tuffs in a sedimentary melange. They are approximately the same age as the ophiolite and may be related to tectonic instability of the Oman margin immediately prior to ophiolite emplacement. Alkaline sills, intrusive into a variety of rocks, including Triassic volcanics, give Jurassic and Cretaceous ages and are interpreted as periodic alkaline activity on the Oman margin throughout passive margin development.
Petrography and provenance of Upper Cretaceous – Palaeogene sandstones in the foreland basin system of Central Nepal
