The Semail ophiolite complex of the northern Oman Mountains was obducted during Cenomanian to Campanian (Upper Cretaceous) times onto the Arabian continental margin. During this time span, schistose metamorphic rocks of amphibolite and greenschist facies exhibiting polyphase deformation were formed and accreted below the ophiolite slice. Whole-rock and mineral analyses confirm that the amphibolites were originally tholeiitic basalts comparable to present day occurrences at mid-ocean ridges. A comparison of trace element geochemistries, however, suggests that the protoliths were not equivalent to the Haybi alkaline and transitional tholeiites found in the underlying structural slice, or to the Semail ophiolite volcanic rocks which lie structurally higher in the sequence. It is therefore suggested that the amphibolites may have been derived from Jurassic or Lower Cretaceous oceanic basalts not recognized elsewhere in the Oman Mountains.
Structurally beneath the amphibolites within the metamorphic sheet, greenschist facies metasedimentary rocks, including quartzites and marbles, are interbanded with metabasite horizons containing actinolite and, less commonly, blue crossitic amphiboles.
It is suggested that the sedimentary and volcanic protoliths were metamorphosed in a subduction zone environment during initial ophiolite displacement, and were accreted to the base of the ophiolite during progressive underthrusting.