Metamorphism at the base of the Samail Ophiolite, southeastern Oman Mountains

Abstract

Metamorphic rocks, showing an inverted metamorphic zonation from upper amphibolite facies to greenschist facies, occur beneath the Samail ophiolite. The metamorphic protolith was a section of marine basalts, Mn‐rich cherts, and argillites. Garnet‐clinopyroxene amphibolites occur within 2 m of the contact, hornblende‐clinopyroxene assemblages occur within 80 m of the contact, and lower amphibolite facies rocks persist to 130 m from the contact. The higher‐grade rocks show polyphase deformation, cataclastic fabrics, and partial retrogression to greenschist facies assemblages; however, lower‐grade phyllites show evidence of only one episode of metamorphic crystallization. Unmetamorphosed sediments in contact with the basal metamorphic sheet do not show evidence of polyphase deformation. The distribution coefficient KD = FeO/MgO gar/FeO/MgO cpx ranges from 4.4 to 5.3, suggesting a T range of 755°– 865°C at 200 MPa. Low jadeite content in clinopyroxene and low glaucophane content of Ca amphiboles are consistent with moderate pressures of crystallization. The metamorphic sheet is inferred to be of composite origin. The highest‐grade rocks were tectonically transported the greatest distance, and the lower‐grade rocks were successively incorporated into the sheet as the peridotite was thrust over progressively cooler rocks. Residual heat from the ophiolite is the dominant heat source for the metamorphism, and this could have been augmented by a limited amount of frictional heat generated during thrusting. These thermal constraints suggest that the metamorphism occurred early in the tectonic transport history of the Samail ophiolite.