The death of an accretion zone as evidenced by the magmatic history of the Sumail ophiolite (Oman)

Abstract

The ophiolitic crustal sequence of the Salahi block in the northern part of Oman shows a complex magmatic history. The oceanic accretion episode led to the formation of a main plutonic sequence consisting of layered cumulates, isotropic gabbros, a sheeted dike complex and a first tholeiitic lava series (V1) depleted in compatible elements. The second magmatic episode immediately following the accretion stage is illustrated by predominantly ultrabasic intrusions crosscutting the main plutonic sequence and by a second volcanic suite (V2) directly linked, as can be seen from field evidence, to more differentiated high-level intrusive plutons. This volcanic unit includes very primitive members and is in general characterized by depletion in incompatible elements. A third volcanic event (V3) exhibiting a transitional character enriched in LREE's. caps the ophiolite. These V3 lavas are separated from the V2 unit by a 15 m thick pelagic sedimentary section. A geotectonic evolution of the Sumail ophiolite is proposed on the basis of field, mineralogical and geochemical data. After the oceanic accretion phase that gave birth to the main ophiolitic sequence, a second-stage melting of the upper mantle is suggested to explain the genesis of the second magmatic episode. Such a model accounts well for the depleted character of the V2 lavas. The compression linked to the beginning of the intra oceanic detachment allows the extraction from the mantle of these second-stage melts which are then emplaced in the crust and partly extruded. The V3 volcanism postdates the intraoceanic detachment of the ophiolite, but precedes its obduction onto the Arabian continental margin. It is interpreted as an intraplate seamount-type magmatism.