STRUCTURAL EVOLUTION OF THE NORTHERN END OF THE OMAN OPHIOLITE AND ENCLOSED GRANULITES

A combined structural field mapping, microstructural and petrographic study has been undertaken to better understand the origin and evolutio n of the northernmost part of the Oman Ophiolite (Khawr Fakkan massif) in the framework of an oceanic spreading centre. Emplacement of a she et of granulite facies metamorphic rocks into the peridotites occurred at temperatures at which the peridotites deformed ductily. The metamo rphic rocks define the southern extension of a block boundary which se parates an eastern peridotite part displaying high-temperature fabrics from a western one with porphyroclastic to mylonitic fabrics. Granito id intrusions representing partial melts from the metamorphic rocks oc cur only in the ophiolitic part overlying the metamorphic rocks, thus delineating the same boundary between the two mantle blocks. This high -temperature boundary is not visible on satellite or aerial photograph s. The granulite facies foliations and lineations in the metamorphic r ocks can be correlated with porphyroclastic textures developed at decr easing temperatures in the associated peridotites and to the obduction of the ophiolite. The final emplacement of the metamorphic rocks in t heir present position was accompanied by retrograde hydration, decompr ession and continuous deformation at temperatures ranging from 800 to 300 degrees C. The hydration is extended along the Wadi Ham fault. Two models are proposed for the emplacement of the metamorphic rocks. In the first model the protolith of the metamorphic rocks was deposited i n a pull-apart basin along a transform fault and then subducted along the transform fault after reversal from transtension to transpression. In this model, the ophiolite obduction occurred later along another t hrust plane. In the second model, the protolith material descends alon g an intraoceanic subduction zone and is emplaced into the ophiolite h anging wall along a secondary thrust caused by transpressive or rotati onal movements during the ophiolite obduction. In this case the ophiol ite obduction occurred along the same intraoceanic subduction plane.