Slow spreading accretion and mantle denudation in the Mirdita ophiolite (Albania)

The Mirdita ophiolite in Albania occupies a N‐S corridor which escaped most Alpine and Cenozoic deformation, possibly due to a thick ophiolitic basement. The sheeted dike complex strikes NS and dips steeply, indicating that the ridge was oriented parallel to the NS corridor and that the ophiolite has not been significantly tilted, although differential motion between individual massifs cannot be excluded. Classically, the western massifs have been considered “lherzolitic” and the eastern massifs “harzburgitic”. Detailed structural mapping reveals that the deep mantle section was harzburgitic and that the major differences between the two are restricted to the uppermost mantle and lower crustal section. These are typically “ophiolitic” in the eastern massifs, being composed of a thick dunitic transition zone rich in basaltic impregnations and chromite deposits overlain by a lower crust of layered gabbros. In contrast, in the western massifs the uppermost mantle is composed of highly strained to mylonitic lherzolites which originate from more depleted harzburgites by impregnation and tectonic dispersion of melt during deformation occurring at 1000°–800°C. Layered gabbros are locally absent, and the crust can be reduced to diabase dikes or sills and extrusives. The diabase intrusions are locally sheared together with peridotites and metamorphosed to amphibolites. The contrast between the eastern and western massifs is ascribed to successive episodes of magmatic and amagmatic spreading in a slow spreading environment. The low‐T, high strain deformation of the western massifs is localized in the dome‐shaped envelopes of these massifs. This structure, and even the present‐day topography of the western massifs, evoke the “turtleback” domes described along the Mid Atlantic Ridge (MAR) and explained by mantle denudation [Tucholke et al., 1998].