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 baseme nt. The sheeted dike complex strikes NS and dips steeply, indicating that t he ridge was oriented parallel to the NS corridor and that the ophiolite ha s not been significantly tilted, although differential motion between indiv idual massifs cannot be excluded. Classically, the western massifs have bee n considered "lherzolitic" and the eastern massifs "harzburgitic". Detailed structural mapping reveals that the deep mantle section was harzburgitic a nd that the major differences between the two are restricted to the uppermo st mantle and lower crustal section. These are typically "ophiolitic" in th e eastern massifs, being composed of a thick dunitic transition zone rich i n 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 fro m more depleted harzburgites by impregnation and tectonic dispersion of mel t during deformation occurring at 1000 degrees-800 degrees C. Layered gabbr os are locally absent, and the crust can be reduced to diabase dikes or sil ls and extrusives. The diabase intrusions are locally sheared together with peridotites and metamorphosed to amphibolites. The contrast between the ea stern and western massifs is ascribed to successive episodes of magmatic an d amagmatic spreading in a slow spreading environment. The low-T, high stra in deformation of the western massifs is localized in the dome-shaped envel opes of these massifs. This structure, and even the present-day topography of the western massifs, evoke the "turtleback" domes described along the Mi d Atlantic Ridge (MAR) and explained by mantle denudation [Tucholke et al., 1998].