Mid-Ocean Ridge and Suprasubduction affinities in the Ophilitic Belts from Albania

Petrological data based on 147 new major and trace element analyses are presented as a preliminary result of the research program on Jurassic ophiolites from Albania. Albanide ophiolites of the Mirdita tectonic zone represent one of the most complete, coherent sequences among all the Mediterranean ophiolites, and are constituted by two NNW-SSE trending subparallel belts. The Eastern Belt is mainly represented by mantle tectonites, predominantly harzburgitic in composition (clinopyroxene: 3-0 %; Kukes, Bulqize, and Shebenik Massifs, from north to south). Westward, tectonites gradually give way to a cumulitic intrusive sequence - particularly well-developed in the northern part - consisting of dunites, (plagioclase-) lherzolites, (olivine-) websterites, (mela-) gabbronorites (with minor troctolites), quartz-diorites and plagiogranites. Accordingly, their crystallization order is olivine + chromite, followed by pyroxenes and then plagioclase. Chromititic layers, pods and lenses - always associated with dunites - occur both within mantle harzburgites and in the ultramafic lower portion of the cumulitic sequence. The associated volcanic suite consists of low-Ti and, more rarely, very low-Ti basalts and basaltic andesites, andesites, dacites, and rhyolites. Sheeted dike complexes occur mainly in the lower part of the volcanic formation. The Western Belt mainly consists of mantle tectonites, lherzolitic in composition (clinopyroxene: 13-5 %), passing upward to cumulitic (plagioclase-) dunites, (plagioclase-) wehrlites, troctolites, (mela-) gabbros, Fe-gabbros, and very scarce plagiogranites. The crystallization order is therefore olivine + chromite, followed by plagioclase and then clinopyroxene. The associated volcanics consist of high-Ti basalts with MORB affinity. Strongly-tectonized metabasites, showing MORB affinity and metamorphosed into greenschist and amphibolite facies, are locally wedged within the ophiolitic units, as well as at their eastern and western borders. In conclusion, the Western Ophiolitic Belt reveals pure MORB affinity, as indicated by the crystallization order and nature of cumulate intrusives, the less depleted composition of mantle tectonites, as well as the geochemical features of associated high-Ti basalts. Its original setting is therefore to be considered an oceanic spreading system, without any influence of subduction-related processes. By contrast, for the Eastern Ophiolitic Belt, the petrological characteristics of the cumulitic sequence and the underlying strongly depleted mantle tectonites, together with geochemical features of the lavas, coherently indicate a suprasubduction generation of parental magmas. The generation of such a magmatic system implies intra-oceanic subduction of a pristine lithosphere, whose remnants could be represented by the Western belt ophiolites as well as by the associated metabasites with MORB affinity. A paleogeographic reconstruction of the Mirdita oceanic basin during the Jurassic should therefore imply a previous creation of MORB-type oceanic lithosphere between the newly formed continental margins after the Triassic rifting phase; later on, an intraoceanic converging plate system would have developed on the eastern side of this basin, with the generation of a suprasubduction oceanic crust and underlying harzburgitic refractory mantle residua. The relatively undepleted lherzolite tectonites of the Gomsiqe, Krrab and Tropoja massifs, parallel to the Shkoder-Pec line, could represent, at least in part, subcontinental upper mantle material protruded and exposed along the bordering transcurrent system of the oceanic basin to the north.