Geology and petrology of ophiolitic sequences in the central Mirdita region (northern Albania)

In Albania, the ophiolite sequences belonging to the Mirdita Nappe, are well represented in the Mirdita region. The Albanian ophiolites are subdivided into two NNW-SSE trending subparallel belts: the Western and the Eastern ones. These belts show different stratigraphical, petrological and geochemical characteristics, which suggest, in turn, two different original geodynamic settings. The majority of the western belt ophiolitic sequences represent a Jurassic oceanic lithosphere generated at midge-oceanic ridge (MOR ophiolites) without any influence of subduction-related processes. Hower, in the central Mirdita region, pillow lava basalts showing low- to high-Ti intermediate geochemical characteristics, as well as very low-Ti basaltic dykes are also found in the easternmost sector of the western ophiolitic belt, suggesting that the influence of subduction-related processes played a certai role in this area. By contrast, the occurrence of the eastern belt ophiolites having a well developed hazburgitic mantle sequence coupled to magmatic rocks showing IAT geochemical characteristics supports their interpretation as a record of an oceanic basin developed over a supra-subduction zone (SSZ ophiolites). The sedimentary cover of the ophiolites sequences is represented only by the Kalur Cherts, which show in both the belts the same age (middle Bathonian at the base to middle Bathonian - early Callovian at the top). In the Mirdita region, the western and eastern ophiolite sequences overlain the Rubik Complex, which consists of an assemblage of continental- and oceanic-derived thrust sheets. Between the ophiolite sequences and the Rubik Complex, slices of metamorphic sole, mainly represented by amphibolites, are recognized. The western and eastern ophiolites are, in turn, overlie by a thick sedimentary sequence, which includes the Tithonian - Valanginian Mirdita Melange and Firza Flysch. These deposits, which are found unconformably over the cherts and the basalts of western and eastern ophiolites, can be regarded as syn-orogenic sequences, deposited after the inception of the deformation of the ophiolites. At the top, the shallow-water, Barremian- Senonian carbonate sequence is regarded as the first post-orogenic deposit. The ophiolites and the associated sedimentary sequences are affected by a Middle Jurassic - Early Cretaceous sequence of deformations that include a development of a metamorphic sole followed by two phases of folding associated to large scale thrusting without metamorphism. The first phase is recognized only in the ophiolites sequences, whereas the second phase affects also the Mirdita Melange and the Firza Flysch. The shallow-water, Barremian- Senonian carbonate sequence is undeformed. This deformation history is interpreted as achieved during the obduction of the Albanian ophiolites, developed by a stage of intraoceanic thrusting followed by the emplacement of slabs of oceanic lithosphere onto the neighbouring continental margin.