THE MANTLE-CRUST TRANSITION ZONE IN THE MIRDITA OPHIOLITE, ALBANIA

Dinaro – Hellenic Alpine fold belt is a mountain range which extends North-South from Slovenia to Greece, and is interpreted as the result of a convergence/collision between the Eurasian and Adriatic plates in Mezosoic– Tertiary times following the closure of the Tethys Ocean. Albania is located in the Dinaro-Hellenic Alpine fold belt and exposes three major geological domains, from West to East, (1) a fold-and-thrust belt of sedimentary rocks derived from the Adriatic continental margin; (2) a central domain characterized by a major ophiolitic nappe occupying more than 4000 Km², the Mirdita ophiolite; and (3) the Korabi – Pelagonian Zone, which is an eastern complex of Ordovician-to-Devonian, Hercynian basement of low-grade metamorphic rocks overlain by a Permian-Jurassic rift. The Mirdita ophiolite is a Jurassic oceanic remnant of neo-Tethys. It was only slightly affected by the Alpine orogeny during the Tertiary and thus displays structures that are mostly related to its primary oceanic setting and subsequent emplacement onto the Adriatic plate. In Albania, the Mirdita ophiolite consists of two segments of oceanic rocks, referred to as the Western-type and Eastern-type ophiolite belts and which were interpreted as typical LOT- and HOT-type ophiolites, respectively. The mantle sequence of the Eastern-type ophiolite belt is composed of harzburgite passing up-sequence into a transition zone and overlying crustal sequence. The transition zone is mainly characterized by ultramafic cumulates (?) (dunite, wherlite, websterite, pyroxenite) overlain by a crustal gabbro sequence showing well-developed igneous layering. The nature of the transition zone (residual or cumulative rocks?) and its boundary with the underlying mantle sequence are currently debated. This study aims to precisely determine the origin and composition of ultramafic rocks of the transition zone by detailed petrographic, microprobe and geochemical analyses (trace elements and REE).