MIRDITA OPHIOLITE (ALBANIA) AS A REMANT OF AN EARLY JURASSIC BACKARC SPREADING CENTER–EXTENSIONAL TRANSFORM ZONE INTERSECTION

The Mirdita ophiolite occurs in the western branch of the Dinaride–Albanide–Hellenide ophiolite belt, which escaped much of the Alpine deformation events possibly due to a thick, in-situ ophiolitic basement. It covers an area of 250 x 40 km² extending to the south by the Pindos and Othrys ophiolites of the Helenide belt and to the north by the Rahovec ophiolite (Kosove) of the Dinaride belt. The Mirdita nappe displays an eastward increasing thickness, and exposes, in a synform structure, mantle rocks along two peripheral eastern and western zones; a discontinuous amphibolitic sole delimits both peridotite zones. The central part of the nappe exposes a crustal section composed of gabbroic plutons and a poorly organized sheteted dyke complex. Previous studies have distinguished western “lherzolitic” massifs that are juxtaposed against the eastern “harzburgitic” massifs in the Mirdita ophiolite. However, our detailed mapping in the last decade has revealed that the mantle section in a structurally deeper level across the entire Mirdita ophiolite is largely harzburgitic and that the major differences in ophiolitic units are restricted to the uppermost mantle and the lower crustal section. In the western massifs, the plagioclase-lherzolite is spatially associated with higly deformed, mylonitic peridotites. Clinopyroxene and plagioclase in them were introduced by tectonic dispersion of gabbroic dykes and melt impregnation patches. Gabbroic rocks underwent strong plastic deformation during tectonic extension. The upper mantle peridotites in the western massifs are locally directly overlain by doleritic dikes and volcanic rocks of the upper crust. The sheeted dyke complex seems to have been variously developed and locally reduced in thickness in the western massifs. The eastern massifs are composed of harzburgite with locally thick dunitic and chromite–rich Moho transition zones. The best exposed transition zone occurs in the northen part of the Kukes massif, which displays a typical lower crust composed of layered olivine–gabbro and norite plutons. The layered gabrro grades upsection into foliated and isotropic, hydrothermally altered amphibole gabbro and a sheeted dyke complex, which consists of basalt, rhyodacite and plagiogranite dikes. The extrusive unit is well developed, consisting of basalt to basalt–andesite lavas and pillow lavas, overlain by rhyodacitic and boninitic extrusives.

This contrast in crustal architecture and rock units in the Mirdita ophiolite is an artifact of the two linked spreading segments of the Mirdita backarc basin formed in the early Jurassic: the NW-SE–trending extensional transform zone in the north and the NE-SW– trending Mirdita spreading center in the south. We infer that the oceanic lithosphere of the Mirdita backarc basin originated from a right–lateral pivoting motion between the Korab–Pelagonian and Apulian microcontinents. The Mirdita spreading center formed a wedge varying in width from 60 km at its northwestern end to zero at its southeastern tip.