TECTONICS OF A PAIRED OPHIOLITE BELT IN ALBANIA AND ITS SIGNIFICANCE FOR THE MODELS OF OPHIOLITE GENESIS

The Jurassic Neotethyan ophiolites in Albania form two ophiolite sequences with different internal structure and chemical compositions and constitute a paired ophiolite belt bounded on both sides by a conjugate passive margin pair. The western zone of the paired belt includes serpentinized peridotites (lherzolite, dunite), plutonic rocks (troctolite, wehrlite, gabbro, ferrogabbro), and massive to pillow lavas in a <6km-thick sequence. Extrusive rocks locally rest directly on the serpentinized peridotites. The eastern zone is ~ 10-km-thick and includes mainly harzburgite, dunite, gabbronorite, olivine gabbro, diorite, plagiogranite, sheeted dikes, and massive to pillow lavas. Contacts between the extrusive rocks and the sheeted dike complex (SDC) and the gabbros and SDC are generally intrusive, reminiscent of a typical Penrose-type ophiolite pseudostratigraphy. The basaltic-basaltic andesitic lavas and gabbroic rocks in the western zone have a MORB chemistry, whereas the lavas and dikes in the eastern zone, ranging in composition from basalts, basaltic andesites, andesites, and dacites, display suprasubduction zone chemistry. Boninitic dikes crosscut tilted dike swarms representing the latest dike intrusions within SDC. Generally NE-running sheeted dikes are crosscut by dike-parallel and dike-orthogonal, brittle to cataclastic extensional faults that are locally mineralized. Lensoidal plagiogranite plutons form late intrusions within SDC that are spatially associated with hydrothermal zones and epidosite formations. The internal structure of SDC suggests NW-SE-oriented extension manifested in faulting and magmatism keeping pace with the seafloor spreading in the Mirdita-Pindos ocean basin. The tectonic setting of this spreading was an embryonic arc developed in an extended forearc environment, as inferred from the progressive change in chemical compositions of the upper crustal rocks towards more silicic end-members in space and time. Thus the paired ophiolite belt and the continental margin rocks in Albania appear to display a record of tectonic and geochemical evidence indicating a transition in the evolutionary history of the Mirdita-Pindos ocean from an initial rift basin to an extended arc-forearc basin in a convergent margin setting within the Neotethyan oceanic realm.