Rift-Drift, Seafloor Spreading, and Subduction Tectonics of Albanian Ophiolites

摘要

Albanian ophiolites in the Dinaride-Albanide-Hellenide mountain system in the Balkan Peninsula represent oceanic remnants of the Mesozoic Pindos-Mirdita basin and structurally overlie the peripheral tectonic units, composed mainly of volcanosedimentary rift assemblages and conjugate passive margin sequences. These tectonic units and the ophiolites collectively record the rift-drift, seafloor spreading, and subduction evolution of the Pindos-Mirdita basin, which developed as a Tethyan seaway between the Apulian and Korabi-Pelagonian microcontinents. The peripheral tectonic units on the west include, from west to east, the Apulian platform (Sazani, Ionian, and Kruja zones), Apulian passive margin sequences (Krasta-Cukali Zone), volcanosedimentary unit (Triassic-Upper Jurassic), subophiolitic melange, and metamorphic assemblages; the Korabi-Pelagonian platform and its passive margin sequences bound the ophiolites on the east. Metamorphic assemblages include amphibolite, micaschist, and marble that generally display inverted field gradients, analogous to metamorphic soles beneath other ophiolites in the eastern Mediterranean region. Some metamorphic assemblages with cooling ages >169 Ma in southern Albania may represent, instead, dynamothermally metamorphosed volcanosedimentary rocks developed during advanced stages of continental rifting, prior to the onset of seafloor spreading. The subophiolitic melange contains blocks and clasts of material derived from ophiolites. metamorphic units, platform carbonates, and passive margin sequences, and represents a subduction-accretion complex formed during initial displacement and tectonic emplacement of the ophiolites. The supra-opholitic melange and the overlying flysch deposits (Tithonian-Valanginian) and neritic limestones (Barramian-Upper Cretaceous) unconformably rest on the ophiolites and the platform carbonates, and constrain the timing of ophiolite emplacement onto the Korabi-Pelagonian continental margin as the latest Jurassic-Early Cretaceous. The Pindos-Mirdita basin initially developed as a broad Iranstentional, intra-continental rift system with highly oblique, ultraslow rifting and spreading. Ophiolites in the Mirdita region show a transition from MORE to IAT and boninitic affinities from west to east and structurally upwards, indicating magma genesis from highly depleted, refractor)' mantle sources increasingly affected by subduction-derived fluids. These spatial, temporal, and chemostratigraphic relations point to a suprasubduction zone origin of the eastern ophiolites above a west-dipping, intra-oceanic subduclion zone, whose rollback resulted in lithospheric extension and the formation of a boninitic "proto-arc" in the upper plate. This subduclion-accretion system was terminated by collision of the Korabi-Pelagonian margin with the trench that facilitated eastward emplacement of the ophiolites. Subsequent oblique collision of Apulia with Eurasia during the Maastrichtian-Paleogene transported the Albanian ophiolites westward onto the Apulian margin. The bidivergent emplacement tectonics of the Albanian ophiolites is hence a result of two separate collisional events at different times.