Metamorphic soles from the Albanian ophiolites:: Petrology, 40Ar/39Ar geochronology, and geodynamic evolution

In the eastern Mediterranean region the Albanian ophiolites are remnants of one branch of the Tethyan oceanic basin obducted on carbonate margins during the Mesozoic. Pressure, temperature, time, and structural data in the metamorphic soles of these ophiolites place new constraints on the closure of this basin and the emplacement history of oceanic rocks. The petrological and structural work documents an "apparently inverted metamorphic gradient" with granulite facies at the top, decreasing downward through amphibolite, then greenschist facies until unmetamorphosed rocks. Thermobarometry indicates that the granulite facies formed at peak temperatures of 800 degrees -860 degreesC for pressures close to 1 Gpa. These conditions point to the subduction of the ophiolites beneath young oceanic lithosphere to a depth of 30-40 km. A large set of 40Ar/39Ar data has been obtained on the metamorphic soles and some ophiolite intrusives using single grains and bulk mineral populations. Ages range from 160 to 174 Ma, corresponding to the middle Jurassic (Bajocian-Bathonian). Within the same sole the convergence of results using single grains and mineral populations of amphibole and muscovite and the lack of internal age gradient within muscovite are interpreted to result from fast cooling of metamorphic soles. The main result documented by this study is a systematic younging of metamorphic sole ages from south to north, with a difference of 14 Ma along the 140 km length of the belt. The age of igneous rocks, such as plagiogranites and mafic dikes in the ophiolites, is equivalent to that of metamorphic soles, which indicates that the ophiolite was still hot and young at the time of metamorphic sole formation. The emplacement of the Albanian ophiolites is the consequence of a complex tectonic evolution: continental rifting, slow mid-ocean spreading followed by west dipping intraoceanic subduction initiated at the ridge axis, symmetric intraoceanic thrusting with opposite vergence, and at last, emplacement onto the continental margin. The geochronological and stratigraphic data suggest that this evolution lasted only a few millions years. Moreover, they indicate that this evolution stopped earlier to the south with the closure of a narrow basin than to the north, where the basin was wider due to faster oceanic spreading.