| Summary: | The Rhodope zone of northern Greece has been involved in an Alpine metamorphic cycle that consists of a high-pressure (eclogite-facies) metamorphism extensively overprinted under medium-pressure conditions in Eocene time. Abrupt differences in the grade of metamorphism within the Rhodope zone allows its subdivision into a lower and an upper tectonic unit. The pelitic rocks of the lower tectonic unit are typically garnet - chloritoid +/- staurolite schists. These rocks preserve the high-pressure paragenesis garnet + chloritoid + chlorite + phengite + quartz + rutile. Minerals formed during exhumation are staurolite, muscovite, Fe-rich chlorite and, rarely, biotite and andalusite. In the upper tectonic unit, the metapelites are generally garnet - biotite +/-kyanite gneisses and schists; the high-pressure paragenesis has been partly obliterated during extensive retrograde overprinting. Peak (minimum) conditions of pressure determined for the lower tectonic unit by use of the phengite geobarometer (for T = 550-degrees to 600-degrees-C) are 13-13.5 kbar. P-T estimates for peak of metamorphism of the upper tectonic unit are uncertain. Textural features and mineral chemical data provide information on metamorphic conditions for various stages of decompression of both units. The exhumation path of the lower tectonic unit was nearly isothermal to a depth of approximately 12 km from the surface. In the upper tectonic unit, exhumation proceeded along a P-T path characterized by cooling. These differences are probably attributed to thrusting of the deeper lying, upper tectonic unit over the lower one at depth during unloading. In this case, the upper unit acted as a warm ''shield'' and prevented the rocks of the underlying lower unit from losing heat. Since the P-T path of Rhodope shows important similarities in form with that of other high-pressure terranes (i.e., the Western Alps, the Seward Peninsula in Alaska, the Cyclades in the Aegean Sea), analogous constraints, such as rapid unroofing tectonics or continuous underthrusting of cold material, can be invoked to explain the cooling that accompanied exhumation.
|
|---|