METAMORPHIC EVOLUTION OF METAPELITES IN THE HIGH-PRESSURE TERRANE OF THE RHODOPE ZONE, NORTHERN GREECE

The Rhodope zone of northern Greece has been involved in an Alpine met amorphic cycle that consists of a high-pressure (eclogite-facies) meta morphism extensively overprinted under medium-pressure conditions in E ocene time. Abrupt differences in the grade of metamorphism within the Rhodope zone allows its subdivision into a lower and an upper tectoni c unit. The pelitic rocks of the lower tectonic unit are typically gar net - chloritoid +/- staurolite schists. These rocks preserve the high -pressure paragenesis garnet + chloritoid + chlorite + phengite + quar tz + rutile. Minerals formed during exhumation are staurolite, muscovi te, Fe-rich chlorite and, rarely, biotite and andalusite. In the upper tectonic unit, the metapelites are generally garnet - biotite +/-kyan ite gneisses and schists; the high-pressure paragenesis has been partl y 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) ar e 13-13.5 kbar. P-T estimates for peak of metamorphism of the upper te ctonic unit are uncertain. Textural features and mineral chemical data provide information on metamorphic conditions for various stages of d ecompression 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 attribut ed to thrusting of the deeper lying, upper tectonic unit over the lowe r one at depth during unloading. In this case, the upper unit acted as a warm ''shield'' and prevented the rocks of the underlying lower uni t from losing heat. Since the P-T path of Rhodope shows important simi larities in form with that of other high-pressure terranes (i.e., the Western Alps, the Seward Peninsula in Alaska, the Cyclades in the Aege an Sea), analogous constraints, such as rapid unroofing tectonics or c ontinuous underthrusting of cold material, can be invoked to explain t he cooling that accompanied exhumation.