PETROGENESIS OF THE OLIGOCENE VOLCANICS FROM THE CENTRAL RHODOPE MASSIF (N GREECE)

The Oligocene volcanic rocks of the Central Rhodope massif range in co mposition from basalts to rhyolites, and belong to orogenic calc-alkal ine and shoshonite series. These lavas were erupted during an extensio nal tectonic event, in a post-collisional setting. The eastern part of the massif, the Kotani-Kalotycho area, includes mainly basic to inter mediate rocks (basalts, basaltic andesites, high-g andesites,latites, trachytes), whereas the western part, the Dipotama-Kotyli and Zarkaden ia areas, is of larger extent and consists of acid rocks only (high-K dacites, rhyolites). Mineralogical, petrological and geochemical data supported by isotopic data (Sr-87/Sr-86 ratios 0.706-0.707 and delta(1 8)O values ranging mainly from + 6.2 parts per thousand to + 7.7 parts per thousand) indicate that the basic to intermediate rocks evolved p redominantly by assimilation/fractional crystallization of a parental magma which originated at the mantle-crust boundary according to the M ASH hypothesis of Hildreth & Moorbath (1988). Geochemical data and Sr acid O isotope ratios (0.708-0.709 and + 8.8 parts per thousand to + 1 2.9 parts per thousand, respectively), as well as the geological evide nce, do not favour a genetic link between the acid and the basic to in termediate rocks that can be explained by simple fractional crystalliz ation. instead, disequilibrium phenocryst assemblages in the high-K da cites and many of the rhyolites lead to the conclusion that these rock s were mainly derived by mixing of a basic and an anatectic silicic ma gma. The basaltic magma which triggered the crustal melting is conside red consanguineous with the parental liquid feeding the Kotani-Kalotyc ho basic to intermediate lavas.