K/Ar dating, geochemistry and evolution of the Tertiary volcanic rocks (Thrace, northeastern Greece)

The Tertiary Evros volcanic rocks in Thrace of northeastern Greece, shows features of calcalkaline to high-K calc-alkaline and, locally, shoshonitic rock series. They comprise basaltic andesite to rhyolite through andesite, trachyandesite, trachydacite and dacite. The new K/Ar ages obtained range from 33.5 to 19.6 Ma, establishing an Oligocene (33.5-25.4 Ma) and a Lower Miocene (22.0-19.6 Ma) period of volcanic activity. Two main groups of rocks, the PxBt group and the HblBt group, indicating different evolutionary histories, have been distinguished. Sr I.R. is 0.7057-0.7074 in the PxBt group, and 0.7071-0.7080 in the HblBt group. The PxBt and the HblBt groups were evolved through an open system process (mixing or MFC) in which basaltic andesite and trachydacite represent the basic and the acid end-members respectively. A similar process, with a hybrid magma of the PxBt group, is suggested for the evolution of the HblBt group. Partial melting of crustal material, under various P-T conditions, could be responsible for the genesis of melts similar to the less evolved rhyolites.