The Xanthi Plutonic Complex (XPC) is one of a series of Oligocene subduction-related plutonic bodies comprising an “acid” group and a “basic” group. Based on mineral compositions and assemblages of the “basic” group, the XPC is assumed to have originally crystallized at a pressure of 5.4 kbar and at a temperature of 1300oC under relatively dry conditions and oxygen fugacity (fO2) near the NNO buffer. As the basic magma migrates to shallower levels and at a temperature of about 870oC, water content increases and oxygen fugacity moves towards the MH buffer. The increase of water content could be the result of open system evolutionary processes. The “acid” group crystallizes at an average temperature of 729oC and at a pressure of 0.7 kbar under oxidizing conditions, between the NNO and MH buffer, suggesting a possibly different origin and/or evolution for the “acid” group.