Petrogenesis of Eocene Agios Gregorious Plutonic complex (Mt. Athos, Greece): interplay between magma mixing, assimilation and fractional crystallization.

The Eocene Gregoriou Plutonic Complex (GPC) is located in the easternmost finger of the three-legged Chalkidiki Peninsula (northern Greece), and intrudes the metamorphic basement of the Serbo-Macedonian Massif to the north, and the Circum Rhodope Belt to the south. The intrusive rocks comprising the GPC are studied on the basis of their mineral chemistry, whole-rock major and trace element geochemistry, and Sr and Nd isotopic composition. The rocks have been subdivided, on the basis of petrography, into a basic- intermediate group bearing hornblende and biotite, with the basic rocks showing not strong evidence of cumulus, and an intermediate- acid group bearing biotite only; common mineralogical assemblage is made up of quartz, perthitic orthoclase, plagioclase, and allanite, epidote, titanite, apatite, and opaques as accessories. On the basis of geochemistry the most basic rocks are cumulates, whereas the others are intermediate and acid in silica. Metamorphic xenoliths are very common, whereas magmatic enclaves are present, yet not widespread, and strong field evidence of magma mixing are restricted to some areas. Some rocks of GPC are similar to rocks of the Sithonia Plutonic Complex on the basis of elemental and isotopic composition. Cumulitic samples show evidence of amp ± bt ± pl, although textural evidence of cumulus processes (e.g. layering) are scarce. A filter pressing process that squeezed out some portions of the residual liquid determined the ‘cumulitic’ signature of some samples. A process of Mixing plus Fractional Crystallization followed by an Assimilation plus Fractional Crystallization process is considered responsible for the evolution of GPC. The first step explains the chemical variation in the less evolved intermediate rocks, whereas the second step explains the geochemical variations of the remaining intermediate-acid rocks. The least evolved rock of GPC represents the mafic end-member, whereas some leucogranites found in the Sithonia Plutonic Complex represent the acid end-member of the first step. The average of less evolved intermediate-acid GPC rocks represents the parental magma of the AFC process, whereas a mica gneiss, found in the basement of Sithonia Plutonic Complex, represents the contaminant. Tectonomagmatic characterization of the GPC rocks indicates continental arc environment, and the mafic magma is presumably the result of melting of a repeatedly metasomatized and enriched in LILE mantle. Accordingly, GPC magmatism is connected with the subduction events, which affected the area since the Jurassic.