The Eocene Sithonia Plutonic Complex (Chalkidiki, Greece), which intrudes the Circum Rhodope Belt and the Serbomacedonian Massif, is divided into a northern part comprising two-mica (TMG) granites and leucogranites (LG) and a southern part comprising hornblende-biotite granodiorites (HBGD), grading into tonalities (HBTON), and biotite granodiorites (BGD). Abundant mafic microgranular enclaves are enclosed in the granodiorites and tonalites. A mixing plus fractional crystallisation process (MFC) is considered responsible for the evolution of the Sithonia Plutonic Complex. Two end-members are considered, an acid represented by leucogranites and a basic one represented by a lamprophyric-like magma which underplated a lower crust of amphibolitic/basaltic composition. At the early stages of the evolutionary process fractional crystallisation was more active than mixing giving rise to tonalitic/monzonitic enclaves while later on mixing was the prevailing process giving the wide spectrum of composition of the southern part of the complex (HBTON, HBGD, BGD).
The aim of this study is the K-Ar mineral geochronology and thermochronometry of the southern part of the Sithonia Plutonic Complex.
Based on the variety of rock types and their spatial distribution, twenty seven samples (mineral separates) of hornblende (3), biotite (12) and K-feldspar (12) were selected and dated. The K-Ar ages obtained range between 45 and 50 Ma for hornblende, 40 and 46 Ma for biotite and 36 and 42 Ma for K-feldspars respectively.
The K-Ar ages yielded and the published Rb-Sr mica ages are used to investigate the thermal history of the complex. The intrusion of the LG affected mostly the northern part of the HBGD (~46 Ma) and disturbed more the K-Ar isotopic system of the biotite than the Rb-Sr system. The last intrusion was that of the BGD at about 42 Ma. The larger age difference between Bt and Kf and the smaller Kf age in BGD in comparison to the rest rock types along with the fact that biotite gives an isochron only in BGD indicate that: 1) The cooling rate of BGD is slower than the rest rock types, and 2) The K-Ar isotopic system for Kf in BGD was open for longer time resulting in a homogenous feeding of biotite with radiogenic Ar. In this way biotite was enriched in 40Ar.
The closure temperatures of hornblende, biotite and K-feldspar, the K-Ar ages obtained and the available Rb-Sr mica ages were used to decipher the thermal history of the southern Sithonia Plutonic Complex. The estimated average cooling rate for HBGD+HBTON was Δ0C/ Δt = 40.20C/Ma, nearly the same as that for the whole south part of the complex (HBGD+HBTON+BGD) estimated to Δ0C/ Δt = 40.10C/Ma. This cooling rate is lower than the cooling rate estimated for the TMG of the northern part of the Sithonia Plutonic Complex (Δ0C/ Δt = 600C/Ma). For the last cooling rate zircon ages have also been used.