K/Ar mineral geochronology of the northern part of the Sithonia Plutonic Complex (Chalkidiki, Greece) and implications for its thermal history

The Sithonia plutonic complex (Chalkidiki, Greece) occupies the greater part of the Sithonia Peninsula intruding the Circum Rhodope Massif to the west and the Serbomacedonian Massif to the east. It comprises an Eocene pluton and so far, its origin and evolution has been studied by many researchers. The subject of the present study is the K/Ar mineral geochronology of the northern part of the pluton which consists of three main bodies, the Two-mica Granite (TMG), the Porphyry Leucogranite (PLG) and the Leucogranite (LG).
   The systematic K/Ar study of the pluton along with existing Rb/Sr mica and U/Pb zircon ages are used to investigate the thermal history of the pluton and shed light on the process that affected it and resulted in discordant Rb/Sr and K/Ar mineral ages. Thirty-nine samples of muscovite biotite and K-feldspar samples were selected according to the lithological characteristics and spatial distribution. These samples yielded K/Ar ages ranging from 38 to 49 Ma for muscovites, 32 to 47 Ma for biotites and 37 to 43 Ma for K-feldspars respectively.
   The K/Ar geochronological results indicate that the mineral ages of TMG and PLG are in accordance with the principles of the isotopic closure temperatures of the K/Ar isotopic system, but the geochronological results of the LG indicate disturbed behaviour.
   The processing of the geochronological data with the K/Ar isochron method, in association with the Rb/Sr data, indicates that a reheating event took place and disturbed the isotopic systems of biotite and Κ-feldspar but did not manage to disturb the isotopic system of muscovite. Regarding the thermal evolution of the LG, it is considered that the voluminous pegmatite intrusions in the LG area disturbed the isotopic systems of the two minerals but the simultaneous or imminent reheating mentioned above caused the resetting of the K-feldspar isotopic system and partly the biotite isotopic system.
   The reheating event, which is probably associated with a tectonic event, that disturbed the mineral isotopic systems exceeded the closure temperature of biotite for the Rb/Sr isotopic system (350 ± 50°C), but did not exceed the closure temperature of muscovite for the K/Ar isotopic system ( 375 ± 25°C).
   The comparison of the K/Ar mineral ages of the present study, the existing Rb/Sr and U/Pb mineral ages and the closure temperatures of the different isotopic systems for the different minerals indicate a high cooling rate for the TMG of the Sithonia pluton which reaches 60 ± 12 °C per million years, received as minimum due to thermal event that caused slightly younger biotite and K-feldspar resultant ages. This is in agreement with the aspect that the extensional collapse of the Hellenides where the Sithonia pluton intrudes started during Eocene.