Magnetic signature of plutons and implications for emplacement conditions: examples from Northern Greece

The magnetization of batholiths is often unstable as a result of slow cooling and unroofing during their formation. These factors favour crystallization of coarse grains and mineralogical changes. Nevertheless, numerous studies have revealed the existence of stable magnetic recordings in batholiths, which can help to unravel the history of the pluton as far as its stability, translation or tilting is concerned. When an accurate isochron is established, it is possible to date the magnetic components through to blocking temperature spectra since isotopic and magnetic closure temperatures can be compatible. Finally, additional examination of other geophysical data and estimation of cooling rates can help the detection of burial conditions of the pluton. The majority of the above conditions apply successfully to large plutonic bodies. The present study focuses on Tertiary plutons in Northern Greece, covering more than 1200 km², classified from intermediate to large (70-430 km²), where extensive paleomagnetic and rock magnetic studies were carried out. Accurate radiometric ages are available for all studied plutons. The new data (Symvolo and Vrondou), along with previously published results (Elatia, Samothraki, Sithonia, Symvolo, Vrondou and Xanthi) compiled a detailed paleomagnetic dataset which constitutes an important step towards distinguishing local rotations from the regional ones. The mean direction values from nearby volcanics were also used in the compilation for an additional test. A major point of attention for this study was the establishment of reliable inclination values, which would reflect the latitudinal variations, if important tiltings could be discarded. Thus, we scrutinized all palaeomagnetic results, by a closer examination at the site level and in comparison with the numerous available radiometric, geothermometry and geobarometry data. Finally, an attempt was made to quantify cooling rates in the area and make precise correlations with the big dataset of laboratory blocking temperatures.
   The quality of the magnetic signature was, in general, satisfactory and the obtained directions (mostly clockwise) could be interpreted in the regional kinematic frame. The information provided by the paleomagnetic and rock magnetic studies has been used in various ways to assess the history of the plutons: (1) Dominance of magnetite, hematite or maghemite with estimation of grain size entails information on cooling rates and mineralogical transformations. The medium to coarse-grain granodiorites and monzonites of Elatia and Vrondou yielded the less reliable results of this study. (2) Laboratory blocking temperatures range from 350-600°C. For a slow cooling of 3°C/Ma this gives a range of natural Tb of 150-400°C which could be compared to detailed radiometric data in Sithonia and Symvolo, enabling us to accurately date the magnetic components. (3) Demagnetization diagrams and stereographic projections suggest minor or no tilt for some of the plutons (Symvolo, Xanthi) and possible tilting during emplacement for Sithonia. (4) The anisotropy of magnetic susceptibility was studied to assess the possible deflections of the palaeomagnetic vectors; in most cases AMS was relatively low while in plutons with higher anisotropy no systematic correlation was observed between irregular directions and increased AMS. The potential field data, where available, gave additional constraints to the above results. For instance, the Xanthi pluton has proved to have a 6-7km lower depth and a shape of a truncated pyramid. Assuming world-wide standard batholith burial depths versus critical isotherms we can estimate that the whole body has cooled ~above 13.5 km, which converges with the Curie isotherm for the area and crystallization depth calculated from geothermobarometry.