THE TRANSITION FROM LOW-P TO CONTACT METAMORPHISM IN CALABRIA RELATED TO PLUTON GROWTH IN THE INTERMEDIATE CRUST

Granitoids represent a large proportion of the Hercynian crustal sections exposed in Calabria. In the Sila and Serre massif the granitoid stack has a cumulative thickness of 9 to 13 km and includes tabular intrusions of tonalitic to granitic composition with minor dioritic and gabbroic bodies. Emplacement of the granitoids occurred sometime between 300 and 290 ± 10 Ma during the extensional tectonic stage, following collapse of the Hercynian chain. The heat advected by granitoids was considered responsible both for contact metamorphism in the upper crust and regional low-P metamorphism in the lower to intermediate crust. This was shown by static and dynamical numerical models. A main limitation of the models was the instantaneous emplacement assumed for the magmas. In the light of the general consensus existing on the idea that large magma chambers can be filled through dykes in a short time (< 100 kyr in Petford, et al., 2000), the instantaneous emplacement was considered a permissible simplification. In recent years, the rapid growth of plutons has been questioned, on the basis of radiometric dating at different levels of the magmatic body (e.g., Glazner et al., 2004). Even though the melt flow entering in the magma chamber can be high, feeding may be discontinuous. Consequently, pluton accretion by repeated melt injections can be completed in some million years (Matzel et al., 2006). Another interesting point raised is the way the pluton grows (downward or upward) because this affects the intensity of thermal perturbation above and below the magmatic body (Annen, 2011). On the basis of these arguments we have formulated a new 2D thermal model, to be applied on the Calabria crustal sections, that takes into account the incremental growth of the pluton. We examined the consequences of the end-member processes of pluton accretion and of different growth rates. In case of under-accretion, contact metamorphism in the upper crust occurs before regional low-P metamorphism in the intermediate to lower crust. Model indicates that pluton growth must be completed in a short time (< 200 kyr) to reproduce observed peak temperatures (540-590 °C) in the contact aureole. Deformation structures related to pluton growth, overprinted by peak T minerals, must be formed in an even shorter time interval. In case of over-accretion, an opposite sequence of low-P and contact metamorphism is produced and peak temperatures can be generated even when pluton construction lasts for 5 Myr. In comparing data of the Calabria crustal sections with results of underand over-accretion models, the second option is preferred. Model results suggest also that the smooth transition from regional low-P to contact metamorphism may reflect upward pluton growth and concurrent exhumation by extensional tectonics.