Effects of incremental growth of granite plutons on the thermal evolution of the continental crust exposed in Calabria

Granitoids represent a considerable proportion of the Hercynian crustal sections exposed in Calabria. In the Sila and Serre massif the granitoids are stacked to make a pile of tabular intrusions with a cumulative thickness of 9 to 13 km. Composition typically ranges from tonalite to granite, but minor dioritic and gabbroic bodies are also present. Magma emplacement took place between 300 and 290 ± 10 Ma during the extensional tectonic stage, following collapse of the Hercynian belt. 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 satisfactorily reproduced by static and dynamical numerical models. However, a limit of the models is the assumption concerning the instantaneous emplacement of the magmas, following the general consensus on the idea that large magma chambers can be filled through dykes in a short time interval (< 100 kyr in PETFORD et alii, 2000). In recent years, radiometric datings obtained from different levels of the magmatic bodies indicate that pluton growth can be completed in a significantly longer time (>1 Myr, e.g. GLAZNER et alii, 2004; MATZEL et alii, 2006) as an effect of the discontinuous melt feeding. Consequently, the way the pluton grows (downward or upward) needs to be examined since this affects the intensity of thermal perturbation above and below the magmatic body (ANNEN, 2011). On the basis of these arguments we have set up a new 2D thermal model, applied to the Calabria crustal sections, that takes into account the incremental growth of the pluton. We analysed the effects of the end-member processes of pluton accretion at 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 peak temperatures (540-590 °C) in the contact aureole observed in the upper crust. A slower growth rate would result in weaker thermal effects. Consequently, deformation structures related to pluton growth and overprinted by peak temperatures porphyroblasts, must be formed in an even shorter time interval. In case of over-accretion, a reverse time sequence of regional low-P and contact metamorphism is produced and observed peak temperatures can be generated even when pluton construction is completed in 5 Myr (Fig. 1) In comparing data of the Calabria crustal sections with results of under- and over-accretion models, the second option is mandatory (Fig. 1). 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.