Estimating P-T metamorphic conditions on the roof of a hidden granitic pluton: an example from the Mt. Calamita promontory (Elba Island, Italy)

Abstract

The metamorphic basement (i.e. Palaeozoic micaschist, Porto Azzurro Unit) cropping out in the Mt. Calamita promontory, in the southeastern Elba Island, represents the roof of the mostly unexposed late Miocene Porto Azzurro granite. A thermobarometrical study was performed on micaschists and interlayered amphibolites to constrain P-T conditions during contact metamorphism. Adopted methods were based on P-T grids, conventional thermobarometry and analysis of fluid inclusions. It was found that the basement was originally located at a depth of 6.5 ± 1.3 km and that the peak metamorphic conditions were close to the muscovite dehydration reaction in micaschist.Therefore, small differences of temperatures conditioned the presence or absence of K-feldspar in the eastern and western side of the Mt. Calamita promontory, respectively. A maximum peak temperature of c. 625 °C was estimated by amphibole-plagioclase thermometry in amphibolite. Fluid inclusions in micaschists quartz lenses document significant episodes of fluid circulation during the cooling history of the metamorphic aureole. Finally, a conductive thermal model was built up to reproduce the thermal evolution during the contact metamorphic event and following cooling. Simulated temperatures resulted significantly lower than the observed peak values. This discrepancy is explained as an effect of the localized heat advection in zones affected by infiltration of magmatic fluids and by the intrusion of numberless felsic dykes.