An Exposed Hercynian Deep Crustal Section in the Sila Massif of Northern Calabria: Mineral Chemistry, Petrology and a P-T Path of Granulite-facies Metapelitic Migmatites and Metabasites

Abstract

In the Sila massif of northern Calabria a continuous section is exposed through a segment of a Hercynian deep continental crust, which has been interpreted by previous workers as a stack of basement nappes (‘Monte Gariglione Complex’). The section consists essentially of metapelitic migmatites and subordinate metabasites and marbles, which were metamorphosed at medium-pressure–high-temperature granulite-facies conditions. A continuous metamorphic gradient through the exposed segment can be deduced from the systematic change in the compositions of ferromagnesian minerals in divariant metapelitic assemblages. This gradient is partly supported by conventional geothermobarometry and by applying the TWEEQU method. However, peak-metamorphic conditions are better defined by dehydration melting reactions, which reveal ∼4 kbar and 740°C at the top and ∼6 kbar and 770°C at the base of the section. Therefore the exposed rocks represent a former crustal depth range of ∼14–21 km at the thermal peak of metamorphism. The metamorphic evolution of the former deep crustal rocks has been reconstructed from reaction textures. The prograde P–T path took place in the stability field of sillimanite. The retrograde path is characterized by a stage of isothermal uplift after peak metamorphism to mid-crustal levels (10–15 km) followed by near-isobaric cooling to greenschist-facies conditions. The deep crustal section in northern Calabria resembles that of the Serre massif in southern Calabria, which represents the lowermost part of an exposed tilted cross-section through the Hercynian continental crust. Postkinematic mineral growth, reaction textures and isotopic age constraints indicate that the thermal conditions in the Calabrian crust during the Hercynian orogeny were mainly controlled by advective heat input through magmatic intrusions. As in southern Calabria, large granitic bodies were emplaced between the granulite-facies lower crust (‘Monte Gariglione Complex’) and the amphibolite- to greenschist-facies upper crust during regional metamorphism.