Petrology of Felsic Granulites, Metapelites, Metabasics, Ultramafics, and Metacarbonates from Southern Calabria (Italy): Prograde Metamorphism, Uplift and Cooling of a Former Lower Crust

A high-grade metamorphic terrane in the southern part of the Calabrian massif (South Italy) has been petrographically mapped and the dominant rock types petrologically investigated. Both methods of investigation have led to the recognition of a continuous section through a former lower crust which is ∼7 km thick. Its lower part consists predominantly of metabasic rocks together with minor felsic granulites, its upper part of metapelites with minor metabasic and metacarbonate rocks. The rocks experienced a common two-stage prograde metamorphic evolution in which the second stage occurred after the last penetrative deformation. The prograde metamorphism which, according to radiometric dates, ended in late Hercynian time, was of the medium-pressure type of Miyashiro (1961), and equilibration occurred in the ‘medium-pressure granulite field’ (characterized by the instability of olivine-plagioclase as well as garnet-clinopyroxene-quartz). Estimates of the highest P—T conditions of prograde metamorphism give 7–8 kb and approximately 800°C at the base, but 5–6 kb and 650–700°C at the top of the section, at which the paragenesis staurolite-quartz indicates the transition to the amphibolite facies. The existence of a metamorphic gradient in the lower crust section is demonstrated by the systematic change in the compositions of ferro-magnesian minerals in divariant metapelitic assemblages. The metamorphic evolution during the excavation history of the former lower crust has been reconstructed using the numerous disequilibrium reaction textures preserved in most rock types. The highest metamorphic conditions ended with a pressure decrease of approximately 1.5 to 2 kb, which was followed by a period of quasi-isobaric cooling in the middle crust. During this cooling, the stability field of the ‘high-pressure granulites’ (garnet-clinopyroxene-quartz) was reached. The pressure decrease, which induced the end of the high-temperature history of the lower crust, is interpreted as reflecting the erosion of the uppermost crustal levels as a response to overlapping of large crustal segments during the Hercynian orogeny. Consequently, the deduced P—T path of the upper, i.e. overthrust crustal segment is thought to have been tectonically controlled.