PETROLOGICAL EVIDENCE FOR CRUSTAL THICKENING AND EXTENSION IN THE SERRE GRANULITE TERRANE (CALABRIA, SOUTHERN ITALY)

Abstract

The paper presents the metamorphic trajectory recorded by metapelitic migmatites of the upper part of the Hercynian lower continental crust of the Serre (southern Calabria, Italy). The relict minerals, reaction textures and phase equilibria define a clockwise P-T path. The prograde metamorphism from temperature of about 500°C and pressure of 4-5 kbar to T<700°C and P~8 kbar stabilized the assemblage Grt+Ky+Bt+Ms(Si/11ox=3.26-3.29) in the uppermost metapelites of the profile. Progressive heating led to H<inf>2</inf>O-fluxed and dehydration melting first of Ms, then of Bt at T<700°C in the stability field of sillimanite. This process was followed by nearly isothermal decompression producing additional melt with a transition from Grt to a Grt+Crd stability field. Further decompression caused the formation of Crd-corona around garnet. Nearly isobaric cooling led to rehydration and retrogression across the stability field of andalusite up to the stability field of kyanite. The lowermost metapelites of the studied profile have lost most of the memory of the prograde P-T path; they record decompression and cooling. High-temperature mylonites occur in which boudinage, elongation and pull-aparts characterize the porphyroclasts. The pull-aparts in the high-T mylonites are filled with low-P minerals (Crd+Spl). The Hercynian metamorphic trajectory and the microtextures are consistent with crustal thickening and subsequent extensional regime. During extension, an important tectonic denudation probably caused the isothermal decompression. Extension also occurred in post-Hercynian times as documented by pull-aparts in sillimanite porphyroclasts filled with chloritoid within a low-grade mylonite.