Metamorphic evolution of Verrucano metasediments in northern Apennines: new petrological constraints

Abstract

The Triassic Verrucano metasediments largely crop out along the mid Tuscan ridge, in the northern Apennines. The metasediments mainly consist of ferriferous quartzites and Al-rich metapelites and contain a specific mineralogy which lead to the reconstruction of their P-T path. The newly-formed metamorphic minerals are syn-to post-kinematics and they consist of magnesiocarpholite, muscovite, chlorite, paragonite, pyrophyllite, sudoite, kaolinite, and chloritoid, Magnesiocarpholite locally occurs only in a few syn-metamorphic quartz veins. Microanalytical techniques (SEM-EDS, TEM) allowed to collect compositional and microstructural information at the micron scale, to recognize the actual mineral reactions and their relative chronology. For Verrucano quartzites the stable paragenesis is chloritoid-chlorite-pyrophyllite, with sudoite and kaolinite being incompatible AFM phases. Thermodynamic calculations for Verrucano quartzites indicates that these rocks experienced a) relative high P/low T metamorphism (peak conditions ranging from 350 degrees C at 8 kbar to 420 degrees C at 10 kbar); b) a retrograde path characterized by cooling during decompression. These data demonstrate the usefulness of metapelites in deciphering the conditions of very low grade metamorphism. Besides, they support the regional geodynamic evolution which outlines the subduction of the continental crust of the Adria microplate under the European plate (Oligocene).