METAMORPHIC EVOLUTION OF VERRUCANO METASEDIMENTS IN NORTHERN APENNINES - NEW PETROLOGICAL CONSTRAINTS

The Triassic Verrucano metasediments largely crop out along the mid Tu scan ridge, in the northern Apennines. The metasediments mainly consis t of ferriferous quartzites and Al-rich metapelites and contain a spec ific mineralogy which lead to the reconstruction of their P-T path. Th e newly-formed metamorphic minerals are syn-to post-kinernatics and th ey consist of magnesiocarpholite, muscovite, chlorite, paragonite, pyr ophyllite, sudoite, kaolinite, and chloritoid. Magnesiocarpholite loca lly occurs only in a few syn-metamorphic quartz veins. Microanalytical techniques (SEM-EDS, TEM) allowed to collect compositional and micros tructural information at the micron scale, to recognize the actual min eral reactions and their relative chronology. For Verrucano quartzites the stable paragenesis is chloritoid-chlorite-pyrophyllite, with sudo ite and kaolinite being incompatible AFM phases. Thermodynamic calcula tions for Verrucano quartzites indicates that these rocks experienced a) relative high P/low T metamorphism (peak conditions ranging from 35 0 degrees C at 8 kbar to 420 degrees C at 10 kbar); b) a retrograde pa th characterized by cooling during decompression. These data demonstra te 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 t he Adria microplate under the European plate (Oligocene).