PETROGRAPHY AND STABLE-ISOTOPE ASPECTS OF COLD VENT ACTIVITY IMPRINTED ON MIOCENE AGE CALCARI-A-LUCINA FROM TUSCAN AND ROMAGNA APENNINES, ITALY

Over 20 occurrences of discontinuous limestone blocks, locally called ''calcari a Lucina,'' were mapped in the Tuscan-Romagna region of the northern Italian Apennines. The limestones, consisting of a variable mixture of authigenic carbonates (calcite, dolomite, and aragonite), sulfides (primarily pyrite), and allogenic silicates, occur in association with turbidite and hemipelagite units that were deposited in foredeep basins during early to late Miocene times. The limestone blocks are interpreted to represent relicts of carbonate buildups formed around methane-rich fluid vents on the basis of their (1) striking petrographic similarities to carbonates from cold vents in the modern oceans; (2) unique chemosynthetic-like fauna, and (3) anomalously negative delta(13)C values (delta(13)C = -16 parts per thousand to -58 parts per thousand PDB). The contemporaneous tectonism of the Apennine orogeny is likely to be the primary cause for the expulsion of the methane-rich fluids to the seabed in a manner analogous to the fluid-flow processes occurring at modern accretionary prisms.