STABLE ISOTOPE EVIDENCE FOR A MARINE ORIGIN OF OPHICALCITES FROM THE NORTH-CENTRAL APENNINES (ITALY)

Carbon and oxygen isotope ratios of 59 samples of Mesozoic ophicalcites from the north-central Apennines were measured. Strontium content and isotope ratios were only determined for selected samples. The data obtained (-4.1 ≤ δ 13C ≤ + 3.0; 16.5 ≤ δ 18O ≤ 30.1; 0.7076 ≤ 87Sr 86Sr ≤ 0.7086; 60 ≤ Sr ppm ≤ 1140) suggest that the carbonate cement of the Apennines ophicalcites had a marine origin. The 18O/16O values of most of the carbonates studied, however, indicate that the rocks have interacted with hydrothermal fluids, dominantly represented by sea water. In particular, the rocks from the northern section of the Apennines apparently reequilibrated their δ 18O content at higher temperature than those from the central zone. Strontium isotope ratios, close to the Cretaceous-Cenozoic ocean water values, are well in agreement with such a kind of sea water/ophicalcites interaction, even if the carbonate cement formed earlier during the Jurassic period. © 1979.