HYDROTHERMAL DOLOMITIZATION OF JURASSIC-CRETACEOUS LIMESTONES IN THE SOUTHERN ALPS (ITALY) - RELATION TO TECTONICS AND VOLCANISM

Dolomitization has affected up to 750 m of the Jurassic and Cretaceous pelagic carbonate sequence of the southern continental margin of the Alpine Tethys; the sequence crops out in the southern Alps of Italy (Monti Lessini). Late Paleocene to Miocene extrusion of basaltic tuffs, breccias, and lavas was contemporaneous with the dolomitization and was associated with extensive tectonism in an ancient back-arc basin. More than 200 samples were analyzed by X-ray diffraction, cathodoluminescence, scanning electron microscopy, stable isotope ratios (carbon, oxygen, strontium), and clay mineralogy. The dolomite contains 40% to 50% MgCO 3 . In thin section, the crystal size distribution is unimodal (about 100 μm), possibly indicating a single nucleation for the main crystallization phase. The δ 13 C of the dolomite is not appreciably different from the undolomitized pelagic limestone (+1.0‰ to +2.0‰ Peedee belemnite [PDB]). The δ 18 O variation (-5.0‰ to-13.0‰ PDB) is due to temperature variation in the system. The 87 Sr/ 86 Sr ratio in the dolomite (0.70839-0.70867) is consistent with the ratio in late Oligocene-Miocene marine water. Clay minerals in limestone and dolomite differ in the presence of neoformed Mg-chlorite, indicating a maximum temperature of about 150 °C for dolomitization. The dolomite is suggested to have a hydrothermal origin. The heat flow associated with volcanism allowed marine water to penetrate the system and circulate in convective cells through the tectonic breccias, locally dolomitizing the limestone. © 1990 Geological Society of America.