Hydrothermal dolomitization of Jurassic-Cretaceous limestones in the southern Alps (Italy): Relation to tectonics and volcanism

Abstract

Dolomitization has affected up to 750m 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 was 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{sub 3}. In thin sections, the crystal size distribution is unimodal (about 100 {mu}m), possibly indicating a single nucleation for the main crystallization phase. The {delta}{sup 13}C of the dolomite is not appreciably different from the undolomitized pelagic limestone (+1.0{per thousand} to +2.0{per thousand} Peedee belemnite (PDB)). The {delta}{sup 18}O variation (-5.0{per thousand} to -13.0{per thousand} PDB) is due to temperature variation in the system. The {sup 87}Sr/{sup 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 150C for dolomitization. The dolomite is suggested more » to have a hydrothermal origin. The heat flow associated with the volcanism allowed marine water to penetrate the system and circulate in convective cells through the tectonic breccias, locally dolomitizing the limestone. « less