Fluid circulation in the Apuane Alps core complex: evidence from extension veins in the Carrara marble

In the Apuane Alps (AA) metamorphic core complex, syn-metamorphic mineral deposits are mainly restricted to extensional shear zones in the Lower Plate Palaeozoic basement. By contrast, the extension structures at upper levels, such as the detachment fault, that are typically the seat of fluid circulation and mineralization in other core complexes, are barren in the AA. Extension veins hosted by the Jurassic Carrara marbles are among the few examples of (minor) mineralization located in the upper levels of the AA core complex. Calcite-dolomite geothermometry and fluid inclusion data suggest that the mineralizing process in these veins began under pressure (P)-temperature (T) conditions close to the metamorphic peak (about 400 degrees C, 3 kbar). Progressive cooling and mixing between metamorphic and late stage meteoric fluids were probably responsible for most of the mineral deposition. Batches of relatively saline fluids presumably resulted from interaction with evaporitic levels located along the detachment fault. In agreement with previous estimates, fluid inclusion constraints on the P-T synmetamorphic path of the AA suggest a relatively rapid cooling of the core complex as a result of uplift. However, the maximum estimated geothermal gradient (about 35 degrees C/km) is considerably lower than in other core complexes, where large-scale hydrothermal circulation was associated with extension and uplift. Hence, in the AA, fluid circulation at shallow levels and mixing among fluids of different origin were not favoured, thus precluding the formation of mineral deposits along major extensional structures.