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

In the Apuane Alps (AA) metamorphic core complex, syn-metamorphic mineral d eposits 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 circul ation and mineralization in other core complexes, are barren in the AA. Ext ension veins hosted by the Jurassic Carrara marbles are among the few examp les of (minor) mineralization located in the upper levels of the AA core co mplex. Calcite-dolomite geothermometry and fluid inclusion data suggest tha t the mineralizing process in these veins began under pressure (P)-temperat ure (T) conditions close to the metamorphic peak (about 400 degrees C, 3 kb ar). Progressive cooling and mixing between metamorphic and late stage mete oric fluids were probably responsible for most of the mineral deposition. B atches of relatively saline fluids presumably resulted from interaction wit h evaporitic levels located along the detachment fault. In agreement with p revious estimates, fluid inclusion constraints on the P-T synmetamorphic pa th of the AA suggest a relatively rapid cooling of the core complex as a re sult of uplift. However, the maximum estimated geothermal gradient (about 3 5 degrees C/km) is considerably lower than in other core complexes, where l arge scale hydrothermal circulation was associated with extension and uplif t. Hence, in the AA, fluid circulation at shallow levels and mixing among f luids of different origin were not favoured, thus precluding the formation of mineral deposits along major extensional structures.