Evolution of metamorphic fluids at the Variscan fold-and-thrust belt in eastern Belgium

Quartz veins in the southern, greenschist facies metamorphic border of the Stavelot-Venn Massif (Belgium) have been examined. A detailed petrographica l study of these quartz veins indicates the presence of eleven quartz stage s. A microthermometric analysis shows the expulsion of three main fluid typ es during the Palaeozoic. In the first quartz stage (I) of Caledonian age, mixed aqueous/gaseous (H2O-CO2-NaCl) fluid inclusions are present. The post -Caledonian and pre-Variscan second stage (II) is characterised by gaseous CO2-N-2 inclusions occurring along inter-crystal grain boundaries and by aq ueous H2O-NaCl intra-crystal inclusions. The quartz stages III-VI, which fo rmed during the Variscan period, contain primary mixed aqueous/gaseous (H2O -CO2-NaCl) inclusions, all with an equal bulk chemical composition. Post-Va riscan quartz stages VII-XI could not be investigated microthermometrically . The combination of microthermometry, chlorite geothermometry and geotherm al gradients reveals minimum trapping conditions of 200 MPa and 200 degrees C for stage I quartz and 175-275 MPa and 385-435 degrees C for quartz stag es III-VI. The trapping conditions of stage II quartz could not be determin ed due to the strong recrystallisation of the quartz veins. At the lower-me tamorphic Variscan thrust front, distinct temperature anomalies are associa ted with thrust faults, as indicated by conodont colour alteration index (C AI), vitrinite reflectance data, chlorite geothermometry and fluid inclusio n microthermometry. The expulsion of metamorphic fluids generated at the so uthern part of the Stavelot-Venn Massif could have caused the distinct temp erature anomalies along Variscan thrust faults. Alternatively, some of thes e anomalies could have been induced by deeply circulating meteoric fluids. (C) 2000 Elsevier Science B.V. All rights reserved.