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

Abstract

Quartz veins in the southern, greenschist facies metamorphic border of the Stavelot–Venn Massif (Belgium) have been examined. A detailed petrographical study of these quartz veins indicates the presence of eleven quartz stages. A microthermometric analysis shows the expulsion of three main fluid types during the Palaeozoic. In the first quartz stage (I) of Caledonian age, mixed aqueous/gaseous (H 2 O–CO 2 –NaCl) fluid inclusions are present. The post-Caledonian and pre-Variscan second stage (II) is characterised by gaseous CO 2 –N 2 inclusions occurring along inter-crystal grain boundaries and by aqueous H 2 O–NaCl intra-crystal inclusions. The quartz stages III–VI, which formed during the Variscan period, contain primary mixed aqueous/gaseous (H 2 O–CO 2 –NaCl) inclusions, all with an equal bulk chemical composition. Post-Variscan quartz stages VII–XI could not be investigated microthermometrically. The combination of microthermometry, chlorite geothermometry and geothermal gradients reveals minimum trapping conditions of 200 MPa and 200°C for stage I quartz and 175–275 MPa and 385–435°C for quartz stages III–VI. The trapping conditions of stage II quartz could not be determined due to the strong recrystallisation of the quartz veins. At the lower-metamorphic Variscan thrust front, distinct temperature anomalies are associated with thrust faults, as indicated by conodont colour alteration index (CAI), vitrinite reflectance data, chlorite geothermometry and fluid inclusion microthermometry. The expulsion of metamorphic fluids generated at the southern part of the Stavelot–Venn Massif could have caused the distinct temperature anomalies along Variscan thrust faults. Alternatively, some of these anomalies could have been induced by deeply circulating meteoric fluids.