Contrasting chemistry and H, O, C isotope composition of greenschist-facies, Hercynian and Alpine metamorphic fluids (Western Carpathians)

Abstract

Microthermometry and stable isotope data enable us to distinguish between two contrasting sets of fluids involved in the Hercynian and Alpine metamorphic events in the Western Carpathians. High-temperature smoky quartz (450–500°C) was probably formed during the syn-kinematic phase of the Hercynian metamorphism, when chemical and isotopic equilibrium had been established between aqueous solutions and the surrounding rocks. This is indicated by fixed Na/Ca ratios and narrow ranges of δ D and δ 18O values in the inclusion fluids (−59 ± 5‰ and + 7 to + 7.5‰ SMOW, respectively). In contrast, Alpine quartz precipitating at temperatures between 300° and 400°C in extension-related structures displayed a non-equilibrium signature characterized by a much larger compositional and isotopic variability. Presence of CO 2 in quartz from granitoid-hosted shear zones indicates externally derived fluids with δ 13C values between −11.9 to −7.7‰ PDB. These values can be interpreted to reflect an admixture of a lighter CO 2 generated by decomposition of organic matter in underlying low- to medium-grade sediments. The essentially identical isotopic composition of the inclusion CO 2 from the presumably Hercynian as well as Alpine metamorphic quartzes supports the concept of Hercynian overthrusting of granitoids onto metamorphites in the crystalline basement of the Western Carpathians.