Characterisation of amphibolite-facies fluids of Variscan eclogites from the Orlica-Snieznik dome (Sudetes, SW Poland)

Abstract

Eclogites from the Orlica-Snieznik dome (Sudetes, SW Poland) exhibit evidence for Variscan high-pressure metamorphism and a subsequent amphibolite-facies overprint. Three main types of fluid inclusions were observed in quartz of partially retrogressed eclogites. Type 1: aqueous inclusions, mostly two-phase inclusions (L + V). Type 2: (a) CO 2 ± N 2 ± CH 4 inclusions, normally one-phase inclusions, some contain graphite as daughter mineral; (b) N 2 ± CH 4 inclusions, one-phase inclusions. Type 3: mixed H 2OCO 2 ± N 2 inclusions, normally two-phase inclusions (LH 2O + LCO 2 ± VCO 2) consisting of 70–80 vol% CO 2. The texturally earliest inclusions are the CO 2 ± N 2 ± CH 4 (type 2a) inclusions, which were followed by H 2OCO 2N 2 (type 3) and H 2O (type 1) inclusions in chronological order. Late aqueous (type 1) and N 2CH 4 (type 2b) inclusions were the last to be trapped. Isochores for fluid inclusions are not conformable with peak-metamorphic conditions which are estimated at temperatures between 640° and 790°C and minimum pressures of >14 kbar. At least three possible explanations must be considered: (1) no fluid phase was present during the eclogite-facies metamorphism; (2) all fluid inclusions which were trapped during the high-pressure event decrepitated during retrogression; (3) only under medium-pressure conditions fluid inclusions were trapped. We did not succeed to identify fluid inclusions which clearly are related to the eclogite-facies metamorphism. However, the isochores derived from microthermometric measurements can be used to constrain the retrograde P-T path. The highest-density isochore of the early CO 2 ± N 2 ± CH 4 inclusions indicates a pressure of 8 kbar at 600°C. The isochores of the H 2OCO 2 ± N 2, the H 2O inclusions and the re-equilibrated CO 2 ± N 2 ± CH 4 inclusions suggest an almost isothermal decompressional uplift to 2 kbar at 600°C, which was followed by isobaric cooling to 200°C. Therefore, the rate of uplift was low relative to the rate of cooling, indicating a relatively long period of tectonic inactivity.