Calculated phase equilibria for phengite-bearing eclogites from NW Spitsbergen, Svalbard Caledonides

Abstract

Abstract Phengite-bearing eclogites occur in the Richarddalen Complex of NW Spitsbergen, Arctic Caledonides. Phase equilibrium modelling and conventional geothermobarometry have been used to constrain the metamorphic evolution of these eclogites. Pseudosections are calculated for the peak-pressure assemblage garnet+omphacite+phengite+amphibole+dolomite quartz+rutile. Compositional isopleths for garnet and phengite constrain the pressure–temperature ( P–T ) conditions to 1.9–2.0 GPa and 720–730 °C, in good agreement with the results obtained from conventional thermobarometry (720–740 °C and 2.4–2.5 GPa). Further P–T pseudosection modelling of clinopyroxene+plagioclase±amphibole±clinozoisite symplectites after omphacite suggests that decompression to c. 1.2 GPa occurred along a steep exhumation path. The eclogite-bearing Richarddalen Complex constitutes the uppermost unit of a simple stack of thrust sheets where the metamorphic grade is increasing structurally upwards in the pile. Thrusting is the favoured uplift mechanism for the initial syn-orogenic exhumation to lower crustal levels. Constrictional north–south stretching in a transpressional regime is interpreted to be responsible for the final exhumation of the assembled stack of thrust sheets. Late Silurian–Early Devonian conglomerates were deposited directly on the eclogite-bearing gneisses of the Richarddalen Complex, and mark the end of exhumation of the nappe stack.