Retrograded eclogites of the Richarddalen Complex, NW Svalbard : petrology and P/T-conditions

Abstract: At least three terranes have been identified in the Svalbard Caledonides, each one having independent structures and tectonothermal evolution. On Biskayerhalvöya in the Northwestern Terrane, eclogites, located within deformed igneous basement rocks, have been dated by the U-Pb zircon method to c. 625 Ma. This study, using the garnet-clinopyroxene Fe/Mg exchange geothermometer, has established the crystallization temperature of these eclogites to be between 640 and 700°C at estimated pressures from 12 to 18 kbars. The eclogites occur in hornblendic gneisses, associated with marbles, calc-silicate bearing psammites and amphibolites; together with the thermobarometric data, this indicates that these high P/T rocks where formed in a continental collision setting within tectonically thickened crust. The petrographical investigation verified that the eclogites at a later stage have undergone extensive retrograde alteration processes where secondary minerals such as horn blende, chlorite, epidote and sphene were formed at the expense of the primary eclogite minerals, omphacite, garnet, quartz and rutile. 40Ar/39Ar and K/Ar isotope studies of the secondary hornblende in the retrograded eclogites indicate that this metamorphism occurred at c. 540 Ma plior to Caledonian deformation and amphibolitefacies metamorphism.