The granulite- to eclogite- and amphibolite-facies transition: a volume and mass transfer study in the Lindås Nappe, Bergen Arcs, west Norway

This study is focused on a specific outcrop in the Bergen Arcs, Norway where the transition between dry granulite and the hydrated eclogite and amphibolite is exposed. In this outcrop the foliation in the granulite is continuous as it passes through eclogite- and amphibolite-facies rocks, presenting a challenge to understanding the nature of these spatial relationships. Although there is no major change in the bulk chemical composition of all three metamorphic-facies rocks, the loss of ignition (LOI) content increases from granulite to the eclogite and to the amphibolite. During hydration and metamorphism, the density changes from c. 3 g cm−3 for the anorthositic granulite to 3.2 g cm−3 for the eclogite, and 2.75 g cm−3 for the amphibolite. Based on the mass balance equation, eclogitization of the granulite shows a reduction of volume of c. 3% whereas amphibolitization of the granulite gains c. 5% in volume. By assuming equilibrium, modelling the phase equilibria provides estimates of the amount of fluid necessary to form the eclogite and the amphibolite assemblages. Results show that both assemblages can be stable at similar temperature and a similar fluid composition but differ in pressure by c. 10 kbar. This study suggests that the stress generated during hydration of the granulite may influence the local mineral assemblage equilibrium.