Abstract 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.

Abstract The northwestern part of Holsnøy island, in the Bergen Arcs, Norway, consists of a granulite-facies protolith partially transformed at depth in eclogite (700 °C, > 19 kbars) and amphibolite (650 °C, 8–10 kbars) facies during the Caledonian orogenesis. Eclogitized zones are mainly planar objects (fractures with parallel reaction bands and cm-to-100 m-scale shear zones). Eclogitic zones are distributed in two sets of orientations and the associated deformation can be described as ‘bookshelf tectonics’. The major shear zones strike around N120 and dip to the North, and show consistent top-to-the-NE shear sense throughout the area. In the large-scale kinematic frame of Caledonian NW-dipping slab, eclogitic shear zones are interpreted as the way to detach crustal units from the subducting slab and to prevent their further sinking. As the retrograde amphibolitic deformation pattern is similar to the eclogitic one, the detached crustal units started their way up along these eclogitic shear zones. Radiometric ages of eclogitic and amphibolitic metamorphism and their comparison with the chronology of Caledonian orogenesis show that the deformation recorded on Holsnøy occurred in a convergent context. The mechanism we propose can thus account for the first steps of exhumation during collision.

The anorthosite arc of the Bergen area contains a thick series of gabbroic to anorthositic rocks, with garnet-pyroxene-plagioclase mineralogy. These rocks are infolded with amphibolite-facies Cambro-Silurian supracrustal rocks, deformed during the Caledonian orogeny. Eclogites occur as thick layers and as crosscutting dikes in the anorthosites. Olivine pods, apparently formed from disrupted layers, have reacted with plagioclase during cooling, passing through the generalized reactions: 1. olivine + plagioclase → aluminous pyroxenes + spinel, and 2. aluminous pyroxenes + spinel + plagioclase → low-Al pyroxenes + garnet. The resultant corona structures have orthopyroxene cores, surrounded successively by shells of low-Al clinopyroxene and garnet (+ inclusions of aluminous clinopyroxene and spinel). Microprobe analyses show the aluminous clinopyroxene to be enriched in Tschermak’s silicate (Ts) and low in jadeite (Jd), whereas the low-Al clinopyroxene has Jd/Ts > ½. Slow uplift has forced simultaneous exsolution of jadeite from the low-Al clinopyroxene and of grossular from garnet, leading to a strong zoning of the garnet shell and eventual appearance of a plagioclase shell between the clinopyroxene and the garnet. The eclogites found as conformable layers are quartz free and the pyroxenes are low in Na, but with high Jd/Ts ratios; the eclogites originally may have been cumulate olivine-pyroxene-plagioclase layers. The crosscutting eclogites are quartz bearing and the pyroxenes are omphacites or chloromelanites, typical of low-T eclogites. The mineralogy of the coronas and eclogites suggests that the anorthositic rocks originally crystallized from a magma at P < 8 kb, and were buried to 10 to 12 kb during cooling. The great difference in metamorphic grade between the anorthosites and the Cambro-Silurian rocks rules out interpretation of the anorthositic rocks as either Caledonian intrusives or as a late Precambian sequence conformably underlying the Cambro-Silurian rocks. Decompression breakdown of garnet and Tschermakitic pyroxene is common in the anorthosites of the Caledonian Jotun nappes, but is absent in the Bergen anorthosites. This difference suggests there was no rapid vertical movement in the P-T history of Bergen anorthosites, and thus that they are tectonically distinct from the anorthosites of the Jotun nappes.