High‐pressure granulite facies re‐equilibration and zoisite–biotite dehydration melting during decompression of an ultrahigh‐pressure garnet clinopyroxenite from the island of Fjørtoft, Norway

Abstract

AbstractA garnet clinopyroxenite from the island of Fjørtoft, Western Gneiss Region (WGR) in Norway, has experienced a complex metamorphic evolution demonstrated by studies involving petrographic observations, mineral chemistry, phase equilibria modelling and geothermobarometry. This rock, originally an eclogite, documents ultrahigh‐pressure (UHP) conditions higher than 3.5 GPa (Stage 1), witnessed by Ca‐rich garnet inclusions in kyanite. The UHP assemblage was pervasively re‐equilibrated at high‐pressure (HP) granulite facies conditions around 1.75 GPa and 870°C, during which and/or somewhat before Ca‐poor garnet, sodian diopside, zoisite and biotite formed (Stage 2a). Dehydration melting of zoisite and biotite (Stage 2b) took place at similar pressure–temperature conditions. The produced melt was poor in Si and rich in alkalis and crystallized mainly to analcime + plagioclase + K‐feldspar. Oriented composite inclusions of quartz + amphibole + phengite ± garnet, found in clinopyroxene, are interpreted to have resulted from interaction between a fluid (hydrous melt and/or aqueous fluid) and clinopyroxene at the early retrogression Stage 3. Stage 1 resulted from deep subduction of the rock. Stage 2 followed after early exhumation probably due to stagnation at a hot and thickened orogenic root. Consequently, other HP granulite facies metabasites, which are similar to the studied garnet clinopyroxenite and widespread in the northwestern WGR, could have been transformed from former eclogite at this stage, too. In addition, this study provides a direct link between the generation of peralkaline melts and crustal anatexis in collision zones and new insights into the retrograde formation of oriented mineral inclusions in HP‐UHP minerals.