Occurrence of glaucophane and crossite in eclogites of the Seve Nappes, southern Norrbotten Caledonides, Sweden

The alkali amphiboles glaucophane and crossite have been identified in metabasic lenses within quartzitic and calcareous schists and calcite marble (Pieske limestone) in the uppermost part of the Seve Nappes (Upper Allochthon), southern Norrbotten, Sweden. During regional prograde metamorphism, these metabasites transforrned from epidote amphibolite (M1) to quartz-zoisite eclogite (M2a); metamorphic conditions of eclogite forrnation are estimated to be T= 610±- 90°C and Pmin = 14.9 ± 1.5kb, i.e. similar
to the conditions reported for eclogites in the Eastern Seve Belt in the Vasterbotten Caledonides further south. Retrogression of the eclogites occurred in two stages along a steep P-T trajectory and under hydrous conditions. An early stage (M2b), preceded by deformation of the eclogite assemblage, is
characterized by the growth of sodic-calcic amphibole (barroisite) porphyroblasts; the glaucophane and crossite are also inferred to have grown during this stage. Subsequent retrogression (M3) is characterized by the onset of greenschist facies conditions. The occurrence of glaucophane and crossite confirms the presence of high-pressure metamorphism under conditions of a low geotherrnal gradient in the rocks forming the outermost part of the Baltoscandian miogeocline.