THE ROLE OF WATER-FLUXED AND WATER-UNDERSATURATED MELTING IN GARNETIFEROUS AMPHIBOLITES ACROSS A METAMORPHIC TRANSECT IN THE EASTERN SEGMENT OF THE SVECONORWEGIAN OROGEN, SW SWEDEN

Two end-member processes can contribute to partial melting in orogenic belts. Water-fluxed melting occurs when H2O enters a rock lowering the solidus while water-undersaturated (dehydration) melting is a peritectic reaction in which a hydrous mineral assemblage forms a melt and anhydrous solid phases. We have attempted to a first evaluation of the relative contributions of these two processes in the partially melted garnet-hornblende-biotite-plagioclase amphibolites in the Eastern Segment of the Sveconorwegian orogen in Southwestern Sweden. Charlotte Möller and Jenny Andersson (2018; J. Metamorphic Geology) identified a continuous westward increase in metamorphic grade from epidote amphibolite-facies to upper amphibolite- and high-pressure granulite-facies in this area. We examined amphibolites along a traverse beginning near the first appearance of leucosome in these rocks near Värnamo and extending 90km westward to Ullared. Concentrations of Ti in biotite and Ti, Na and K in hornblende tend to increase westward indicating increasing metamorphic grade. Leucosome is dominated by plagioclase and quartz with grain sizes up to 10 times that of the amphibolite host. Analysis of outcrops surfaces indicate that proportions of leucosome vary from 0 to 25 % by area. More variation in the proportion of leucosome from outcrop to outcrop occurs in the east than the west. Plagioclase (predominantly oligoclase) is slightly more sodic (2 to 5 mole percent) in leucosome compared to amphibolite host in 1/3 of the samples investigated and nearly identical in the remainder. Occasional biotite and hornblende megacrysts are associated with leucosome in the eastern portion of the traverse, orthopyroxene and garnet megacrysts appear 75 km from the eastern end of the traverse and clinopyroxene appears at the western end. Existing experimental data and thermodynamic models indicate that water-undersatured melting of amphibolite should produce pyroxenes at lower pressure and pyroxene + garnet at higher pressure. The lack of pyroxene in the eastern portion of the traverse and the variation in leucosome proportions suggest that water-fluxed melting was the predominant processes in the eastern portion of the traverse while water-undersaturated melting occurred in the hotter western portion.