TEMPERATURE-PRESSURE INVESTIGATION OF THE SOUTHERN PART OF THE SOUTHWEST SWEDISH GRANULITE REGION

Mafic rocks in SW Sweden were metamorphosed in the granulite and upper amphibolite facies during the Sveconorwegian-Grenvillian orogeny. Acc ording to previously published age data, this event took place approxi mately 900 Ma ago. Geothermometry based on Fe2+-Mg exchange among coex isting garnet-clinopyroxene, gamet-orthopyroxene, and garnet-hornblend e indicates temperatures for the granulites and upper amphibolite faci es rocks in the ranges 700-800 degrees C and 680-750 degrees C, respec tively; Estimates from Al-solubility data from orthopyroxene coexistin g with garnet suggest unreasonably high temperatures. This is consider ed an artifact of the low Al-content of the orthopyroxene. Independent evidence shows that peak metamorphic temperatures can be only slightl y higher than the Fe2+ Mg exchange temperatures. Pressure was determin ed from coexisting garnet-clinopyroxene-plagioclase-quartz, garnet-ort hopyroxene-plagioclase-quartz, and garnet-hornblende-plagioclase-quart z parageneses. Resulting pressures are in the range 0.85-1.17 GPa for the granulites and 0.85-1.06 GPa for the amphibolites. The present res ults are considered to be near the peak of metamorphic conditions. Pre ssure and temperature appear to decrease towards the southwest. The re trograde history is recorded by chemical zoning preserved as thin rims around the garnets and pyroxenes. A prograde P-T path is indicated by well-preserved coronitic textures, newly developed metamorphic pyroxe nes and plagioclase in relict igneous textures, and the prograde zonin g of garnet. Using the moderately Ca-rich garnets (X(gross) = 0.17-0.2 9) and wide Fe-Mg range (X(py) = 0.12-0.25), the application of differ ent thermometers and barometers yields an apparent temperature and pre ssure variation up to similar to 135 degrees C and similar to 0.25 GPa , respectively.