Temperature-pressure Investigation of the Southern Part of the Southwest Swedish Granulite Region
Xiang-Dong, Wang; Lindh, Anders
published: Feb 22, 1996
manuscript accepted: Oct 26, 1995
manuscript received: May 8, 1995
ArtNo. ESP147050801007, Price: 29.00 €
Abstract Mafic rocks in SW Sweden were metamorphosed in the granulite and upper amphibolite facies during the Sveconorwegian-Grenvillian orogeny. According to previously published age data, this event took place approximately 900 Ma ago. Geothermometry based on Fe2+-Mg exchange among coexisting garnet-clinopyroxene, garnet-orthopyroxene, and garnet-hornblende indicates temperatures for the granulites and upper amphibolite facies rocks in the ranges 700-800°C and 680-750°C, respectively. Estimates from Al-solubility data from orthopyroxene coexisting with garnet suggest unreasonably high temperatures. This is considered an artifact of the low Al-content of the orthopyroxene. Independent evidence shows that peak metamorphic temperatures can be only slightly higher than the Fe2+-Mg exchange temperatures. Pressure was determined from coexisting garnet-clinopyroxene-plagioclasequartz, garnet-orthopyroxene-plagioclase-quartz, and garnet-hornblende-plagioclase-quartz 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 results are considered to be near the peak of metamorphic conditions. Pressure and temperature appear to decrease towards the southwest. The retrograde 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 pyroxenes and plagioclase in relict igneous textures, and the prograde zoning of garnet. Using the moderately Ca-rich garnets (Xgross = 0.17-0.29) and wide Fe-Mg range (Xpy = 0.12-0.25), the application of different thermometers and barometers yields an apparent temperature and pressure variation up to ~135°C and -0.25 GPa, respectively.