Detta kandidatarbete i geologi undersöker berggrunden på Utös nordöstra udde i Stockholms skärgård, med målet att fastställa metamorfa tryck-, temperatur- och fluid-XCO2-förhållanden. Utös berggrund metamorfoserades ca 1,87-1,78 Ga när en öbåge kolliderade med den arkeiska kratonen. Tidigare deponerade karbonater, vulkaniter och gråvackor blev metamorfoserade i samband med kollision och senare exstensionell kollaps. I studieområdet hittas karbonater omvandlade till marmor och skarnbergarter, som i vissa fall, troligtvis är hydrotermalt omvandlade till sulfidbergarter i samband med intrusioner i orogenesens senare skede. Gråvackor hittas omvandlade till metapeliter bärandes indexmineral. Kalcit-dolomit-termometri på tremolit-, talk- och flogopitbärande skarnbergarter gav temperaturer på 379°C respektive 412°C som kunde härledas till fluid-XCO2-koncentrationer på 0,0039 och 0,012. Geotermobarometri för metapeliter i området resulterade i osäkra tryck- och temperaturvärden p.g.a. en begränsad och retrograd mineralsammansättning, men antyder att metamorf grad tidigare varit högre. Skarn och relaterade sulfidbergarter kunde med vissa undantag klassificeras som Zn-Pb-skarn utifrån mineralogisk sammansättning av zinkblände, blyglans och pyrit. Retrograda förhållanden inom greenschist facies uppskattades utifrån framräknade temperaturer samt förekomst av reaktionstexturer.

This geothermobarometric investigation of St Persholmen, Utö, in the south central part of Sweden presents an attempt at determining the metamorphic conditions of this important part of the Svecofennian province. Belonging to the geology of the Bergslagen area, Utö historically represent part of Sweden’s vast ore resources with concentrations of iron, copper and sulfides. Rock types from this area are around 1.91-1.89 Ga old (Stephens et al. 2009) and as such Paleoproterozoic in age. The rocks on Utö are considered representative of Bergslagen and record the closing of an ocean starting with subduction followed by volcanic episodes and orogeny (Talbot 2008). The bedrock we observe at Persholmen is thought to represent the remains of the aforementioned orogeny where greywackes from the oceanic stage have been preserved at the base of the mountain range (Stålhös 1982). The two rock types of interest at Persholmen which have been evaluated in this study are 1) normal greywackes and 2) greywackes which have been migmatised either because of the influence of fluids, reworking in an accretionary prism or melted at the base of a mountain range. In this project the area of study has been mapped and samples have been retrieved in order to distinguish the mineralogy and metamorphic history of the bedrock. After petrographic analysis I have determined mineral chemistry by the use of a SEM (Scanning Electron Microscope). These chemical data have then been entered into the computer programs AX and THERMOCALC for determination of temperature and pressure. For the normal/migmatised greywackes a temperature of 538 ±36/756±133°C and a pressure of 3.1±1.3/3.8±3.2 kbars respectively have been estimated. Two generations of muscovite provide evidence of fluid-rock interactions and at the north coast of Persholmen the occurrence of sillimanite indicates a high grade of metamorphism.

Berggrunden på norra Utö har karterats i fält som en del av projektet Metamorphic map of Sweden. Vidare har stuffer analyserats i tunnslip och mikrosond, för att med datorprogrammen THERMOCALC, AX samt Anovitz & Essenses kalcit-dolomit-geotermometer (1987) för kunna fastställa vilken grad av metamorfos området utsatts för. Berggrunden visade sig utgöras främst av pelagiska metasediment som karbonatbergarter och gråvackor, med avbrott i form av felsiska vulkaniter som följer lagringen. I fält såväl som mikroskop indikerade resultaten en relativt låg grad av metamorfos. De geotermobarymetriska uträkningarna som utförs i rapporten stöder detta resultat och bestämmer temperaturen till mellan 430 och 697°C samt trycket 4.5±2.3Kbar, vilket placerar metamorfosen inom ”Amphibolite facies”. Undersökningar kring XCO2-värdet i den fluid som verkat under metamorfosen uppvisade resultat med stora osäkerheter. / metamorphic map of sweden

This thesis attempts to interpret the metamorphic condition and fluid composition experienced by metacarbonates on Utö, located in the south-eastern parts of the Stockholm archipelago. Utö is a part of the Svecofennian domain, and the area Bergslagen, that has hosted several mines over the last millennia. The bedrock in this area has been dated by the U-Pb technique to 1.90-1.87 Ga, placing it in the Paleoproterozoic era (Allen et al. 1996, Lundström et al. 1998). The rocks, of the studied area on the island of Utö, are mainly metacarbonates with a varying purity and thin layers of volcanic ash. These rocks become more felsic towards the north-western coast as the layers of felsic ash become more dominant. To determine the P-T-XCO2 of metamorphism, metacarbonates were examined, in the field, in thin sections and mineral chemistry was determined by SEM analysis. Three samples were collected along a 1km transect, along which the assemblage calcite + dolomite + quartz + tremolite + diopside was observed. Petrographic and SEM analysis were performed to gather chemical data from coexisting calcite and dolomite in order to calculate temperature using the calcite-dolomite geothermometer. Chemical data from the SEM analysis were also run with AX and THERMOCALC together with pressure data received from a study by Engström (2011) of the adjacent island, Persholmen, to generate a T- XCO2 diagram. Pressure was estimated to 3.1 +/- 1.3 kbars, temperature calculated to 442°C 30°C and XCO2 to range from 0,00067-0,0038 with the standard deviation taken in to account. These results record equilibration with a CO2-bearing hydrous fluid at greenschist facies conditions. / Metamorphic map of Sweden