In the eastern, external part of the Grenvillian Belt in SW Sweden, five formations of granitic rocks were found in the basement of the Dalslandian Snpracrustal Group. The granitic rocks have been strongly recrystallized but have preserved most of their granitic texture in the process. Most magmatic crystals have been pseudomorphed by metamorphic minerals: quartz, albite, chlorite, biotite, white mica, epidote, titanite, hematite, pyrite and carbonate. Two of the formations have subsequently been affected by a cataclastic deformation and at present consist of mylogneisses.
δ18O whole-rock values for the granitic rocks vary from + 3.0
‰ to + 11.1 ‰. Quartz-apatite, quartz-zircon and quartz-titanite pairs
show 18O/16O fractionations corresponding to equilibrium
temperatures of 550-700 ºC, which are believed to reflect in the main continued
closed-system isotopic exchange at high temperatures following solidification.
In contrast highly positive 18O/16O fractionations
quartz-K-feldspar, quartz-biotite, quartz-chlorite and quartz-sericite pairs in some granitic samples indicate that these rocks have exchanged oxygen with heated, meteoric, H2O dominated fluids. Other granitic samples, however, show virtually undisturbed magmatic 18O/16O fractionation values for the same mineral pairs, even though these rocks are equally altered.
It is concluded that all granitic rock units recrystallized under greenschist facies conditions during the infiltration of fluids under the influence of hydrothermal convection systems set up by the intrusion of the granitic plutons. The fluids probably had a range of δ18O values from ca. - 14‰ to ca. + 10‰, indicating the mixing of distinct fluid reservoirs, one of meteoric origin and the others of magmatic and/or metamorphic origin. The temperature of alteration is estimated at 450-500 ºC.
Estimation of pre-alteration δ18O whole rock values for the five granitic units suggests that three units should be assigned a dominantly S-type origin, where as the other two units may partly or wholly have an I-type origin.