Oxygen-isotope systematics of a strongly recrystallized granitic rock complex,Grenvillian Belt,SW Sweden

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 Supracrustal 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. ¹⁸O 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° C, 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 for 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 ¹⁸O 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° C.Estimation of pre-alteration ¹⁸O 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.