Imprints of multiple fluid regimes on episyenites in the Bohus granite, Sweden

• A number of episyenite occurrences within the Neoproterozoic Bohus granite, southwestern Sweden, were investigated in order to elucidate their genesis and past associated fluid regimes. Fluid inclusion microthermometry and O-H isotope systematics integrated with information from detailed mineralogy reveal a paragenetic succession within individual episyenites, including: (1) albitization of plagioclase, accompanied by dissolution of quartz, with a vuggy feldspathic reservoir rock as a result; (2) resumed infilling and replacement of igneous Fe-Ti-phases by various phyllosilicates (muscovite, chlorite and/or illite) +/- hematite assemblages and quartz, due to rift-related reactivation and (3) sporadic argillitic alteration and precipitation of fracture calcite. Although sparse owing to later overprinting, there are a few remnants that provide arguments for the involvement of meteoric water, shifted to a more O-18-rich composition, during the initial stage of dissolution and albitization. The main vug-filling phase is proposed to be associated with the ingress of surface-derived fluids with negative delta O-18 values, during the regional thermal anomaly accompanying the development of the nearby Permo-Carboniferous Oslo Rift at similar to 250 Ma The fluids involved during this event show consistently a marked, but variable O-18-depletion and resemble the modern-day shield brines, with modal salinities of 11-24 wt.% NaCl eq., low NaCl/(NaCl + CaCl2) weight ratios and no discernible CO2. Even though no P-T path can be defined for this phase, the fluid pressure can be restricted to 03-0.8 kbar and temperature to 160-280 degrees C, assuming a sedimentary overburden of similar to 3 km. An overall variability in the delta O-18 and delta D data can mainly be ascribed to (1) precipitation from fluids affected by isotopic exchange with the host rock and/or sedimentary overburden under highly variable water/rock ratios, and to some extent with (2) varying degrees of partial re-equilibration during subsequent hydrothermal interaction at lower temperatures, as emphasized by the presence of fluid inclusions of suspected secondary origin in quartz.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Geologi (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Geology (hsv//eng)

NATURVETENSKAP  -- Geovetenskap och miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences (hsv//eng)

Nyckelord

Episyenite

Quartz dissolution

Bohus granite

fluid inclusions

Stable isotopes

Shield brines

Episyenite; Quartz dissolution; Bohus granite

fluid inclusions; Stable isotopes; Shield brines

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