High Salinity Retrograde Metamorphic Fluids in the
High Pressure-Low Temperature Gneisses of the
Sopron Area (W-Hungary)

K. Török Department of Petrology and Geochemistry, Eötvös University, Múzeum krt. 4/a, H-1088 Budapest, Hungary


Aqueous fluid inclusions were found in low temperature/high pressure blueschist facies gneisses of the Grobgneiss Series (Eastern Alps). The gneisses contain quartz, potash feldspar, albite, white mica, ± biotite ± grossularite rich garnet (up to 52% grossularite content) ± chlorite ± zoisite. Accessories such as zircon, monazite, ilmenite and apatite also occur. The high pressure conditions are marked by the high Si content of the white mica (up to 3.51 p.f.u.) in association with potash feldspar and biotite. Grossularite-rich garnet also can be the indicator of the high pressure in metagranites and gneisses.

The inclusions can be found mostly in quartz veins and segregations, but matrix albite also contains a few inclusions in some cases.

Fluid inclusions can be classified in two main groups according to their salt content. 1. Low to intermediate salinity, NaCl dominated aqueous inclusions. Two subgroups can be distinguished within this main group according to their mode of occurrence. 1.1. Primary inclusions in the core of matrix albite with solid inclusions of phengitic muscovite and abundant dark, probably decrepitated inclusions. The albite have clear rim without any fluid or solid inclusions. The inclusions contain medium salinity (10.9-12.4 NaCl eqv. wt%) aqueous solution with homogenisation temperature from 178.4 to 217.2oC. Though the inclusions are quite small (less than 8-10 microns) some eutectic temperatures (between -28 and -35.4oC) show the presence of other cations than Na+ (possibly Mg++ and/or Ca++). These inclusions probably contain the fluid which may have been present under high pressure metamorphism and than reequilibrated under lower PT conditions. 1.2. Secondary inclusions in the quartz veins, occurring along healed fractures. The inclusions contain 7.1-12.4 NaCl eqv. wt % salt. Homogenisation temperatures range from 85.4 to 182.0oC. The brines contain predominantly NaCl, with a minor amount of other, divalent cations, as it is shown by the eutectic temperatures (between -22.6 and -31.5oC ) close to the NaCl-H2O eutectic.

2. Intermediate to high salinity aqueous inclusions which can be modelled with the CaCl2 -H2O±NaCl- system. None of the inclusions in this group are supersaturated. All of these inclusions are secondary, occur along healed fractures in quartz veins and segregations. The common in the behaviour of this inclusions is that all of them have eutectic temperature close to -50oC or even lower (down to -76oC). The inclusions of this group can be classified in three groups according to their behaviour on cooling and subsequent heating.
2.1. Inclusions in this group freezed below 100oC and have very low eutectic points (-63.5 - -68oC). Sometimes melting of a bright phase (possibly antarcticite) can be observed at about -51oC. Formation of NaCl hydrate can not be observed. These inclusions can be modelled in the CaCl2 -H2O system. The salinity is between 20 and 28 CaCl2 eqv. wt %. Homogenisation temperatures range between 60.7 and 170.8oC. 2.2. Inclusions of this group have low eutectic temperature (between -56.3 and -78 oC), than ice melting
(-23.5 - -35.7 oC), and the last low temperature phase change is the disappearance of the NaCl hydrate between -7.4 and -23.1 oC. Homogenisation temperatures range between 113.0 and 204.9oC. 2.3. Inclusions of this group have also low eutectic points (-40.7 - -58.4), followed by the disappearance of the NaCl hydrate in the temperature range of -24.4 and -25.7 oC. The last low temperature phase change is the melting of the ice between -14.4 and -23.5oC. Homogenisation temperatures range between 69.7 and 206.6oC.

Fluids in the type 1.1. inclusions are thought to be the witnesses of the preexisting high pressure fluids which may have stretched or opened and reequilibrated during the early stages of the retrograde metamorphism. Similar fluid (Type 1.2.) was present at later stages of the retrograde metamorphism, together with high salinity, but not supersaturated CaCl2 -H2O±NaCl brines.


The author acknowledges the financial support received from the "Magyary Zoltán" postdoctoral scholarship sponsored by the "Foundation for the Hungarian Science and Higher Education". This work was also supported by OTKA grant, F015886.