FLUID INCLUSIONS IN ROCKS FROM THE CENTRAL KOLA GRANULITE AREA (BALTIC SHIELD)

Abstract

Metamorphic rocks of the Archean Central Kola granulite area, Baltic Shield, contain two major fluid-inclusion types: gaseous and salt-bearing aqueous (brines). Most of the abundant gaseous inclusions are CO2-rich, containing up to 5-35 mol.% N2. Almost pure N2-bearing (with occasionally up to 7.9 mol.% CH4) and CH4-rich inclusions also occur. Brine inclusions contain CaCl2 (up to 21.8-28.3 eq.wt.%)- or NaCl (9.9-30.5 eq.wt.%)-rich fluids. Late, low-salinity aqueous inclusions (mostly pure H2O, containing occasionally up to 6.1 eq. wt.% NaCl) have also been observed. N2-rich and CO2-rich fluids with maximum N2 contents occur only in BIF and closely related enderbites, together with CaCl2-bearing brine inclusions. Other fluid types occur in all other investigated samples (pelitic gneiss and plagiogneiss, enderbites). The increase of N2 content (from 5 up to 35 mol.%) in the CO2-rich fluid causes a regular decrease of both homogenization (Th, always to liquid) and final melting (Tm) temperatures, from -17 to -45.4°C and -57.7 to -61.7°C, respectively. In pure to almost pure (less than 5 mol.% N2) CO2 inclusions, density variations for synchronous (syngenetic) inclusions (GSI = groups of synchronous inclusions) are essentially due to post-trapping changes, either volume changes (notably volume decrease, leading to a density increase of the enclosed fluid) and/or leakage (density decrease). The Central Kola granulite area has been submitted to three metamorphic events: M1 (670 ± 20°C, 5.1 ± 0.5 kbar), M2 (565 ± 15°C, 4.0 ± 0.5 kbar) and lower-grade M3. From the location of the inclusions in minerals equilibrated at a given metamorphic stage, and from comparison between inclusion isochores and mineral P-T data, it is inferred that N2-, CaCl2- and most CO2-rich inclusions are related to the M1 event. Some primary, lower-density CO2 inclusions in M2 garnet represent M2 fluid. They indicate CO2 pressure at M2 conditions lower than metamorphic pressure (by about 1.1 kbar), whereas some isochores for M1 and M2 inclusions (V = 46.7 and 51.2 cm3/mole, respectively) correspond to fluid pressures higher than metamorphic pressure. This is explained by a combination of partial H2O leakage and volume decrease of the inclusion during post peak-metamorphic conditions. The overall interpretation leads to a model of cyclic alternation of stable (quiet) subisobaric cooling periods (post M1 and M2), followed by short episodes of decompression during uplift (leading to M2 and M3, respectively).