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 CO 2 -rich, containing up to 5-35 mol.% N 2 . Almost pure N 2 -bearing (with occasionally up to 7.9 mol.% CH 4 ) and CH 4 -rich inclusions also occur. Brine inclusions contain CaCl 2 (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 H 2 O, containing occasionally up to 6.1 eq. wt.% NaCl) have also been observed. N 2 -rich and CO 2 -rich fluids with maximum N 2 contents occur only in BIF and closely related enderbites, together with CaCl 2 -bearing brine inclusions. Other fluid types occur in all other investigated samples (pelitic gneiss and plagiogneiss, enderbites). The increase of N 2 content (from 5 up to 35 mol.%) in the CO 2 -rich fluid causes a regular decrease of both homogenization (Th, always to liquid) and final melting (Tm) temperatures, from -17 to -45.4 degrees C and -57.7 to -61.7 degrees C, respectively. In pure to almost pure (less than 5 mol.% N 2 ) CO 2 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+ or -20 degrees C, 5.1+ or -0.5 kbar), M2 (565+ or -15 degrees C, 4.0+ or -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 N 2 -, CaCl 2 - and most CO 2 -rich inclusions are related to the M1 event. Some primary, lower-density CO 2 inclusions in M2 garnet represent M2 fluid. They indicate CO 2 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 cm 3 /mole, respectively) correspond to fluid pressures higher than metamorphic pressure. This is explained by a combination of partial H 2 O 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).
