Fluid inclusions in rocks from the Central Kola granulite area (Baltic Shield)
Fluid inclusions in rocks from the Central Kola granulite area (Baltic Shield) (in ECROFI XIV (European Current Research on Fluid Inclusions) symposium, Michel Cathelineau and Jean Dubessy)
European Journal of Mineralogy (December 1998) 10 (6): 1181-1200
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 (sub 2) -rich, containing up to 5-35 mol.% N (sub 2) . Almost pure N (sub 2) -bearing (with occasionally up to 7.9 mol.% CH (sub 4) ) and CH (sub 4) -rich inclusions also occur. Brine inclusions contain CaCl (sub 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 (sub 2) O, containing occasionally up to 6.1 eq. wt.% NaCl) have also been observed. N (sub 2) -rich and CO (sub 2) -rich fluids with maximum N (sub 2) contents occur only in BIF and closely related enderbites, together with CaCl (sub 2) -bearing brine inclusions. Other fluid types occur in all other investigated samples (pelitic gneiss and plagiogneiss, enderbites). The increase of N (sub 2) content (from 5 up to 35 mol.%) in the CO (sub 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 (sub 2) ) CO (sub 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 (sub 2) -, CaCl (sub 2) - and most CO (sub 2) -rich inclusions are related to the M1 event. Some primary, lower-density CO (sub 2) inclusions in M2 garnet represent M2 fluid. They indicate CO (sub 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 (super 3) /mole, respectively) correspond to fluid pressures higher than metamorphic pressure. This is explained by a combination of partial H (sub 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).