Evolution of a Metamorphic Fluid During Progressive Metamorphism in the Joroinen-Sulkava Area, Southeastern Finland, as Indicated by Fluid Inclusions

Matti Poutiainen
Department of Geology, University of Helsinki, Finland

Abstract: Fluid inclusions in the progressively metamorphosed rocks of the Joroinen-Sulkava area, located in the south-eastern end of the Raahe-Ladoga zone near the Archaean-Proterozoic boundary, southeastern Finland, fall into four main categories: (1) H2O-rich, (2) CO2-rich, (3) mixed H2O-CO2 and (4) CH4-N2 inclusions. The samples were collected from quartz veins associated with different deformation phases (D2-D4) and from metapelites. The progressive stage of metamorphism took place mainly during the D2 deformation. The age of metamorphism and D2 deformation becomes younger with increase in metamorphic grade from amphibolite to granulite facies.

Regional distribution of the different fluid types indicates a change in fluid regime from H2O to CO2-dominant during the progressive stage of the metamorphism. H2O entered preferentially into the anatectic melt. The possibility of CO2 infiltration from deeper crust can not be excluded, because granulite facies rocks occur most probably below the lower grade zones. A zone enriched in CH4-N2 fluids is located near the lineament zones caused by the D3 deformation. This fluid type dominates the Au-bearing D2–D3 quartz lenses in the K-feldspar-sillimanite zone. Density data of early CO2 inclusions in combination with estimates of metamorphic temperatures (645–750°C) in the different metamorphic zones indicate a pressure range of 3.0–4.5 kbar, which is consistent with data derived from mineral geobarometry. The diversity of fluid types encountered in the D2–D4 quartz veins are a result of the passage of different fluids through veins at different times without re-equilibrating with the wall rocks. However, it is supposed that the CH4-N2 fluid is derived from a CO2-rich fluid with XCH4 ⩽ 0.4 by re-equilibration during its passage through the rocks.

Keywords: metamorphic fluid • fluid inclusions • progressive metamorphism • Finland

Mineralogical Magazine; June 1990 v. 54; no. 375; p. 207-218; DOI: 10.1180/minmag.1990.054.375.07
© 1990, The Mineralogical Society
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