Fluid inclusions in rocks of the Archean high-grade Varpaisjärvi-Lapinlahti area, central Finland

Abstract

Fluid inclusions both in matrix quartz of metamorphic rocks and in quartz veins from five samples from the Archean high-grade Varpaisjärvi-Lapinlahti area were examined with microthermometric methods. Two types of fluid inclusions were observed: CO2-rich and H2O-rich. Carbonic inclusions are dominant in the granulites and aqueous inclusions in the amphibolite facies terrain. Mineralogic P-T determinations for the granulite area yield 8–10 kb/700–850°C for the peak of the metamorphism. Densities of early CO2-rich inclusions (1.00–0.95 g/cc) are too low for the postulated conditions of formation for the inclusions. Instead, these early CO2-rich inclusions were trapped at relatively high P-T conditions (5–6 kb/775°C) during uplift and erosion of the granulite terrain. This agreement of fluid trapping P-T and the mineral equilibria may indicate fluid-present metamorphism during the late Archean about 2.7 Ga ago. Late CO2-rich inclusions were trapped along the isochores of 0.95 to 0.76 g/cc at pressures and temperatures below 5 kb and 750°C. Aqueous fluids of low-salinity (<3.0 eq.wt.% NaCl) locally penetrated the rocks of the high-grade area during uplift/erosion and contributed to the retrograde hydration effects observed in the rocks.

Keywords

• Fluid inclusions
• metamorphic rocks
• microthermometry
• metamorphism
• Archean
• Varpaisjärvi
• Lapinlahti
• Finland