P–T–t development of Archaean granulites in Varpaisjärvi, Central Finland: I. Effects of multiple metamorphism on the reaction history of mafic rocks

Medium pressure granulites are exposed as fault-separated blocks in the Varpaisjärvi area, central Finland. They underwent a multi-stage metamorphic development from the mid-Archaean to Palaeoproterozoic. Partial melting of compositionally basaltic and andesitic rocks produced garnet, plagioclase and pyroxene bearing restitic granulites at 9–11 kbar and 800°C–900°C. Ferromagnesian phases in leucosomes are iron rich, giving slightly lower pressures than those in mesosomes, which suggests that decompression promoted melting. `Peakʹ conditions were followed by cooling and decompression to around 700°C and 7 kbar. Fe-rich and Mg-rich whole rock compositions show different reactions during decreasing pressure and temperature, reflecting the dP/dT slope differences between Mg and Fe endmember reactions. In Mg-rich compositions garnet broke down in the reaction grt+cpx+qtz=opx+pl producing pseudomorphs after garnet, whereas in Fe-rich rocks garnet was a product of reactions cpx+pl=grt+qtz and scp+cpx=grt+CO2. Mg-rich basaltic mafic granulites have small abundances of internal leucosomes, whereas in more Fe-rich andesitic compositions migmatization was more intense. Small quartz–plagioclase films are often present between pyroxenes, amphiboles and garnets representing remnants of melt that was expelled from the restitic mafic granulites. In the northwestern parts of the granulite terrane pressures are higher than elsewhere, which indicates that they represent a deeper section of the crust. The second metamorphic event at lower pressure conditions produced orthopyroxene/hornblende-anorthitic plagioclase symplectites on garnet rims and fractures, and opx–pl and cpx–pl symplectites on hornblende rims. This seems to be connected with the semibrittle fracturing of the bedrock either during the emplacement of the Palaeoproterozoic dolerites at 2.3–2.1 Ga or during the Svecofennian orogeny at 1.89–1.88 Ga. Hydration connected with this event produced epidote filled fractures.