Internal Differentiation of the Archean Continental Crust: Fluid-Controlled Partial Melting of Granulites and TTG–Amphibolite Associations in Central Finland

Fault bound blocks of granulite and enderbite occur within upper amphibolite-facies migmatitic tonalitic–trondhjemitic–granodioritic (TTG) gneisses of the Iisalmi block of Central Finland. These units record reworking and partial melting of different levels of the Archean crust during a major tectonothermal event at 2·6–2·7 Ga. Anhydrous mineral assemblages and tonalitic melts in the granulites formed as a result of hydrous phase breakdown melting reactions involving amphibole at peak metamorphic conditions of 8–11 kbar and 750–900°C. A nominally fluid-absent melting regime in the granulites is supported by the presence of carbonic fluid inclusions. The geochemical signature of light rare earth element (LREE)-depleted mafic granulites can be modelled by 10–30 wt % partial melting of an amphibolite source rock leaving a garnet-bearing residue. The degree of melting in intermediate granulites is inferred to be less than 10 wt % and was restricted by the availability of quartz. Pressure–temperature estimates for the TTG gneisses are significantly lower than for the granulites at 660–770°C and 5–6 kbar. Based on the P–T conditions, melting of the TTG gneisses is inferred to have occurred at the wet solidus in the presence of an H2O-rich fluid. A hydrous mineralogy, abundant aqueous fluid inclusions and the absence of carbonic inclusions in the gneisses are in accordance with a water-fluxed melting regime. Low REE contents and strong positive Eu anomalies in most leucosomes irrespective of the host rock composition suggest that the leucosomes are not melt compositions, but represent plagioclase–quartz assemblages that crystallized early from felsic melts. Furthermore, similar plagioclase compositions in leucosomes and adjacent mesosomes are not a ‘migmatite paradox’, as both record equilibration with the same melt phase percolating along grain boundaries.