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Internal Differentiation of the Archean Continental Crust: Fluid-Controlled Partial Melting of Granulites and TTG-Amphibolite Associations in Central Finland |
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| Authors: | Nehring Franziska; Foley Stephen F; Holtta Pentti; Van Den Kerkhof Alfons M |
| Institution: | 1Johannes Gutenberg University, Institute of Geosciences, C/O Stephen Foley, Becher-Weg 21, 55099 Mainz, Germany 2Johannes Gutenberg University, Institute of Geosciences, Becher-Weg 21, 55099 Mainz, Germany 3Geological Survey of Finland, Betonimiehenkuja 4, FI-02151 Espoo, Finland 4Georg August University of GöTtingen, Geoscience Centre, Goldschmidtstr. 3, 37077 GöTtingen, Germany |
| Abstract: | Fault bound blocks of granulite and enderbite occur within upperamphibolite-facies migmatitic tonalitic–trondhjemitic–granodioritic(TTG) gneisses of the Iisalmi block of Central Finland. Theseunits record reworking and partial melting of different levelsof the Archean crust during a major tectonothermal event at2·6–2·7 Ga. Anhydrous mineral assemblagesand tonalitic melts in the granulites formed as a result ofhydrous phase breakdown melting reactions involving amphiboleat peak metamorphic conditions of 8–11 kbar and 750–900°C.A nominally fluid-absent melting regime in the granulites issupported by the presence of carbonic fluid inclusions. Thegeochemical signature of light rare earth element (LREE)-depletedmafic granulites can be modelled by 10–30 wt % partialmelting of an amphibolite source rock leaving a garnet-bearingresidue. The degree of melting in intermediate granulites isinferred to be less than 10 wt % and was restricted by the availabilityof quartz. Pressure–temperature estimates for the TTGgneisses are significantly lower than for the granulites at660–770°C and 5–6 kbar. Based on the P–Tconditions, melting of the TTG gneisses is inferred to haveoccurred at the wet solidus in the presence of an H2O-rich fluid.A hydrous mineralogy, abundant aqueous fluid inclusions andthe absence of carbonic inclusions in the gneisses are in accordancewith a water-fluxed melting regime. Low REE contents and strongpositive Eu anomalies in most leucosomes irrespective of thehost rock composition suggest that the leucosomes are not meltcompositions, but represent plagioclase–quartz assemblagesthat crystallized early from felsic melts. Furthermore, similarplagioclase compositions in leucosomes and adjacent mesosomesare not a ‘migmatite paradox’, as both record equilibrationwith the same melt phase percolating along grain boundaries. KEY WORDS: Archean continental crust; fluid inclusion; granulite; migmatite; partial melting |
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