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

Abstract

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 2Á6^2Á7 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 750^9008C. 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 660^7708C and 5^6 kbar. Based on the P^Tconditions, melting of theTTG gneisses is inferred to have occurred at the wet solidus in the presence of an H 2 O-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.

Mechanisms of Archean crust formation inferred from high-precision HFSE systematics in TTGs

“…likely to control the Nb-Ta budget at amphibolite facies conditions (e.g., Foley, 2008), whereas ilmenite (Nehring et al, 2009a) or rutile (e.g., Bohlen and Mezger, 1989) are most likely to control the Nb-Ta budget at granulite facies conditions. This is supported by recently published melt/ mineral partition coefficients obtained on granulite facies samples by Nehring et al (2009b) indicating a strong fractionation of Nb/Ta towards higher Nb/Ta in coexisting melts (D Nb /D Ta ilmenite/melt : 0.61), slightly lower than experimentally derived ilmenite/melt partition coefficients for mafic systems (D Nb /D Ta ilmenite/melt : 0.65-0.70; Klemme et al, 2006). Hence, fluid-and melt-enriched middle crustal rocks may potentially represent a high Nb/Ta reservoir, if compared to the composition of the lower crust (Fig.…”

The sources of energy for crustal melting and the geochemistry of heat-producing elements

“…The peraluminous sources are six examples from the Peña Negra Anatextic Complex of Central Iberia (Bea et al, 1994), four from the Kinzigite Formation of the IvreaVerbano zone, Italy, , one from the Greenland Caledonides (Kalsbeek et al, 2001) and one from the Southern Appalachians (Coler et al, 1997). The metaluminous sources are two examples from partially molten gabbro-diorites from the Urals (Fershtater et al, 1998), one from similar materials in Cuba (Lázaro and García-Casco, 2008), three from Finland (Nehring et al, 2009), and three from Sardinia (Cruciani et al, 2008). Fig.…”

High‐grade metamorphism and partial melting in Archean composite grey gneiss complexes

“…This is largely due to their relatively simple mineral assemblages and relative insensitivity to changes in PÀT (e.g. Nehring et al, 2009;Johnson et al, 2013), with metabasic and metapelitic rocks typically being more amenable to classic thermobarometric methods. However, a number of experimental studies have been undertaken on broadly intermediate to tonalitic compositions at high temperatures and pressures to investigate the partial melting of such compositions (e.g.…”