Importance of water for Archaean granitoid petrology: a comparative study of TTG and potassic granitoids from Barberton Mountain Land, South Africa

Abstract

A new model for Archaean granitoid magmatism is presented which reconciles the most important geochemical similarities and differences between tonalite-trondhjemite-granodiorite (TTG) and potassic granitoids. Trace element abundances reveal a strong arc magmatism signature in all studied granitoids from Barberton Mountain Land. Characteristic features include HFSE depletion as well as distinct enrichment peaks of fluid-sensitive trace elements such as Pb in N-MORB normalisation, clearly indicating that all studied granitoids are derived from refertilised mantle above subduction zones. We envisage hydrous basaltic (s.l.) melts as parental liquids, which underwent extensive fractional crystallisation. Distinctive residual cumulates evolved depending on initial water content. High-H 2 O melts crystallised garnet/amphibole together with pyroxenes and minor plagioclase, but no olivine. This fractionation path ultimately led to TTG-like melts. Less hydrous basaltic melts also crystallised garnet/ amphibole, but the lower compatible element content indicates that olivine was also a liquidus phase. Pronounced negative Eu-anomalies of the granitic melts, correlating with Na, Ca and Al, indicate plagioclase to be of major importance. In the context of our model, the post-Archaean disappearance of TTG and concomitant preponderance of granites (s.l.), therefore, is explained with secular decrease of aqueous fluid transport into subduction zones and/or efficiency of deep fluid release from slabs.

TTG-type plutonic rocks formed in a modern arc batholith by hydrous fractionation in the lower arc crust

“…Rocks with adequate trace element concentrations are produced after 70-90 vol% fractionation, resulting in volumetrically important amounts of granitoids. Furthermore, the modeled trace element characteristics of Group 2 granitoids are essentially identical to those observed in many Archean TTG gneisses, indicating that high-pressure hydrous fractionation of garnet-bearing lithologies in the lower arc crust could lead to the formation of TTG suites in accordance with the model proposed by Kleinhanns et al (2003).…”

“…Accordingly, granitoids, including the TTG suite, could equally be formed by derivative liquids resulting from hydrous fractional crystallization of basaltic melts at medium to high pressure (e.g., Jagoutz 2010; Kleinhanns et al 2003). Interestingly, even though a pressure dependence of the Na concentration of partial melts has been postulated, experimental data indicate that the high Na/K composition of Archean TTG granites cannot be explained by partial melting or fractional crystallization processes at different depths alone (Moyen and Stevens 2006).…”

Petrology and Geochemistry of the Granitoid Rocks of the Johannesburg Dome, Central Kaapvaal Craton, South Africa

“…TTG suites and GGM .suites have strong similarities such as over enrichment in fluid sensitive elements such as Pb (Martin, 1994;Kleinhans et al, 2003). KJeinhans et al (2003) suggested the characteristic features of TTG's include HESE depletion and distinct Pb enrichment signature seen on the chondrite-normalised REE diagram, as reflected in all JD suites, is evidence that these rocks are derived frotn refertilised mantle above subducion zones.…”

“…Average of 31 Closepet-typf granites (<62% SiOjJ (Martin et al, 2005) 7. Nelspruit pluton (Kleinhans et al, 2003) 8. Average of 267 High Silica Atlakitcs (Martin et al, 2005) 9.…”

“…Partial melts of hydrated mantle were shown to contain a high Ni, Co and Cr concentration, which can be attributed to the high concentrations of these elements in the source region combined with a high degree of tnelting rather than partial tnelting of garnet-amphibolite or edogite, which are bcjth depleted in these elements (Kleinhans et al, 2003). Partial melts from garnetamphibolite and eclogite would be even tnore depleted in these elements and could only acquire elevated Ni, Co and Cr contents through assimilation of peridotite.…”