Tonalite-Trondhjemite-Granodiorite
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Tonalite–trondhjemite–granodiorite (TTG) series are an aggregation of rocks that are formed by melting of hydrous mafic crust at high pressure. It is widely accepted that most Archaean granite–greenstones are dominated by TTG, although Late Archaean terranes, such as in the Yilgarn Craton, are dominated by potassium-rich granitoid rocks that are derived through remelting of older felsic TTG-dominated crust. According to this model, a much greater degree of crustal reworking has occurred in the Pilbara craton than is required by TTG-dominated crust.
According to Rapp (1999): There is a convincing enough correlation between the composition of many Archean granitoids, and liquids produced in laboratory experiments on natural basalts at 1-4 GPa, to conclude that "wet melting" of garnet-amphibolite and/or eclogite was an important element of early continent formation (i.e., cratonization). Like many early Archean TTG (Tonalite-trondhjemite-granodiorite), wet melts of basalt are sodium-rich, high-Al2O3 granitoids which have high La/Yb, Sr/Nd, and Sr/Y ratios.
Experimental liquids have Mg-numbers (where Mg# = mole% Mg/(Mg+Fe)) that range from 25 to 45, whereas Mg-numbers of early-Archean TTG range from 25-53. Late-Archean, high-Mg tonalites from the Slave Province and members of the "Sanukitoid Suite" from the Superior Province have Mg-numbers in the range 45-63, clearly inconsistent with a simple "basalt melting" origin.
In order to reconcile these discrepancies between the experimental "TTG" liquids and Archean TTGs we have performed a series of experiments in which "TTG" melts are allowed to react with and assimilate a small amount of natural peridotite simulating melt-rock reaction in a "proto-continental" setting. Peridotite assimilation by TTG melts produces hybridized, tonalitic liquids with elevated Mg-numbers (53-56), but trace element ratios that are largely unaffected by melt-rock reaction (i.e., high Mg-number tonalite with TTG trace element characteristics). As the melt:rock ratio in the experiments is reduced (i.e., more peridotite) even further, hybridized liquids evolve to high-Mg andesites compositions comparable to "parental" sanukitoid magmas, with Mg-numbers of 68-71.
A full continuum of processes is envisioned, from the generation of "pristine" TTG magmas by wet melting of garnet amphibolite/eclogite, to hybridization of these melts by assimilation of peridotite, and the consequent metasomatism of the sub-continental mantle. The Mg-number of Archean granitoids is perhaps the best indicator of TTG lineage, and the extent to which the mantle was involved in early continent formation. The evidence thus far suggests an increasingly important role in the late-Archean, with crustal, oceanic basaltic sources dominating in the early-Archean.
[edit] References
Rapp, Robert P. (1999) "First Origins of Archean Continental Crust: Assessing Experimentally the Roles of Mafic Versus Ultramafic Sources." Dept. of Geosciences, State University of New York.[1]
