Friday, December 16, 2016

A Scenario for the Formation of the First Continental Crust During the Archean


Palin et al


Rocks of tonalitic–trondhjemitic–granodioritic (TTG) composition preserved in Archaean terranes represent fragments of the Earth’s earliest-formed continental crust, and are thought to have formed via partial melting of hydrated metabasalt. The geodynamic environments in which such high-grade metamorphism and anatexis may have occurred in the early Earth is strongly debated. Constraining the pressure (P) and temperature (T) conditions at which melts of appropriate composition can be derived from protoliths containing plausible mineral assemblages is central to addressing this question. Phase equilibrium modelling has been undertaken for an enriched Archaean tholeiite bulk composition—a suggested protolith for early-Earth TTG magmas—using newly parameterised thermodynamic models that were specifically developed to evaluate the anatectic behaviour of metabasalt. Assuming minimal H2O saturation at the wet solidus, the potential fertility of the studied metabasalt is greatest if the solidus is crossed at a pressure of ∼11 kbar, where the solidus temperature reaches a minimum of ∼610 °C. Major-element compositions and proportions of calculated partial melts show the best correlation with those of natural Archaean TTGs when in the P–T range ∼800–950 °C and ∼10–18 kbar, which we suggest are optimal conditions for their petrogenesis. Normative geochemistry suggests that these melts would crystallise to broadly trondhjemitic or tonalitic lithologies. Calculated modal proportions of garnet, plagioclase, amphibole, and rutile in the residuum, which control diagnostic trace-element signatures in the melt, also show the best agreement with natural and experimental data at these conditions. Importantly, although partial melts calculated outside of this P–T range still produce TTG-like major-element compositions and normative mineralogies, they would poorly match the diagnostic trace-element signatures of natural Archaean examples owing to the absences of key minerals in the residuum. This optimal P–T range of 800–950 °C and 10–18 kbar defined herein is most likely to characterise metamorphism of hydrated basalt at the base of a 40-km-thick Archaean oceanic plateau/overthickened crust, or else tectonic underplating via shallow subduction, which we therefore suggest are the most likely tectonic settings for the formation of the first voluminous continental crust on the early Earth.

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