Archean andesites in the east Yilgarn craton, Australia: Products of plume-crust interaction?

The timing of onset of modern plate tectonics on Earth is one of the fundamental unsolved problems in geology: How similar were the tectonic processes on early Earth, when the mantle was hotter and the crust more ductile, to those operating today? A key line of evidence for Archean (pre-2.7 Ga) plate tectonics rests on the presence of andesites, intermediate lavas that are the signature rock type of modern subduction zones. The 2.7 Ga Eastern Goldfields superterrane of the Yilgarn craton (herein east Yilgarn craton) in Western Australia is a richly mineral-endowed crustal element that has been a prime focus of debate between proponents of an uniformitarian, plate-tectonic-driven interpretation, and advocates of an alternative model wherein the entire assemblage of igneous rocks is derived ultimately from mantle plume activity. Andesites are a key component of the volcanic stratigraphy and potentially provide critical clues to the evolution of this piece of Archean lithosphere. Whereas east Yilgarn craton andesites have incompatible trace-element characteristics similar to those of modern island-arc andesites, they are distinguished by unusually high Ni, Cr, and MgO contents. Numerical modeling of fractionation of plume-related tholeiitic basalts, coupled with contamination by contemporaneous partial melts of preexisting continental crust, provides a good fit to this feature, along with all of the essential major-and trace-element characteristics of the east Yilgarn craton andesites. Thus, a rock type previously taken as a key line of evidence for plate-tectonic processes in the east Yilgarn craton can be explained just as well by a plume-driven mechanism, which is more consistent with the overwhelmingly plume-derived character of basalts and komatiites across the entire craton. This explains a paradox noted in many pre-2.7 Ga volcanic rock sequences around the world, namely, that apparently subduction-related rocks are interleaved with voluminous basaltic magmatism derived from 1000-km-scale plume-head arrival events. The problem is moot if Archean andesites are products of plume, not subduction-zone, magmatism.