The Punctuated Continuum of Plate Tectonics Revealed by Global Igneous Rocks

Abstract:

Plate tectonics imparts a distinctive chemical signature to continental crust. Previous studies have demonstrated several major changes in crustal composition through time – particularly at ca. 3.0–2.5 Ga, and again at ca. 1.2 Ga – but their nature is debated: some argue for wholesale plate tectonic shifts, whereas others ascribe these changes to continuous underlying phenomena. Here, we performed a suite of multivariate statistical analyses to determine how continental igneous rocks are compositionally distributed through time. We demonstrate that continental crust is compositionally bimodal throughout the rock record – broadly corresponding to “mafic” and “felsic” – and that crustal or lithologic averages represent mixtures of discrete, separable signals. Mafic rocks record an uninterrupted, ~3.5–4.0 Gy lineage of basalt formation, with a ca. 2.5 Ga discontinuity broadly recording continental emergence above wave base, the development of continental lithospheric mantle, and the rise of modern intracrustal differentiation processes (i.e., arc cumulate delamination and sediment relamination). Felsic rocks record a shift in dominance from basalt/greywacke to mature sediment melting after ca. 2.5 Ga, and a later (ca. 1.2 Ga) maximum in basal heat flux to the continents. These trends indicate the plate-tectonic formation of continental crust since ca. 3.5 Ga, with both gradual and punctual changes arising from continuous secular cooling. Finally, we observe linear to exponential increases in continental mafic-rock V/Sc, Fe³⁺/Fe_tot, and P₂O₅ since the early Archean, suggesting the progressive oxidation and fertilization of the continents in response to gradual crustal thickening and changes in mantle melting. Although not necessarily causal, these trends firmly link the chemical evolution of continental lithosphere to atmospheric oxygen and the rise of life.