The petrological record constrains Archean tectonics and transitions between Earth’s tectonic modes

Earth’s tectonic mode during the Precambrian is controversial: one school argues that plate tectonics started in the Hadean and has probably operated continuously since, whereas another considers Earth's tectonic mode prior to the Proterozoic to have been different due to high ambient mantle temperature. Implicit in the second view is the necessity of transitions between different tectonic modes. Is there evidence of such transitions preserved in the petrology of the continental lithosphere? If so, does the petrological record allow us to constrain the timing of the transitions and differentiate between these two schools of thought? Whatever the tectonic mode was early on, there is clear petrological evidence of a change in tectonic mode after the first 700-1000 Myr of Earth evolution, which is marked by the onset of mantle depletion and the first appearance of substantial volumes of continental crust in the geological record. What then was the tectonic mode after this transition?

From an igneous perspective, based on geological relationships and Th–Nb systematics, purported Archean ‘ophiolites’ do not represent oceanic crust and subduction-related rocks are rare before the Proterozoic. Much of the extant Archean crust was likely generated by plumes, with limited lithospheric extension and convergence, and only short-lived episodic subduction (Brown et al., 2024, JGS). From a metamorphic perspective, Archean crust is characterized by a unimodal distribution of metamorphic T/P, consistent with plume-driven mantle dynamics (Brown et al., 2024, JGS). Further, the eclogite record provides an important constraint on Archean tectonics. Xenolithic (mantle) eclogites, scavenged by younger carbonated magmas as they rise through the lithospheric mantle roots of cratons, are mostly older than Mesoproterozoic and represent oceanic crust that was subducted to mantle depths during the later stages of craton formation (Brown et al., 2026, in review). By contrast, all reliably dated orogenic (crustal) eclogites are post-Archean and are generally found in sutures or accretionary complexes (Brown et al., 2026, in review). The presence of xenolithic eclogites in the mantle roots of Archean cratons suggests that moderate late-stage thickening was driven by subduction, whereas orogenic eclogites occur in sutures between cratons that form the composite continental fragments formed in the first supercontinent cycle.

Paleomagnetic data from Archean cratons require periods of lithospheric mobility at rates like those in the Phanerozoic (cm/yr), prolonged periods of stasis and brief periods of rapid mobility (up to tens of cm/yr), and differential motion between cratons, requiring active tectonic boundaries between them. This apparent contradiction between a dominantly plume origin for cratonic crust and periods of lithospheric mobility can be reconciled if tectonic units were larger than the preserved cratons, and subduction was off craton in an episodic mode. Thus, xenolithic and orogenic eclogites record complementary information about Archean subduction (mostly warmer and episodic, may result in soft collisions but does not generate orogenic eclogites) and Proterozoic subduction (mostly colder and continuous, may result in subduction of continental margins to eclogite facies conditions), where the change relates to the global emergence of plate tectonics.