Eclogites and eclogites: Implications for Archean tectonics

Eclogites are generally divided into two types, xenolithic (mantle) and orogenic (crustal). Xenolithic eclogites are late Archean (ca. 3.0−2.5 Ga) or Paleoproterozoic (ca. 2.1−1.7 Ga) in age and are widely interpreted to have been entrained in younger carbonated magmas as they rose through the lithospheric mantle roots of cratons. By contrast, all reliably dated orogenic eclogites are post-Archean, occurring in three periods ca. 2.2−1.7 Ga, 1.2−0.85 Ga, and 0.7 Ga and are generally found in sutures or accretionary complexes. Although the absence of orogenic eclogites from Archean crust may be due to a tectonic mode dominated by plumes, paleomagnetic data from several Archean cratons indicate periods of lithospheric mobility interspersed with periods of stasis, demonstrating differential motions that require active tectonic boundaries between them. The 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 cratonic nuclei in a tectonic mode with episodic subduction. The presence of xenolithic eclogites in the mantle roots of cratons suggests that moderate late-stage thickening was driven by convergence during a transition to global plate tectonics. By contrast, the earliest orogenic eclogites occur in sutures between cratons that form the composite continental fragments familiar to the supercontinent cycle.