THE SIGNIFICANCE OF SUBDUCTION-RELATED ACCRETIONARY COMPLEXES IN EARLY EARTH PROCESSES

Abstract

The transition from Hadean-style convective overturn, driven by heating from below, and Phanerozoic-style plate tectonics, driven by the sinking of cool lithospheric slabs, was a major turning point in the thermal evolution of the Earth. Prior to this transition the formation of stable, long-lived crust was rare; after this transition, the formation and amalgamation of continental crust became a central theme of tectonic processes. Evidence for Phanerozoic-style plate tectonics includes (1) the formation of oceanic crust at mid-ocean ridge spreading centers, (2) the formation of island arc volcanic and plutonic complexes, and (3) the formation of accretionary mélange complexes during the subduction of oceanic crust. Rock assemblages characteristic of oceanic crust and island arc suites may also be formed during Hadean-style convection, making the recognition of accretionary mélange complexes the most reliable indicator of Phanerozoic-style tectonics. Accretionary mélange complexes result from the juxtaposition of oceanic crust and pelagic sediments in a matrix of arc-derived constituents and may be recognized even after high-grade metamorphic events. At least three potential Archean mélange complexes have been recognized to date: the Dongwanzi ophiolite mélange of the North China craton, the Schreiber-Hemlo mélange of the Superior province, and the Farmington Canyon complex of the Wyoming province; it has also been proposed that the Isua supracrustals of the Greenland craton represent an accretionary complex, but this proposal has been disputed. Each of these terranes contains lithotectonic elements that could not have formed in a single tectonic setting but are now intimately intermixed. Oceanic components include mafic and ultramafic volcanic rocks, gabbros, harzburgite tectonites, and pelagic sediments (chert); arc components are represented by the mélange matrix, consisting of felsic gneiss with a greywacke composition. These studies of Archean crust show that recognition of accretionary mélange complexes may be a reliable indicator of Phanerozoic-style plate tectonics and that the initiation of recognizable plate-tectonic processes occurred by 3.0 Ga, which coincides with the oldest Phanerozoic-style ophiolite assemblages. This transition may have started as early as 3.8 Ga, which coincides with the end of the terminal lunar bombardment. © 2006 Geological Society of America.