Pre-plate tectonics and origin of continents

Abstract

<p indent="0mm">Established in the mid-1960s and regarded as a revolution in Earth sciences, the plate tectonics theory can reasonably interpret nearly all geological phenomena, processes, and events that happened during post-Archean time (from 2.5 billion years ago to the present), and has also been applied to explain the formation and evolution of continents. According to the plate tectonics theory, the mafic lower crust and the felsic upper crust of continents can develop from an island arc that formed by subduction of one oceanic crust beneath another, where the mafic lower crust of continents can be extracted from the mantle through the partial melting of the mantle wedge in the subduction zone, whereas the felsic upper crust of continents can form by the partial melting of the already-formed mafic lower crust. However, such an island arc model under a plate tectonic regime cannot well explain the magmatic, metamorphic and structural features of Archean (&gt;2.5 billion years) continents. For example, island arc models fail to explain the presence of ~1600°C komatiites, absence of andesites that is dominant in post-Archean arcs, nearly coeval and craton-scale emplacement of tonalite-trondhjemite-granodiorite (TTG) rocks, dominancy of dome-and-keel structures, and lack of ultrahigh-pressure rocks, paired metamorphic belts and ophiolites, etc. All of these imply that Archean continents may not have been formed under plate tectonic regimes, but were originated from some pre-plate tectonics (non-plate tectonics). So far, researchers have proposed a number of pre-plate tectonics models, of which the most representative ones are mantle plumes, sagduction, heat-pipes and stagnant lids. Although each of these pre-plate tectonics models can satisfactorily interpret some features of Archean cratons, none of them is successful in explaining all lithological, structural and metamorphic features of Archean cratons. For example, although the mantle plume-derived oceanic plateau models can well explain many features of Archean cratons, oceanic plateaus formed by mantle plumes may not provide enough water (H₂O) for aqueous partial melting of basaltic rocks to create TTG magmas. It is the same case with heat-pipe models. As the sagduction models assume the existence of an old felsic continental crust, they are not suitable for discussing the origin of Archean continents. As for stagnant lids, they just describe the state of a single lithosphere plate which itself cannot provide any geodynamic mechanisms for making a mafic crust be partially melt to form TTG magmas. Therefore, none of available pre-plate tectonics models proposed so far has been successfully applied to interpret the origins of Archean continents. Thus, a number of research groups in the world are conducting extensive and comprehensive investigations on this important scientific conundrum. Although Chinese geologists missed a chance to have made contributions to establishing plate tectonic theory in the 1960s, they have a great potential for breakthroughs in establishing a pre-plate tectonics theory since they have done tremendous work on the early geodynamic mechanisms for the formation and evolution of Archean continents and produced large amounts of new data and competing interpretations in the past four decades.