Origin of continents
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    Abstract:

    It is well known that all solid planets in the solar system have a core-mantle-crust structure,but a continental crust with a felsic composition is unique to the Earth inhabited by humans.The Archean continental cratons consist mainly of orthogneisses and supracrustals that were metamorphosed from tonalite-trondhjemite- granodiorite (TTG) plutons and mafic-ultramafic volcanic rocks with minor sedimentary rocks,respectively. Available data suggest that these rock assemblages were derived from oceanic crust or its partial melts.The oceanic crust is generally divisible into ocean basin,mid-ocean ridge,island arc and oceanic plateau (ocean island),of which the ocean basin and mid-ocean ridge have an average crustal thickness no more than 5~10 km,which is too thin to generate Archean TTG magmas. Therefore, continents with felsic composition must have been originated either from island arcs under a plate tectonic regime or from oceanic plateaus derived from mantle plume.The formation of Archean TTG rocks can be explained by the island arc model in which the Archean high-pressure-type TTG rocks is considered to have been derived from the partial melting of subducted slabs,whereas the Archean low-pressure-type TTG rocks equivalent to calc-alkaline granitoids were derived from the partial melting of the juvenile basaltic crust which itself formed by the partial melting of the mantle wedge with addition of fluids released from the subducted slabs.However, the island arc model fails to explain the absence of andesites from the Archean greenstone terranes,the presence of komatiites with temperatures of 1600℃,nearly coeval emplacement of TTG plutons on a cratonic scale,dome-and-keel structures and anticlockwise P-T paths involving isobaric cooling that characterize the deformation and metamorphism of Archean continental cratons.In contrast,the mantle- plume oceanic plateau model can reasonably interpret the origin of the bimodal volcanic assemblages in Archean greenstones of which the tholetiites and komatiites were derived from partial melting of the head and tail of a mantle plume, respectively,whereas the felsic dacite, rhyolitic dacite and rhyolite were derived from the partial melting of the crust.According to the oceanic plateau model, Archean TTG magmas were derived from the partial melting of basaltic rocks from the lower part of oceanic plateau, which can rationally explain why voluminous Archean TTG magmas were produced within a short period and emplaced over the whole craton without any accretionary patterns like those in the Phanerozoic magmatic arcs. The mantle-plume oceanic plateau model can also well explain the Archean dome-and-keel structure,anticlockwise P-Tpaths,and absence of blueschist and paired metamorphic belts that are considered to be earmarks of modern island arcs. On the basis of timely and synthetic review on the island arc and oceanic plateau models, this paper proposes a new two- stage oceanic plateau model for the origination of felsic continents.

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ZHAO Guochun, ZHANG Guowei.2021. Origin of continents[J]. Acta Geologica Sinica,95(1):1-19

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History
  • Received:August 01,2020
  • Revised:September 05,2020
  • Adopted:October 09,2020
  • Online: January 25,2021
  • Published: