大陆的起源

太阳系固体星球都有类似的核-幔-壳结构,但唯独人类居住的地球具有长英质组成的大陆壳。太古宙大陆克拉通主要由英云闪长岩(Tonalite)-奥长花岗岩(Trondhjemite)-花岗闪长岩(Granodiorite)为主的TTG深成侵入体变质而成的正片麻岩和由基性-超基性酸性火山岩及少量沉积岩变质的表壳岩(绿岩)组成。已有的资料显示这些太古宙大陆岩石组合起源于大洋壳的部分熔融。大洋壳分为大洋盆地、洋中脊、岛弧和洋底高原(大洋岛)。前两者地壳的平均厚度只有5～10km,不可能成为形成太古宙TTG深成侵入体的场所。因此,长英质大陆或起源于板块构造体制下的岛弧,或起源于地幔柱体制下的洋底高原。板块构造体制下的岛弧模式能够很好地解释太古宙克拉通TTG深成岩的成因,即俯冲大洋板片部分熔融所形成的埃达克岩相当于太古宙高压(高Al₂O₂)型TTG,而俯冲板片脱水导致地幔楔部分熔融形成的玄武质地壳再次熔融所形成的钙碱性花岗质岩石相当于太古宙低压(低Al₂O₂)型TTG。然而,板块构造体制下的岛弧模式不能令人满意地解...

Origin of continents

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-T paths,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.