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Mafic Archean continental crust prohibited exhumation of orogenic UHP eclogite

Geoscience Frontiers 2021年第5期12卷 411-420页

作者： Richard M.Palin James D.P.Moore Zeming Zhang Guangyu Huang jon wade Brendan Dyck Department of Earth Sciences University of OxfordOxford OX13ANUnited Kingdom Earth Observatory of Singapore Nanyang Technological UniversitySingapore Institute of Geophysics SGEESVictoria University of WellingtonWellingtonNew Zealand Institute of Geology Chinese Academy of Geological SciencesBeijing 100037China State Key Laboratory of Lithospheric Evolution Institute of Geology and GeophysicsChinese Academy of SciencesBeijing 100029China Department of Earth Environmental and Geographical SciencesUniversity of British ColumbiaKelowna V1V 1V7Canada

The absence of ultrahigh pressure(UHP)orogenic eclogite in the geological record older than c.0.6 Ga is problematic for evidence of subduction having begun on Earth during the Archean(4.0-2.5 Ga).Many eclogites in Phanerozoic and Proterozoic terranes occur as mafic boudins encased within low-density felsic crust,which provides positive buoyancy during subduction;however,recent geochemical proxy analysis shows that Archean continental crust was more mafic than previously thought,having greater proportions of basalt and komatiite than modern-day ***,we show via petrological modelling that secular change in the petrology and bulk composition of upper continental crust would make Archean continental terranes negatively buoyant in the mantle before reaching UHP *** or delaminated Archean continental crust passes a point of no return during metamorphism in the mantle prior to the stabilization of coesite,while Proterozoic and Phanerozoic terranes remain positively buoyant at these *** orogenic eclogite may thus readily have formed on the Archean Earth,but could not have been exhumed,weakening arguments for a Neoproterozoic onset of subduction and plate ***,isostatic balance calculations for more mafic Archean continents indicate that the early Earth was covered by a global ocean over 1 km deep,corroborating independent isotopic evidence for large-scale emergence of the continents no earlier than c.3 *** findings thus weaken arguments that early life on Earth likely emerged in shallow subaerial ponds,and instead support hypotheses involving development at hydrothermal vents in the deep ocean.

关键词： Archean Eclogite Exhumation Secular change Geodynamics Density

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