Zusammenfassung
Die chemische Zusammensetzung der gesamten kontinentalen Kruste, die zu 57% aus der Unterkruste (60% felsische und 40% mafische Granulite) und zu 43% aus Oberkruste besteht, wurde neu ermittelt. Die Proportionen der Krusteneinheiten beruhen auf petrologischen Beobachtungen (Bohlen &Mezger, 1989). Die geschätzte Zusammensetzung der Gesamtkruste liegt zwischen Andesit und Tonalit. Sie ist höher in den Gehalten an Si, K, Rb, Sr, Zr, Nb, Ba, LREE, Pb, Th und niedriger im Mg, Ca, Sc, Mn, Fe als die vonTaylor &McLennan (1985) mitgeteilten mittleren Krustenwerte. Die chemischen Unterschiede zwischen Ober- und Unterkruste werden ausgeglichen, wenn man die Substanz von 12,5% S-Typ-Granit von der Oberkruste abzieht und zur Unterkruste hinzufügt. Als typisches Beispiel der Abtrennung granitischer Partialschmelzen im wasseruntersättigten System wird das der variskischen Metamorphose von Metasedimenten in der Ivreazone (Nord-Italien) angesehen.Schnetger (1988) konnte hier mit einer chemischen Bilanz zeigen, daß die Umwandlung von amphibolitfaziellen zu granulitfaziellen Gesteinen mit dem Verlust von etwa 35% granitischer Schmelze verbunden war. Die negative Eu-Anomalie der Oberkruste ist weltweit doppelt so groß wie die positive Anomalie der Unterkruste. Diese in der Zeit vom Archaikum bis heute vergrößerte Diskrepanz läßt sich nur mit dem Verlust von Ca-reichen Restiten aus der Kruste an den Mantel erklären. Die chemische Zusammensetzung der kontinentalen Kruste hat sich sonst seit dem Archaikum nicht systematisch geändert, wie am Beispiel Grönlands gezeigt wird.
Abstract
A new estimate of the bulk continental crust is reported consisting of 57 percent lower crust (60% felsic and 40% mafic granulites) and 43 percent upper crust. The proportions of crustal units are based on petrological observations (Bohlen &Mezger, 1989). The estimate of a bulk composition is intermediate between andesite and tonalite and is higher in Si, K, Rb, Sr, Zr, Nb, Ba, LREE, Pb, Th concentrations and lower in Mg, Ca, Sc, Mn, Fe than the crustal abundances reported byTaylor &McLennan (1985). Equal chemical composition between the upper crust and the felsic part of the lower crust is attained in balance computations if one restores a fraction of 12.5 percent S-type granite from the upper into the lower crust. An example of water-undersaturated partial melting and separation of a fraction of about 35 percent granitic magma at the conversion from amphibolite-into granulite-facies metasediments has been balanced bySchnetger (1988) in the Ivrea area (N. Italy). The worldwide observed discrepancy between a larger negative Eu anomaly in the upper crust compared with the half as large positive anomaly of the lower crust increasing from the early Precambrian to present has been explained by recycling of Ca-rich restite into the upper mantle. The composition of the Archean crust (example: Greenland) does not differ systematically from the post-Archean crust.
Résumé
Cette note propose une nouvelle estimation de la composition chimique d'ensemble de la croûte continentale, constituée pour 57% de croûte inférieure (60% de granulites felsiques et 40% de granulites mafiques) et pour 43% de croûte supérieure. Les proportions de ces unités crustales sont basées sur les observations pétrologiques (Bohlen etMezger 1989). La composition d'ensemble proposée est intermédiaire entre celles d'une andésite et d'une tonalite; par rapport aux abondances crustales données parTaylor etMcLennan (1985), les teneurs sont plus élevées en Si, K, Rb, Sr, Zr, Nb, Ba, LEE, Pb, Th et moins élevées en Mg, Ca, Sc, Mn, Fe. Si on tranfère de la croûte supérieure à la croûte inférieure la matière correspondant à 12,5% de granite de type S, la différence de composition entre ces deux croûtes disparaît. Un exemple typique de fusion partielle granitique en système sous-saturé en eau est fourni par le métamorphisme varisque de métasédiments dans la zone d'Ivrée (Italie du Nord). D'après ce bilan chimique établi parSchnetger (1988), le passage des roches du faciès des amphibolites à celui des granulites s'accompagne de la production d'environ 35% de magma granitique. L'anomalie négative en Eu de la croûte supérieure est partout le double de l'anomalie positive de la coûte inférieure. Cette différence, qui s'est accrue depuis l'Archéen jusqu'aujourd'hui, s'explique par le passage de restites riches en Ca dans le manteau supérieur. La composition d'ensemble de la croûte continentale ne s'est toutefois pas modifiée depuis l'Archéen, comme le montre l'exemple du Groenland.
Краткое содержание
Заново определили хи мический состав обще й континентальной Зем ной коры, нижняя часть которой составляет 57% е е (60% фельзических и 40% мафических пород), а в ерхняя часть -43%. Соотношения эти, как и приведенное подразд еление основаны на петролог ических исследовани ях (Bohlen & Mezger, 1989). Оценка состава общей материковой ко ры разрешает помести ть ее между андезитами и тоналитами. При сравнении полученны х новых данных с таков ыми Taylor & McLennan (1985) о средней материк овой коре отмечают в ней по вышенный состав Si, K, Rb, Sr, Zr, Nb, Ba, LREE, Pb, Th и пониженные содержан ия Mg, Ca, Sc, Mn, Fe. Химические различия между нижней и верхне й частью материковой к оры сглаживаются, есл и 12,5% гранитов типа S, обычн о причисляемых к верхней части ее, при писать к нижней части. Герцинский метаморф изм метаседиментов зоны Иврей — северная Италия, считают типич ным примером отделения г ранитных локальных расплавов в системе, н едосыщенной водой. Исходя из химическог о состава Schnetger (1988) доказал, что преобраз ование пород амфибол итовой фации в фацию гранули тов связано с потерей гранитных ра сплавов на 35%. В верхней части материковой ко ры на всем Земном шаре отмечается отри цательная аномалия Европия, которая в два раза выше положитель ной ее аномалии в нижней ч асти коры. Такое расхождение, все увел ичивающееся, начиная с архея и продолжающее ся до наших дней, можно объяснить толь ко переходом рестито в, богатых кальцием, из к оры в мантию. На примере Гренландии п оказано, что химическ ий состав материковой к оры с архея не претерп евал значительных измене ний.
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Wedepohl, K.H. Chemical composition and fractionation of the continental crust. Geol Rundsch 80, 207–223 (1991). https://doi.org/10.1007/BF01829361
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DOI: https://doi.org/10.1007/BF01829361