Zusammenfassung
Die spätpräkambrischen Shadli-Metavulkanite in der südöstlichen Eastern Desert von Ägypten sind eine schwach metamorphe bimodale Basalt-Rhyodazit-Abfolge, die bisher als Ausdruck eines Inselbogen-Vulkanismus gedeutet wurde. Zwei Basalt-Typen können aufgrund ihrer unterschiedlichen chemischen Zusammensetzung unterschieden werden: der stratigraphisch untere Typ ist ein N-MORB ähnlicher Ferrobasalt, während der überlagernde Typ Charakteristika eines leicht fraktionierten E-MORB aufweist. Die beiden Basaltvarietäten werden von stark an inkompatiblen Elementen verarmten Schmelzen aus den oberen 60–75 km des Mantels abgeleitet. Die Entstehung der sauren Metavulkanite ist nicht eindeutig geklärt; sowohl Fraktionierung aus einer mafischen Schmelze als auch Aufschmelzung juveniler Kruste vom Inselbogen-Typ sind denkbar. Die mafischen und felsischen Gesteine definieren zusammen ein Rb-Sr-Isochronenalter von 712 ± 24 Ma, das wir als den Zeitraum der Eruption deuten. Die Spurenelement-Verteilung der Shadli-Metavulkanite weist keine der charakteristischen Merkmale von Subduktionsmagmatismus auf, und wir sehen daher keinen direkten Zusammenhang mit einer Inselbogen-Entwicklung. Wir interpretieren den Shadli-Vulkanismus als Resultat eines Riftprozesses in junger kontinentaler Kruste, ähnlich dem Rio Grande-Rift oder dem Afar-Dreieck, wo starke Lithosphärendehnung die Förderung großer Lavamengen ermöglichte. Diese Interpretation stellt das einfache Schema einer panafrikanischen Krustenbildung durch Inselbogen-Addition im arabisch-nubischen Schild in Frage und erfordert eine Neubewertung bisheriger Modellvorstellungen.
Abstract
The late Precambrian Shadli Metavolcanics of SE Egypt constitute a slightly metamorphosed bimodal sequence that has been previously interpreted as manifesting volcanic activity at an island arc. We report the first Rb-Sr geochronologic, trace element (including REE), and Nd isotopic data for these rocks. Two types of basalt are recognized, the stratigraphically lower suite having compositions like N-MORB ferrobasalt while the overlying basalt is similar to slightly fractionated E-MORB. The two basalt types were derived from melting of a strongly depleted source, most likely within the upper 60–75 km of the upper mantle. The origin of the felsic melts is problematic, and these could either have fractionated from a mafic melt or resulted from melting of juvenile crust. The mafic and felsic lavas yield a Rb-Sr isochron age of 712±24 Ma that probably represents the time of volcanic eruption. The trace element characteristics of both mafic and felsic members of the Shadli Metavolcanics show few of the hallmarks of subduction-related melts, and we reject the hypothesis that these formed at an island arc. Instead, the field and geochemical data are most consistent with the hypothesis that these rocks originated in a magmatic rift, where the eruption of large volumes of lava accompanied large-scale lithospheric extension. This inference suggests that the tectonic setting of the important 700–715 Ma crust-forming event in NE Africa and Arabia needs to be critically reexamined.
Résumé
Les roches volcaniques faiblement métamorphisées du Précambrien tardif de Shadli dans le Sud-Est de l'Egypte se caractérisent par une séquence bimodale qui était jusqu'ici interprétée comme résultant d'un volcanisme d'arc insulaire. Nous présentons ici les premières données isotopiques (Rb-Sr, Nd) obtenues pour ces roches ainsi que des données d'éléments en trace (y compris les Terres Rares). Deux types principaux de basaltes peuvent être distingués: les basaltes qui se trouvent dans la partie inférieure de la colonne stratigraphique, ont une composition comparable aux ferrobasaltes de type N-MORB, tandis que les roches se trouvant dans la partie supérieure de la séquence s'apparentent plutôt aux E-MORB légèrement fractionnés. Les deux types de basalte proviennent de la fusion partielle d'un manteau fortement appauvri, et ce à une profondeur probable de 60 à 75 km dans le manteau supérieur. L'origine des laves acides pose, quant à elle, quelques problèmes: elles peuvent s'être formées par cristallisation fractionnée à partir d'un liquide basique, ou alors elles sont le produit de la fusion partielle d'une croûte juvénile. La combinaison des données Rb-Sr obtenues pour les roches acides et basiques permet l'obtention d'une isochrone définissant un âge de 712 ± 24 Ma. Cet âge est interprété comme datant les éruptions volcaniques. Les données d'éléments en traces obtenues pour les laves basiques et acides ne présentent aucune des caractéristiques associées au volcanisme d'arc insulaire. Par conséquent, nous rejetons l'hypothèse selon laquelle les roches volcaniques de Shadli se seraient formées dans un contexte d'arc insulaire. Les données géochimiques et de terrain seraient plutôt en accord avec une hypothèse selon laquelle les roches volcaniques métamorphiques de Shadli se seraient formées dans une zone de rift où de très volumineux épanchements de laves auraient succédé à une extension à grande échelle de la lithosphère. Cette interprétation des données nous conduit à suggérer un réexamen critique du contexte tectonique entourant la période de formation crustale se situant entre 700 a 715 Ma, dans le nord-est de l'Afrique ainsi qu'en Arabie.
Краткое содержание
Позднекембрийские м етавулканиты Shadli, наход ящиеся на юго-востоке восточ ной пустыни Египта, являются слаб о метаморфизированн ыми свитами бимодальног о базальт — риодацита, которые до сих пор счи тали образованиями вулканизма островно й дуги. По химическому составу различают зд есь два типа базальто в: подстилающий ферроб азальт, напоминающий N — MORB и перекрывающий ег о базальт ЕMORB, проявляющий слабое ф ракционирование. Оба типа базальтов, у к оторых отмечается истощение взаимоиск лючающих элементов, происходят из верхни х 60–75 км мантии. Происхождение кислы х метавулканитов еще полностью не выяснен о; возможным считают или фракционировани е из мафического расп лава, или возможное распла вление ювенильной коры типа островных д уг. Изохронный возрас т мафических и фельзич еских пород, определе нный с помощью метода Rb/Sr, сос тавляет 712 ± 24 Ма. Предполагают, чт о и эрупция имела место в это врем я. Распределение малы х элементов в метавулк анитах Shadli не несет хара ктера субдукционного магм атизма; поэтому авторы не видят здесь прямой взаимосвязи с развитием островной дуги, но рассматриваю т метавулканиты Shadli, как с ледствие образовани я рифта в поздней матер иковой коре, подобно рифту Рио Гранде, или т реугольнику Афар, где сильное растяжен ие литосферы способс твовало поднятию больших кол ичеств магмы. Такая интерпретация ставит под сомнение простую схему формир ования панафриканск ой коры в результате при соединения островны х дуг к арабо-нубийском у щиту; возникает необ ходимость пересмотреть сущест вующие модели и представления.
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Stern, R.J., Kröner, A. & Rashwan, A.A. A late Precambrian (∼ 710 Ma) high volcanicity rift in the southern Eastern Desert of Egypt. Geol Rundsch 80, 155–170 (1991). https://doi.org/10.1007/BF01828773
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DOI: https://doi.org/10.1007/BF01828773