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Geochemistry and geochronology of Early Mesozoic tholeiites from Central Morocco

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Zusammenfassung

Aus Zentralmarokko (Zentraler Hoher Atlas, Mittlerer Atlas, Haute Moulouya) wurden, in kontinentale Rotserien des frühen Mesozoikums (Trias-Lias) eingeschaltete Tholeiite geochemisch und geochronologisch untersucht. Haupt-, Spurenund Seltene Erden-Elementgehalte klassifizieren die Vulkanite als basaltische bis andesitisch-basaltische, quarznormative Tholeiite. Einige Spurenelementverhältnisse (Zr/Nb, Zr/Y, Y/Nb, Ti/V) weisen auf eine MORB-ähnliche (P- bis N-typisch) Zusammensetzung hin. Die erhöhten LILE-Gehalte, die negative Nb-Anomalie und die87Sr/86Sr-Anfangsverhältnisse (0.7064–0.7069) zeigen eine krustale Komponente an (13–17 Gew%). Unterschiedliche87Sr/86Sr-Verhältnisse karbonatischer Mineralseparate aus verschiedenen Lavaströmen weisen auf mehrere Alterationszyklen hin, deren Alterationslösungen keine Meerwasserzusammensetzung besaßen.40Ar/39Ar-Datierungen an Plagioklasen ergaben Extrusionsalter zwischen 210.4 ± 2.1 Ma und 196.3 ± 1.2 Ma. Stratigraphisch umfaßt dies den Zeitraum zwischen Nor (Rhät?) und oberem Sinemur.

Abstract

From Central Morocco (Central High Atlas, Middle Atlas, Haute Moulouya) continental tholeiites were investigated geochemically and geochronologically. These tholeiites are intercalated within continental redbeds of the Early Mesozoic (Triassic-Liassic). The major, trace and rare earth element contents classify these volcanic rocks as basaltic to andesitic-basaltic, quartz-normative tholeiites. Some trace element ratios (e.g. Zr/Nb, Zr/ Y, Y/Nb, Ti/V) suffer a heterogeneous source with a composition similar to MORB (P- to N-type). The enriched LILE contents, the negative Nb anomaly and the inital87Sr/86Sr ratios (0.7064–0.7069) reveal the presence of a crustal component up to 13–17 wt%.87Sr/86Sr ratios of carbonate mineral separates from different lava flows show different cycles of alteration; however, the major and trace element chemistry together with Sr isotope evidence, indicate that the alteration phases are not submarine in origin.40Ar/39Ar age determinations on translucent plagioclase phenocrysts yield extrusion ages which range between 210.4 ± 2.1 Ma and 196.3 ± 1.2 Ma. These ages correspond to a stratigraphic period between the Norian (Rhaetian?) and the Upper Sinemurian.

Résumé

Des tholéiites du Maroc central (Haut Atlas central, Moyen Atlas, Haute Moulouya) ont fait l'objet d'une investigation géochimique et géochronologique. Ces tholéiites sont intercalées dans des couches rouges continentales d'âge mésozoïque inférieur (Trias-Lias). Les teneurs en éléments majeurs, en éléments en trace et en terres rares classent ces roches volcaniques comme tholéiites basaltiques à andésito-basaltiques, à quartz normatif. Certains rapports d'éléments en traces (p. ex.:Zr/Nb, Zr/Y, Y/Nb, Ti/V) indiquent une composition analogue au MORB (type P à N). La teneur élevée en LILE, l'anomalie négative du Nb et les rapports initiaux87Sr/86Sr (0,7064 à 0,7069) révèlent la présence d'un composant crustal dans la proportion de 13 à 17% en poids. Les rapports87Sr/86Sr de minéraux carbonatés séparés de diverses coulées de lave montrent l'existence de plusieurs cycles d'altération; toutefois, le chimisme des majeurs et des traces, ainsi que les données isotopiques du Sr, indiquent que ces phases d'altération ne sont pas sous-marines. Les mesures d'âge par40Ar/39Ar effectuées sur des plagioclases fournissent des âges d'extrusion compris entre 210,4 ± 2,1 Ma et 196,3 ± 1,2 Ma. Ces âges correspondent à une période située entre le Norien (Rhétien?) et le Sinémurien supérieur.

Краткое содержание

Провели геохимическ ие и геохронологичес кие исследования толеит ов, включенных в материковые красноц ветные свиты раннего мезозоя триаслейас и з центрального Марок ко. На основании распред еления главных и микр оэлементов, а также Редких Земель считают, что эти вулканиты мож но отнести к базальто вым до андезитно-базальт овым толеитам с нормальным содержан ием кварца. На основан ии соотношения микроэл ементов Zr/Nb, Zr/Y, Y/Nb, Тi/V предполагают сост ав, подобный MORB типа P до N. Повышенное с одержание LILE, отрицательная анома лия Nb и исходные соотношения изотопо в стронция87Sr/86Sr= 0,7064–0,7069 указывают на прису тствие компонентов коры в них (13–17 вес.-%).

Различия в соотношен иях содержания строн ция в карботных минерала х из различних потоков лав говорят о многократных циклич еских изменениях, происход ивших в эти промежутк и времени, причем содер жание элементов во внесенных раствор ах не соответствовал и составу морской воды. Определ ение возраста по изотопам аргона на пл агиоклазах указываю т на то, что возраст экст рузий составляет 210,4 ±2,1 Ма - 196,3± 1,2 Ма. Стратиграф ически этот возраст соответ ствует норийскому яр усу (рэтскому ?) и верхнему с инемюрскому ярусу.

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  1. L. Fiechtner, H. Friedrichsen & K. Hammerschmidt

    Present address: FR Geochemie, Institut für Mineralogie, Boltzmannstr. 18–20, W-1000, Berlin 33, FRG

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    Fiechtner, L., Friedrichsen, H. & Hammerschmidt, K. Geochemistry and geochronology of Early Mesozoic tholeiites from Central Morocco. Geol Rundsch 81, 45–62 (1992). https://doi.org/10.1007/BF01764538

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