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Rb-Sr whole-rock dating of late diagenetic to anchimetamorphic, palaeozoic sediments in southern France (Montagne Noire)

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Abstract

Rb-Sr whole-rock and mineral analyses are reported from Early Paleozoic sediments of the southern rim of the French Central Massif (Montagne Noire).

The sediments—mainly pelites, silt- and sandstones—can be separated into three different zones (A, B, and C) according to their varying degrees of deformation and transformation, which took place during the Hercynian (= Variscan) orogeny (Carboniferous).

For the least deformed, folded but not schistose, fossiliferous Lower Ordovician sediments (zone A), a scatter of the data points resulted, at least partly due to the varying amount of detrital muscovites, which did not equilibrate their Sr isotopes with those of the clay minerals.

Fossiliferous Middle Cambrian sediments (zone B), however, are colinear in an isochron plot. The rocks are also folded, and show a beginning schistosity in the pelitic layers. The age of 333±16 m.y. therefore does not correspond to the time of sedimentation, as one would expect from most of the interpretations given in the literature. It is, however, in close agreement with the stratigraphically known time of folding and anchimetamorphism.

The same result, within analytical error, of 326±17 m.y. was obtained for Lower Ordovician sediments (zone C), which show a schistosity not only in the pelites but also in some psammitic layers. As for the sediments of zone B, this result indicates open whole-rock systems for Rb and/or Sr during folding and anchimetamorphism.

Minerals extracted from psammites of zones B and C gave the following results: albite coincides with the isochrons; detrital muscovites do not coincide and yield ages of about 650 m.y. (Cadomian orogeny) for both zone B and C sediments; detrital chlorites from zone B sediments seem to be only partly reset by anchimetamorphism and therefore plot closer to the whole-rock isochron. From this it must be concluded that an increasing amount of these detrital micas could significantly shift the position of the total-rock points, as might be the case for zone A sediments.

From the whole-rock results of zone B and C sediments and the mineralogical and petrological changes during diagenesis and anchimetamorphism we conclude that many of the isochrons obtained from stratigraphically uncontrollable Precambrian sediments should be reinterpreted in favour of metamorphic and not sedimentary ages, especially if the metamorphism reached greenschist facies conditions.

Devonian and Cambrian limestones intercalated between the pelites, silstones and sandstones contain Sr of a variable and slightly radiogenic isotopic composition in contrast to the expected ancient sea water Sr. This fact is interpreted again in favour of open whole-rock systems during the postdepositional history of the rock, rather than primary scatter of ancient, radiogenic sea water Sr.

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Authors and Affiliations

  1. Laboratory for Isotope Geochemistry and Mass Spectrometry, Swiss Federal Institute of Technology, Zurich

    D. Gebauer & M. Grünenfelder

  2. Institut für Kristallographie und Petrographie, E.T.H., Sonneggstr. 5, CH-8006, Zürich, Switzerland

    D. Gebauer

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  1. D. Gebauer
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  2. M. Grünenfelder
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Gebauer, D., Grünenfelder, M. Rb-Sr whole-rock dating of late diagenetic to anchimetamorphic, palaeozoic sediments in southern France (Montagne Noire). Contr. Mineral. and Petrol. 47, 113–130 (1974). https://doi.org/10.1007/BF00372113

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