Magmatische Würfelquarze in Rhyolithen (Quarzkeratophyren) des Rheinischen Schiefergebirges

Zusammenfassung Nach einer kurzen Erläuterung der Erkennungsmöglichkeit einfacher Kristallformen zur Unterscheidung von Hoch-und Tiefquarz wird das Auftreten von eindeutigen Würfelquarzen (Rhomboeder), und damit von Tiefquarzen, in Rhyolithen (Quarzkeratophyren) des südlichen Rheinischen Schiefergebirges (Bundesrepublik Deutschland) beschrieben. Diese Quarzrhomboeder-führenden Vulkanite lassen sich petrographisch wie auch chemisch von den übrigen Rhyolithtypen jenes Gebietes unterscheiden. Eine eindeutige genetische Aussage zur Bildung dieser Tiefquarze in einem Vulkanit ist nicht zu machen, es werden dazu zwei alternative Modelle vorgestellt.

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Pseudo-cubic quartz crystals of magmatic origin in rhyolites (quartz keratophyres) from the Rheinisches schiefergebirge

Summary Recognition of simple crystal forms for the distinction between high-and low-quartz is briefly outlined. Pseudo-cubic crystals of low-quartz are described from rhyolitic rocks (quartz keratophyres) of the southern Rheinisch Mountains (Federal Republic of Germany). These volcanics, carrying idiomorphic quartz rhombohedrons, can be distinguished by petrographic as well as by chemical means from the other rhyolites in that area. However, there is no decisive genetic solution to the origin of low-quartz in those volcanic rocks. Two alternative models are proposed.

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Mit 6 Abbildungen     

Sheath formation inOscillatoria agardhii Gomont

Sheath formation was observed in laboratory culturedO. agardhii. The sheath serves as a protective cover against bacteria. Some comments on sheath formation by this alga are given.

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Zusammenfassung Scheidenbildung beiOscillatoria agardhii Gomont Im Kulturmaterial vonOscillatoria agardhii Gomont wurde die Bildung von Scheiden beobachtet. Diese dienen als Schutz gegen Bakterien. Es werden weitere überlegungen über die Scheidenbildung bei dieser Alge angestellt.

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Résumé Formation de gaines chezOscillatoria agardhii Gomont La formation d’une gaine chezOscillatoria agardhii Gomont fut observée dans une culture. Ces gaines protègent les algues contre les bactéries. Des considérations ultérieures sont faites sur le phénomène des gaines chez cette algue.

     

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

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.