Zusammenfassung
Glimmerbärte in schwach metamorphen Sedimenten sind syntektonische Verwachsungen von Glimmer, Chlorit, Quarz oder Feldspat auf detritischen Körnern. Die drei hier untersuchten Proben enthalten Bärte aus Hellglimmer und Quarz (oder Feldspat). Die Glimmerorientierung zeigt Variationen in Relation zum umgebenden Schieferungsgefüge (Matrix). Die Bildung der Bärte fand mit syntaxialem Wachstum während der Schieferung statt. Dreidimensional haben die Bärte eine abgeplattete Gestalt, so daß sie in allen Schnittlagen parallel zur Verkürzungsrichtung zu beobachten sind. Dies weist auf plättende Verformung um detritische Körner während der Bildung der Bärte hin.
„Bulk inhomogeneous shortening“ führt zu einem linsigen Schieferungsgefüge und kann bei gleichzeitiger Annahme einer nicht volumenkonstanten Deformation als Erklärung für die im Korngefüge beobachtete Dehnung parallel zur Faltenachse herangezogen werden. Im regionalen Maßstab wird aber eine konstante Länge in Richtung der Faltenachse und damit ebene Deformation angenommen.
Abstract
Beards are intergrowths of mica, chlorite and quartz or feldspar on detrital grains in weakly metamorphosed sedimentary rocks. The three samples studied contain beards of mica and quartz (or feldspar). Their internal morphology shows a variation in mica orientation related to the external foliation (matrix). Formation occurred during foliation development with a syntaxial growth history. In three dimensions, the beards have oblate shapes, so that they are present in all planes cut parallel to the shortening direction. This indicates flattening strain conditions around the detrital grains during the beard formation.
Bulk inhomogeneous shortening, resulting in an anastomosing foliation, can explain the local extension within the plane containing the fold axis and the shortening direction. Concurrently, an approximately constant lenght in the direction of the fold axis is maintained on a regional scale (plane strain). Volume loss probably plays an important role during the formation history.
Résumé
Les barbes de mica sont des intercroissances de quartz (ou feldspath) et de mica sur des grains détritiques dans les roches sédimentaires peu métamorphisées. Elles ont été étudiées dans trois échantillons. Leur morphologie interne montre une variation dans l'orientation du mica en relation avec la schistosité externe (matrice). Leur formation s'est produite pendant le développement de la schistosité, avec une histoire de croissance «syntaxiale». Les barbes ont une forme oblongue en trois dimensions, de sorte qu'elles sont visibles dans toutes les sections parallèles à la direction de raccourcissement. Cela indique, pendant la formation des barbes, l'existence d'un aplatissement autour des grains détritiques.
Le raccourcissement non homogène, qui entraîne une foliation anastomosée peut être invoqué, conjointement à une déformation avec perte de volume, pour expliquer l'allongement que présente localement la microstructure parallèlement à l'axe des plis. A l'échelle régionale, toutefois, on admet la constance de la longuer selon la direction des axes et, partant, le caractère plan de la déformation.
Краткое содержание
В слабо метаморфизир ованных отложениях н айдены синтектонические ср ащения слюды, кварца и полево го шпата на зернах кла стического материала (Glimmerbärte). В
исследованных проба х эти „Glimmerbärte“ состояли и з светлой слюды и кварца (полево го шпата). Ориентация осе й слюды колеблется по отношению к окружающему строен ию сланца — матрицы. Обра зование этих „Glimmerbärte“ свя зано с синаксиальным ростом во время проце ссов расслаивания. Эт и „Glimmerbärte“ являются трехмерным и, но кажутся очень упло щенными, так что во все х срезах оказываются располо женными параллельно к направ лению укорочения. Это указывает на утоньшения вокруг зерен кластического матер иала во время образов ания этих „Bulk inhomogeneous shortening“ создает линзеподобн ое строение сланцева ния и, если предположить деформ ацию, имевшую место при изменении и х объема, то может дать объяснение простира нию зерен параллельн о оси складки. В региона льном масштабе предп олагают постоянную длину в направлении оси скла дки и плоскостную деф ормацию.
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van der Pluijm, B.A. Mica beards in three slates: Morphology, formation and bearing on the strain history. Geol Rundsch 73, 1037–1053 (1984). https://doi.org/10.1007/BF01820887
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DOI: https://doi.org/10.1007/BF01820887