Vers une Meilleure Connaissance du Problème des Spilites à Partir de Données Nouvelles sur le Cortège Spilito-Keratophyrique Hercynotype

Abstract

The paper deals with the petrographic and chemical study of some spilite-keratophyre associations of the northern part of the Hercynian orogeny of Western Europe.

The first part concentrates on the analysis of the facts, after a brief introduction to the geology of the areas under investigation (Cornwall and Devon; Lahn-Dill, Northern Vosges). The different petrographic facies of each area are examined in detail on the basis of samples taken at typical locations.

The comparison of the results of the three areas makes it possible to derive analogies and differences between each province and to draw a sketch of the distribution of the facies, starting from the center of the provinces under consideration.

Subsequently, the authors derive mineralogical and textural characteristics of the spilites and the keratophyres which represent the facies typical and dominating in the Hercynian association.

The primary nature of the textures is reconfirmed and the spilitic paragenesis is given in detail: albite-chlorite-calcite-(K-feldspar)-(augite)-(quartz)-iron oxydes, whereas the absence of olivine is underlined as typical.

The chemical investigation, including 17 5 analyses, is used to comment on the distribution of the different major elements, as well as on the very important role of H₂O and CO₂. For each facies group important conclusions are derived from the comparison of the analytical results and their chemical properties are thus characterized. A number of diagrams show the general geochemical tendency of the association.

In the second part, the problem of the origin of the spilites and the keratophyres is treated. After a summary of the major observational facts, the primary character of these Hercynian provinces is reconfirmed. Consequently, the existence of a spilitic magma is taken into consideration. The origin of this magma is discussed and the authors defend the hypothesis of a basaltic magma contaminated with H₂O and CO₂ derived to a larger part from the geosynclinal environment. The genesis of the keratophyres and their connections with the spilites is also examined. All through this investigation a magmatism of “folds” (Schirmeck type) to a magmatism of “Graben” (Lahn-Dill type) is made evident. It reflects the importance of paleogeographic and paleostruc-tural conditions on the nature of the production of geosynclinal magmatism.