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Contribution à l'étude des skarns de Costabonne (Pyrénées orientales, France) et à la théorie de la zonation métasomatique

Bernard Guy 1, 2
2 LMV-ENSMSE - Laboratoire Magmas et Volcans
Mines Saint-Étienne MSE - École des Mines de Saint-Étienne, SPIN, CNRS - Centre National de la Recherche Scientifique : UMR6524, LMV - Laboratoire Magmas et Volcans : UMR 6524
Abstract : In this work, one will find a description of some natural rocks as well as a discussion of some theoretical tools that may be used to approach metasomatism i. e. chemical transformation of rocks. At Costabonne (Pyrenees, France), skarns are developped within the lower levels of the Cambrian "Canaveilles" sequence, at the contact with the Hercynian Costabonne granite. Several types of rocks undergo skarnization: granites, calcitic and dolomitic marbles, calcic hornfelses, pelitic schists and brucite marbles. The most characteristic systems of zones that are visible within the first four types of rocks are described. The transformations must occur during a large interval of physical and chemical conditions and this is responsible for changes with time of the metasomatic zonings. This is particularly conspicuous in the case of skarns on dolostones. These conditions may have varied from (P > 2kb, T> 680°C) for the beginning (probably magmatic (or metamorphic ?) waters) down to (P < 2kb, T < 200*C) for the last stages (two types of meteoric waters). These estimates lie on data on fluid inclusions and on geochemistry of stable isotopes (H, O, C, S) for which a synthesis is given. The transformation of marbles is mainly guided by the addition of silica whereas that of granites (and of schists) is guided by the addition of calcium. The spatial behavior of some chemical elements is described. The several types of zonings are illustrated by photographs. The model of chromatography as set forth by Korzhinskii (1970) allows a qualitative understanding of the formation of metasomatic rocks, especially concerning the propagation of sharp fronts that are a characteristic feature of skarns. This model has been revisited and made more clear from a theoretical point of view. The very conditions for the appearance of sharp fronts (that lie in the conflict between the velocities of the different compositions) are precisely stated for arbitrary initial and boundary conditions. It is convenient to picture these fronts by true discontinuities from a mathematical point of view and this requires to rewrite the equations (in the framework of "distributions", and following what is usually done on that kind of problem or "hyperbolic problem"). The existence and propagation of discontinuities are integrated in the model through a condition that is derived from the second law of thermodynamics and that must be added to the mass-balance equation: a front thus expresses that some compositions are unstable. This model has a more general interest and allows one to discuss the onset of discontinuities whenever there is a differential movement between two entities (aqueous fluid / solid, magmatic liquid / solid) with a tendancy to local equilibrium. Some possible applications to other fields (diagenesis, magmatism) are briefly described. One expects that in some specific conditions, "strata" may appear during compaction, or that a magma may separate into distinct groups with different composition, along its transport. A model allowing porosity variations is set forth. Examples of numerical simulations for one and two chemical components are given. In the case of two components, the effect of couplings is discussed. In some particular conditions, the zonings may show alternations ABAB and so on, and the situation of this type of zoning in the framework of the overall model is discussed. At last two other approaches are explained: in the first one the number of degrees of freedom according to the phase rule is established and makes the connection of the overall system of zones appear. In the second approach a preliminary proposition is set forth: a metrics can be defined in chemical potential diagrams by the use of the phenomenological coefficients Lij; in this framework a least distance principle (under some conditions that are stated) allows to connect two points of the diagram that picture contrasted chemical situations and thus to define a path. The work is ended by some epistemological remarks, concerning, in particular the problem of the "scientific" understanding of singular events. These results are based on the works of a whole group, mainly in Saint-Etienne.
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Submitted on : Wednesday, September 24, 2014 - 5:45:13 PM
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  • HAL Id : tel-00594768, version 2

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Bernard Guy. Contribution à l'étude des skarns de Costabonne (Pyrénées orientales, France) et à la théorie de la zonation métasomatique. Géochimie. Université Pierre et Marie Curie - Paris VI, 1988. Français. ⟨tel-00594768v2⟩

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