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Magmatic 87Sr/86Sr relicts in hydrothermally altered quartz diorites (Brabant Massif, Belgium) and the role of epidote as a Sr filter

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Abstract

The porphyritic quartz diorites of the Caledonian Brabant Massif have been totally altered. Ca, Rb, Sr, Zr, Ce, Y measurements and Sr-Nd isotopic analyses were performed on the Quenast plug and the Lessines sill, in an attempt to study the relative mobility of Sr and evaluate the extent, direction and magnitude of the 87Sr/86Sr alterations. Sr electron microprobe analyses of epidote were also carried out to assess its role in the Sr distribution.

The initial 87Sr/86Sr ratio is shown to have had an unsteady behaviour during the studied water/rock interactions since it has been sometimes enhanced, sometimes depressed and occasionally not modified. The possibility and magnitude of the 87Sr contamination turn out to be strictly related to the degree of Sr accommodation in the secondary minerals. Epidote in particular has proved to be the main trap for the hydrothermal Sr and this mineral is thus regarded as the major controlling factor of 87Sr hydrothermal contamination. The epidote-poor rocks (albite+chlorite-rich rocks) seem to have been unaffected by any Sr interchange with the aqueous solutions. Therefore, as alteration quickly follows the crystallization of the magma, their initial 87Sr/ 86Sr ratio, which is deduced from an isochron, might be a primary petrogenetic feature enabling interpretation of the genesis of their parental magmas. On the other hand, in the epidote-rich rocks, this ratio has been readily altered; it could thus generally be used only to trace the origin of the hydrothermal solutions. As a consequence, these rocks should not be selected for dating an alteration event by the Rb-Sr method.

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  1. Laboratoires Associés de Géologie-Pétrologie-Géochronologie, Université Libre de Bruxelles, avenue F.D. Roosevelt, 50, B 1050, Brussels, Belgium

    Luc André & Sarah Deutsch

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  1. Luc André
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  2. Sarah Deutsch
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André, L., Deutsch, S. Magmatic 87Sr/86Sr relicts in hydrothermally altered quartz diorites (Brabant Massif, Belgium) and the role of epidote as a Sr filter. Contrib Mineral Petrol 92, 104–112 (1986). https://doi.org/10.1007/BF00373968

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