Geochemistry of evaporite-bearing series: A tentative guide for the identification of metaevaporites |
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| Authors: | B Moine P Sauvan J Jarousse |
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| Institution: | (1) Laboratoire de Pétrographie, ERA 805 du CNRS, Université Claude Bernard, 27-43 Bd du 11 Novembre, F-69622 Villeurbanne Cedex, France;(2) Centre de Recherches Pétrographiques et Géochimiques, 15 rue N.D. des Pauvres, F-54500 Vand uvre-Nancy, France;(3) Société Nationale Elf Aquitaine (SNEA(P)), centre Micoulau, Avenue du Président Angot, F-64000 Pau, France |
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| Abstract: | Literature on metasedimentary rocks indicates that former evaporites have been assumed where brines are present as fluid inclusions or minerals such as scapolite, tourmaline and lazurite, which are considered as evidence of equilibria with brines. Very few works take into account the chemical composition of rocks, because chemical data on evaporite-bearing sedimentary series are scarce.This paper is a contribution to a geochemical approach: 172 analyses of major and trace elements are given on argillites, dolomitic marls and mixed sulphate rocks from some large evaporitic series, mainly from the Triassic of France, the Devonian of the Canning Basin and the Proterozo c of the Amadeus Basin in Australia. Compared with rocks from common platform series, these samples clearly exhibit specific chemical features: high Mg contents, high Mg/Ca ratios except for Ca-sulphate-rich rocks and relatively low iron contents caused by the widespread occurrence of Mg-rich clay minerals. Though sulphate-bearing rocks often show low Mg/Ca ratios, their chemical composition clearly indicates Mg-rich clay fractions. K content is high while Na is low, except in a few halite-bearing rocks. Argillites are characterized by high Li and F contents, and high Li/Mg and B/A1 ratios, while major amounts of Sr are recorded in sulphatebearing rocks.Application to metamorphic series is discussed with some illustrative examples. Though chemical changes during metamorphism are probably frequent in series of evaporitic origin, some relations between Al, Fe, Mg, Ca are often maintained. Under nearly isochemical conditions, the resulting metasediments exhibit abundant phlogopite through a large part of the metamorphic column, owing to high Mg, K and F contents. Such rocks are, for instance, well developed in the Zambian Copperbelt, Central Africa. Although further investigations are needed both on nonmetamorphic and metamorphic evaporite sequences, the efficiency of this approach for identifying an evaporitic origin is well established. |
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