K,Rb, Sr,Ba, U and Th geochemistry of the Lapland Granulites (Fennoscandia). LILE fractionation controlling factors |
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| Authors: | P Barbey M Cuney |
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| Institution: | (1) Laboratoire de Pétrologie, Université de Nancy I, BP 239, 54506 Vandoeuvre les Nancy Cédex, France;(2) Institut de Géologie, C.A.E.S.S., Université de Rennes, 35042 Rennes Cédex, France;(3) Centre de Recherches sur la Géologie de l'Uranium, BP 23, 54501 Vandoeuvre les Nancy Cédex, France;(4) Centre de Recherches Pétrographiques et Géochimiques, BP 20, 54501 Vandoeuvre les Nancy Cédex, France |
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| Abstract: | The LILE geochemical patterns of the three main lithological units (graywacke-shale metasedimentary sequence, tholeiitic metaigneous rocks and migmatitic rocks) of the Lapland Granulite belt are described. K, Ba, Sr and Th concentrations in metasediments are nearly similar to average continental crust, whereas Rb and U are unevenly impoverished. In particular graphitic metashales and calcsilicate rocks are not significantly depleted in uranium. Tholeiitic metaigneous rocks comprises metavolcanics which present K/Rb ratios similar to metasediments, and metaplutonics with LILE abundances close to those of the low-K-tholeiites. Migmatites show wide range in LILE content. Metatexites and diatexites have higher K, Rb, Th and U concentrations and similar K/Rb ratios with respect to equivalent unmobilized rocks. Potassic pegmatoïds are strongly enriched K, Rb, Ba and Th but moderately in Sr and U. Plagioclasic pegmatoids and ferromagnesian restites are rich in Sr and poor in other LIL elements.A comparative review of the LILE geochemistry between Lapland granulites and equivalent lithological units taken from non metamorphosed to high grade terrains suggest that fractionation processes are not systematic but controlled by original lithology and mineralogy, mineral — fluid equilibria during progressive (or retrogressive) metamorphism and mineral-melt-fluid equilibria during anatexis. Moreover, statistical analysis on K-Rb distribution patterns in these various rock types shows that there is no metamorphic trend characteristic of granulite facies terrains as previously suggested.Large Ion Lithophile Elements |
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