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Petrologic significance of rare earths distribution in granites

Abstract

Granitic rocks of various ages and composition are found in the Schwarzwald region of West Germany. These granites range in age from Upper Devonian to Upper Carboniferous (370-280 m.y.) and in composition from granodiorites to alkali feldspar granites. 14 representative samples of twelve different types were analysed for their La, Ce, Nd, Sm, Eu, Tb, Dy, Yb, Lu by instrumental neutron activation. The studies reveal that there are characteristic differences between the different types of granitic rocks both as regards to their total REE content as well as the distribution pattern of these elements. These differences can not be directly related to the variations in the major element chemistry or the mineralogy of the granites. On the other hand, a relationship is found between the age of the granitic rocks and the total REE as well as their distribution pattern. In general the ΣREE varies from 22 to 215 ppm in different types. The ΣLa-Lu increases gradually in the direction Upper Devonian→Lower Carboniferous, however, in the granitic rocks of the Upper Carboniferous this trend is reversed and there is again a marked depletion in the content of REE. The chondrite normalised patterns of all the older types give a smooth concave curve with decrease of concentration from La to Lu. All the Upper Carboniferous granites on the other hand are characterised by a progressive pronounced negative Eu anomaly. The gradual increase of the ΣREE in the older granites is related to their evolution by progressive anatexis, whereas, the decrease in the total REE content in case of the younger Upper Carboniferous granites is due to processes of magmatic differentiation. The depletion of Eu in these K-feldspar rich types of granite is probably related to the breakdown of biotite in the anatectic starting material.

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We are much obliged to Dr. R.K. Srivastava (Karlsruhe) for his kind support in preparing the English text. We are also grateful to Prof. Dr. H. Puchelt and Dr. H.Vogg (both Karlsruhe) for critical comments and helpful suggestions.

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Emmermann, R., Daieva, L. & Schneider, J. Petrologic significance of rare earths distribution in granites. Contr. Mineral. and Petrol. 52, 267–283 (1975). https://doi.org/10.1007/BF00401457

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  • DOI: https://doi.org/10.1007/BF00401457

Keywords

  • Distribution Pattern
  • Devonian
  • Gradual Increase
  • Major Element
  • Neutron Activation