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Changes in the Isotopic Composition of Whole-Rock Pb During Different Stages of Retrogression of Late Archaean Granulite Facies Gneisses from Kangimut Sammisoq, Southern West Greenland

  • D. Bridgwater
Part of the NATO ASI Series book series (ASIC, volume 281)

Abstract

Pb-Pb whole rock isotopic and total Pb and U concentration data are presented from samples from a controversial outcrop of late Archaean gneisses affected by circa 2.82 Ga granulite facies metamorphism followed by different degrees of retrogression in the period 2.82 to 2.4 Ga. The retrogression occured when the granulite facies gneisses were thrust over an amphibolite facies terrane which contains units of both early and late Archaean gneiss. Both U and Pb were added during retrogression but in different proportions in individual samples. The isotopic composition of the Pb from the least retrogressed granulites is comparatively radiogenic. The small amount of Pb present has a low proportion of Pb derived from an early Archaean source. This is interpreted as due to post-granulite introduction of U but little or no introduction of Pb in the first stage of retrogression. Partly retrogressed gneisses contain higher total Pb. This is less radiogenic than that seen in the least retrogressed rocks and a higher proportion of the Pb is derived from older crust. The most highly retrogressed rocks contain 5–10 times the total Pb present in the least retrogressed rocks. The Pb in these rocks is comparatively radiogenic and has a low proportion of early Archaean Pb. It is suggested that the changes in Pb isotopic composition reflect different proportions of Pb and U added to different samples during retrogression, combined with different sources of the introduced Pb. This may be due to more than one period of retrogression associated with different episodes of thrusting or to changes in the source of the fluids causing retrogression.

Keywords

Isotopic Composition Granulite Facies Basic Dyke Granulite Facies Metamorphism Planetary Institute 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© Kluwer Academic Publishers 1989

Authors and Affiliations

  • D. Bridgwater
    • 1
  1. 1.Geological MuseumCopenhagen KDenmark

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