Use of Rb-Sr isotope data to constrain the time of deposition of Precambrian metasediments: an example from Hamborgerland, West Greenland

F Kalsbeek

doi:10.34194/rapggu.v159.8216

Abstract

In high-grade metamorphic terrains it is often not possible to determine the relative age of metasedimentary units by field investigation. However, the time of deposition of the original sediment can be constrained by consideration of the Sr-isotopic evolution of the rocks on the scale of an outcrop. An outline of the method is given, and Rb-Sr data for high-grade (granulite facies) metasediments from HamborgerIand, West Greenland, are discussed as an example. Sm-Nd model age data indicate that these rocks were derived by erosion of a 3000–3200 Ma basement. Deposition took place not long before 2700 Ma ago, and closure of the Rb-Sr isotope system after high-grade metamorphism occurred at about 2600 Ma.

Highlights

In high-grade metamorphic terrains it is often not possibie to determine the relative age of metasedimentary units by field investigation
Rb-Sr data for unmetamorphosed sediments are often difficult to interpret because isotopic equilibration between clastic grains and diagenetic minerals is not normally achieved
Rb-Sr data on diagenetic minerals have been used to date the time of diagenesis

Summary

Feiko Kalsbeek

In high-grade metamorphic terrains it is often not possibie to determine the relative age of metasedimentary units by field investigation. Rb-Sr whole rock isochron dates of high-grade metapelitic sediments yield information on the time of metamorphism, but give no indication of the time of deposition of the original sediment. This is because Sr is mobile during metamorphism, especiaIly in rocks which are rich in volatiles, so that partial Sr-isotopic equilibration is common. Rb-Sr isotope data mayaiso be used to constrain the time of sediment deposition, by consideration of the Sr-isotopic evolution of the rocks with time This method has commonly been used for igneous rocks and their metamorphic equivalents

MANIITSOQ SUKKERTOPPEN

Supracrustal rocks

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