Abstract
SHRIMP U-Pb isotope data on zircon crystals from a gneiss sample near Danmarkshavn, where the presence of Archaean rocks has earlier been documented, show that the rock has undergone a complex history of igneous and metamorphic zircon growth. At least three generations of zircon are present with ages of c. 3000 Ma, c. 2725 Ma and 1967 ±8 Ma (2 α). Apparently the rock was formed from an Archaean protolith which underwent high grade metamorphism during the early Proterozoic. Another sample from the easternmost exposures of the Caledonian basement, collected further north, yielded only early Proterozoic zircons with an age of 1963 ± 6 Ma. Together with a SHRIMP U-Pb zircon age of 1974 ± 17 Ma reported earlier, these results give evidence of a major igneous and metamorphic event in North-East Greenland about 1965 Ma ago.
Highlights
SHRIMP U-Pb isotope data on zircon crystaJs from a gneiss sample near Danmarkshavn, where the presence of Archaean rocks has earlier been documented, show that the rock has undergone a complex history of igneous and metamorphic zircon growlh
The present SHRIMP U-Pb zircon study had two aims: first, to seek confirmation of the Archaean age of the banded gneisses at Danmarkshavn; second, since the Danmarkshavn gneiss studied by Steiger et al (1976) was collected in the eastern part of the exposed Caledonian basement (Fig. l), to test whether more extensive units of Archaean gneisses existed in the easternmost parts of the investigated region
The results clearly demonstrate that a c. 3000 Ma or slightly older component is present, and that in the late Archaean either a new igneous component was added or high grade metamorphism led to zircon growth in a c. 3000 Ma rock
Summary
The results for the Danmarkshavn sample (GGU 365358) show that this rock was forrned from an Archaean protolith, in agreement with the findings of Steiger et al (1976) These authors believed that the banded gneisses were of sedimentary origin, and it must be considered whether the Archaean zircons could be clastic grains in an early Proterozoic sediment. The tonalite (GGU 363175) collected further north (age 1963 ± 6 Ma) was emplaced at about the time when high-grade metamorphism took place at Danmarkshavn. Another tonalitic gneiss (GGU 344937) which has been studied earlier, yielded a comparable age (1974 ± 17 Ma; Kalsbeek et al, 1993). Caledonian metamorphism, documented by mineral isotope data (Steiger et al, 1976; Brueckner & Gilotti, 1993; Dallmeyer & Strachan, 1994) is in this study only recorded by small degrees of Pb loss from parts of the analysed zircons
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