Isotopic age and metamorphic history of the banded gneiss at Danmarkshavn, East Greenland

Along the northern part of the East Greenland coast the Caledonian structures are superimposed on an older fold system called the "Carolinidian". Traces of this Carolinidian belt are preserved in a few isolated fragments within the Caledonian fold belt. According to Haller (1970) one of these fragments exhibiting the typical Carolinidian NNW to NW-trending infrastructural folds is the peninsula of Germania Land which is accessible near the Danish weather station Danmarkshavn. The rock sampled there is a banded gneiss of granodioritic composition with steeply inclined, NNW-trending layers. Isotopic age determinations yielded essentially two groups of ages: 1) 3,000±150 m.y. (zircon suite and Rb/Sr whole rock analyses of layers) and 2) 320-380 m.y. (Rb/Sr mineral isochrons, U-Th-Pb on sphene, K/Ar on hornblende and biotite). The egg-shaped zircons support a sedimentary origin of the banded gneiss and in conjunction with the Rb/Sr whole rock ages determine the age of formation of the banded gneiss (or its last high grade metamorphism) some 3,000 m.y. ago. No other Precambrian metamorphism or orogeny is recorded in the rock. The ages between 320-380 m.y. date a thermal event of lower amphibolite facies grade related to a late Caledonian spasm. The new isotopic data reveal the existence of very old rocks in the hinterland - away from the direction of thrusting-of the East Greenland Caledonian belt. With respect to the age of the Carolinidian fold system three geological interpretations are compatible with the results of this study: 1. the Carolinidian fold system formed approximately 3,000 m.y. ago; 2. the banded gneiss represents part of an old basement: the Carolinidian orogeny is younger than 3,000 m.y. Its marked structural imprint on the Danmarkshavn area was accompanied by very low grade metamorphism not recognised in the isotopic record; 3. the fold structures observed in the Danmarkshavn area are not of Carolinidian age but belong to an earlier fold system preserved in the basement.