Abstract

The Gardar Province in southern Greenland is believed to have experienced three episodes of rifting, all associated with the extrusion of large volumes of alkalic magma. A nepheline-syenite of the Motzfeldt Center yields a U-Pb apatite age of 1.35 ± 0.01 Ga. An augite-syenite from the Ilímaussaq Complex yields a Sm-Nd pyroxene, alkali feldspar, magnetite age of 1.13 ± 0.05 Ga. On the basis of field relationships, the Eriksfjord Formation lavas have been interpreted as the oldest manifestation of Gardar magmatism. Sm-Nd plagioclase, pyroxene, wholerock ages of 1.17 ± 0.03 Ga and 1.20 ± 0.03 Ga for the middle member of the Eriksfjord Formation, however, indicate that these lavas may be part of the youngest magmatic activity. The available chronological data indicate that the Gardar Province was active for at least 200 m.y., from 1.35 to 1.12 Ga. It is difficult to reconcile this protracted history with plume models for the genesis of such alkaline flood-basalt provinces.

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