CALEDONIAN MAGMATISM IN EAST GREENLAND - AN UPDATE

The Caledonides of the North Atlantic region are in most plate tectonic reconstructions presented as an asymmetric orogen with Baltica being under-thrusted Laurentia. The presence of late- and post-orogenic magmatism in E Greenland and not in Scandinavia has supported this view, as has the presence of UHP/HP rocks in western Norway. Recent structural and geochronological work in E Greenland do not support this simplistic view. The new data show that most of the plutonic rocks in the eastern part of the orogen (towards the foreland) are restricted to a thrust sheet and that they do not penetrate the autochthonous/parautochthonous basement with its Neoproterozoic cover. The thrust sheet is sub-divided into; (i) crystalline complex of Paleoproterozoic to Mesoproterozoic age; (ii) variable metamorphosed and deformed Mesoproterozoic supracrustal sequence (Krummedal sequence) with some Grenville age (c. 930 – 1000) augengranites, and (iii) more than 15 km thick Neoproterozoic-lower Paleozoic sedimentary succession. Two-mica- and biotite-granites are common throughout the lower part of the Neoproterozoic succession and the Krummedal sequence and, but appear to be absent in the underlying gneisses. An extensional shear zone (detachment) is commonly observed between the two supracrustal sequences when not obscured by Caledonian intrusives. Both field observations and the chemical signature of the granites indicate that they are formed by anatexis of the Krummedal sequence. U-Pb dating of monazite from syn-extensional granites in the lower part of the Neoproterozoic sequence indicate a crystallization age of 430-424 Ma. This age overlaps or is slightly younger than monazite and zircon ages from anatectic melts in the underlying high-grade Krummedal sequence.

Less precise, slightly older U-Pb and Rb-Sr ages (c. 440-450 Ma) in Liverpool Land further east (hinterland) may indicate that the relatively large plutonic complexes present here, and in the Stauning Alps, represent remnants of a late Ordovician magmatic arc. It is speculated that the other fragments of this magmatic arc are to be found in the Uppermost Allochthon of the Scandinavian Caledonides.