Layered Gabbro Complexes in East Greenland

Christian Tegner¹ and Stefan Bernstein^2
1Department of Earth Sciences, University of Aarhus, Aarhus, Denmark
²Avannaa Ressources Ltd., Geological Museum, Copenhagen, Denmark

The East Greenland Tertiary volcanic rifted margin exposes the intrusive roots of a large igneous province. Although flood the basalts erupted extremely rapidly, probably in less than 300.000 years at continental rupture 55 million years ago, the intrusive roots witness a much longer and complex history. The Skaergaard intrusion is famous for displaying extreme closed-system fractionation and immiscibility into iron- and silica-rich silicate melts in a tholeiitic system. However, this small intrusion (c. 80 km²) is the oddball. The typical gabbro complexes are larger (up to 360 km²) such as Kap Edvard Holm and Imilik. Here, volcanics and early dykes are rotated ~45° seawards as part of a regional coastal flexure and are cut by at least three generations of layered gabbros. The oldest gabbros are deformed, hydrothermally altered during coast-parallel faulting and dyke emplacement, and could not be dated. The second generation consists of layered oxide-gabbros similar to and coeval with Skaergaard. The third, dominant generation is much younger (50-47 Ma) and composed of olivine-gabbro with slumped layering, fine-grained quench zones, restricted mineral compositional variation and intrusive wehrlite plugs and sills that witness frequent magma recharge and tapping. While the syn-breakup gabbros such as Skaergaard are linked to the main flood basalt and mantle melting event, the post-breakup gabbros are enriched in incompatible elements and are explained as renewed mantle melting caused by a westward jump of the oceanic spreading-ridge system back to the rifted margin, possibly controlled by mantle plume-lithosphere interaction.

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