Layered Gabbro Complexes in East Greenland
Christian Tegner1 and Stefan Bernstein2
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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