Skaergaard: Geologic Summary

This was a field trip of the 2001 International Geological Correlation Project 427, led by T. Neil Irvine, Jens Christian Andersen, and C. Kent Brooks. These three have worked many years on the Skaergaard and provided the trip participants a fantastic experience. I wish to thank them for their efforts.

The Skaergaard is exposed on the central east Greenland coast, just north of the Arctic Circle.

Model for the emplacement of the Skaergaard intrusion (from Irvine, 1992).

The Skaergaard is an inmtrusive body of igneous rock, the magma for which was emplaced and crystallized in the middle Tertiary (Eocene). It was emplaced, apparently as a single magma batch, into Archean basement gneiss and related rocks, and into Cretaceous sediments and a thick pile of Eocene lavas. The pluton is ~7.5 km wide, ~10 km long, and of somewhat uncertain thickness. Gravity modeling (Blank and Gettings, 1973) indicates a maximum remaining thickness of ~3.5 km, which is not too different from the exposed thickness. The exposed thickness can be calculated by summing the currently exposed Upper Border Series (950 m), the Layered Series (2500 m), and the Marginal Border Series at the northern, deepest part of the pluton (100 m): 3550 m (thicknesses from Irvine et al., 2001a).

A structural reconstruction was proposed by Irvine (1992), shown above, that takes into account the gravity modeling. The original Skaergaard magma was already considerably evolved (composition modified by fractional crystallization), which presumably took place in a deeper magma chamber prior to emplacement at the current exposure level. The Skaergaard intrusion has an estimated volume of ~250 km3. The whole pluton has been tilted at an angle averaging ~20° to the south. Subsequent to solidification, the Skaergaard intrusion was cut by large numbers of dikes of various sorts, the largest being the Vandfaldsdalen macrodike which probably fed the Basistoppen sheet, shown here.

Model for circulation and deposition within the Skaergaard intrusion (from Irvine et al., 1998).

On this field trip I saw only a small portion of the whole Skaergaard outcrop surface, and doubtless missed many important features. Certainly there are many features described in the literature that I did not notice. I therefore turn the reader to here for further information.

As the pluton lost heat to its upper crustal surroundings, it crystallized on its roof, floor, and walls. Accumulation was aided by the deposition of crystals from density-driven convection currents. These deposits have a wide range of appearance, depending on the location within the pluton. In addition, portions of the magma chamber roof periodically collapsed, permitting roof zone autoliths and xenoliths to drop into the magma chamber, and larger ones to impact onto the floor. Much of our understanding of the roof zone comes from the autolith blocks, as most of the pluton roof has been eroded away and access to the rest is difficult.