In situ fractionation and inward migration of the solidification front in the Skaergaard intrusion, East Greenland

Troels F.D Nielsen

doi:10.34194/geusb.v35.4939

Abstract

For more than 80 years the Skaergaard intrusion, 68°N in southern East Greenland, has been a foremost natural laboratory for the study of the crystallisation and fractionation of basaltic magma. This process has been of prime importance in the evolution of the Earth and other stony planets. Models that have been developed and refined during numerous studies of this particular intrusion have been part of the foundation for petrogenetic modelling for decades. In later years, vast amounts of new data have been added, due to systematic sampling in the field and from analysis of exploration drill cores. Methods for the study on grain-size scale have advanced, and the quest for a wellsupported genetic model for the PGE-Au mineralisation of the intrusion has intensified. The new data and insight question the applicability of conventional petrogenetic modelling, and as a consequence, increasing importance is placed on in situ crystallisation and fractionation in mush zones at the roof, walls and floor of the intrusion.

Highlights

For more than 80 years the Skaergaard intrusion, 68°N in southern East Greenland, has been a foremost natural laboratory for the study of the crystallisation and fractionation of basaltic magma
The Upper Border Series (UBS) and LS meet at the Sandwich horizon (SH)
All three series are subdivided on the basis of a parallel evolution in liquidus parageneses (Salmonsen & Tegner 2013 and references therein)

Summary

The Skaergaard intrusion

The Skaergaard intrusion (Wager & Brown 1968) is a comparatively small but well-preserved and well-exposed layered gabbro intrusion (Fig. 1A). It is 56 Ma old (Wotzlaw et al 2012) and was emplaced during the opening of the North Atlantic. It is 7 × 11 km in surface exposure, has a total structural height of c. The intrusion crystallised concentrically inward from the margins (Fig. 1B) with the Layered Series (LS, LZ, MZ and UZ) in the bowl-shaped floor, the Marginal Border Series (MBS) on the walls, and Upper Border Series (UBS) below the roof. All three series are subdivided on the basis of a parallel evolution in liquidus parageneses (Salmonsen & Tegner 2013 and references therein)

Layered Series

The importance of in situ fractionation

Toward a new solidification model