The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition

Abstract

Re-examination of the Skaergaard intrusion in the context of its regional setting, combined with new data from exploration drilling, has resulted in a revised structural model for the intrusion. It is modelled as an irregular box, c. 11km from north to south, up to 8km from east to west, and 34---4km from the lower to the upper contact. The walls of the intrusion are inferred to follow pre-existing and penecontemporaneous steep faults, and the floor and roof seem largely controlled by bedding planes in the host sediments and lavas, similar to regional sills. The suggested shape and volume are in agreement with published gravimetric modelling. Crystallization along all margins of the intrusion concentrated the evolving melt in the upper, central part of the intrusion, best visualized as an ‘onion-skin’ structure inside the box. The total volume is estimated to c. 280 23km 3 , of which 137% are referred to the Upper Border Series (UBS), 164% to the Marginal Border Series (MBS) and 699% to the Layered Series (LS). In the LS, the Lower Zone (LZ) is estimated to constitute 668%, the Middle Zone (MZ) 135% and the Upper Zone (UZ) 197%. The new volume relationships provide a mass balance estimate of the major and trace element bulk composition of the intrusion. The parental magma to the Skaergaard intrusion is similar to high-Ti East Greenland tholeiitic plateau basalts with Mg number c. 0.45. The intrusion represents the solidification of contemporary plateau basalt magma trapped and crystallized under closed-system conditions in a crustal reservoir at the developing East Greenland continental margin.