Crystallization and Layering of the Skaergaard Intrusion

Abstract

Solidification of large slowly cooled intrusions is a complex process entailing progressive changes of rheological properties as the crystallizing magma passes through successive stages between a viscous Newtonian fluid and a brittle solid rock. Studies of this transition in the Skaergaard intrusion indicate that most crystallization took place in an advancing front of solidification against the floor, walls, and roof where crystals nucleated and grew in a static boundary layer, much in the manner proposed by Jackson in 1961. The non-Newtonian properties of the crystallizing magma account for the fact that plagioclase, which was lighter than the liquid, is a major component of rocks on the floor, while mafic minerals that were heavier than the liquid accumulated under the roof. Crystals that nucleated and grew in these zones were trapped by an increasingly rigid zone that advanced more rapidly than the crystals sank or floated. If any crystals escaped entrapment, they were those of the largest size and density contrast. The rates of accumulation in different parts of the intrusion were not governed by rates of gravitational accumulation so much as by the nature of convection and heat transfer. Cumulate textures, preferred orientations of crystals, and layering, all of which have been taken as evidence of sedimentation, can be explained in terms of in situ crystallization. Layering cannot have been caused by density currents sweeping across the floor; it is well developed on the walls and under the roof, lacks the size and density grading and mineralogical compositions that would be expected, and shows no evidence of having been affected by obstructions in the paths of the currents. We propose an alternative origin of layering that is based on processes governed by the relative rates of chemical and thermal diffusion during cooling. Intermittent layering resulted from gravitational stratification of the liquid, and cyclic layering was produced by an oscillatory process of nucleation and crystal growth. The effects of differentiation during in situ crystallization are strongly dependent on relative rates of diffusion of individual components, and some of the compositional variations in different parts of the intrusion can be explained in terms of these differences.