The Skaergaard layered series. Part III. Non-dynamic layering
Layering in the Skaergaard Intrusion has been divided into two general types, one produced by magmatic flow and another by processes resulting from variations of rates of nucleation and crystallization, and, in the case of the Layered Sereis, by compactionrelated processes. Modal variations caused by shifts of cotectic proportions produce thick layers which, in the Layered and Upper Border Series, are diffuse and normally lack strong foliation and lineation. In the Marginal Border Series, the layers are thinner and sharper; possibly because the rate of accumulation was slower. Oscillatory nucleation may have played a role in producing finescale cyclic layers, but it was less important than solution and reprecipitation during slow cooling and Ostwald ripening. Evidence for compaction is found in deformed plagioclase laths and a relative deficiency of incompatible elements in rocks formed on the floor. Layering related to compaction becomes sharper with increasing height in the Layered Series until it suddenly disappears above the trough horizon near the base of Upper Zone b. Mechanical sorting during compaction may have produced crude layering, but if it did the evidence has long since been destroyed by the superimposed effects of solution and reprecipitation when interstitial liquid rose through the overlying crystals and re-equilibrated with them. Numerical simulations illustrate how small differences of surface energy caused by variations of grain size, textural dependence of solubility, and pressure solution can cause segregation of minerals into layers during solution and reprecipitation.
Boudreau, AE; McBirney, AR
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