The Skaergaard Layered Series. Part III. Non-dynamic Layering

Abstract

Layering in the Skaergaard Intrusion has been divided into two INTRODUCTION general types, one produced by magmatic flow and another by Recent studies of the structural and textural features of processes resulting from variations of rates of nucleation and crysthe Skaergaard Intrusion (McBirney & Nicolas, 1997) tallization, and, in the case of the Layered Series, by compactionhave distinguished two broad types of layering, one related processes. Modal variations caused by shifts of cotectic produced by the dynamic effects of magmatic flow and proportions produce thick layers which, in the Layered and Upper another by processes that operate in situ such as varied Border Series, are diffuse and normally lack strong foliation and nucleation and growth of crystals, recrystallization, or by lineation. In the Marginal Border Series, the layers are thinner and compaction-related mechanisms. We refer to layering sharper; possibly because the rate of accumulation was slower. formed by these latter processes as ‘non-dynamic’ to Oscillatory nucleation may have played a role in producing fineemphasize that it is not the result of fluid dynamic scale cyclic layers, but it was less important than solution and processes. Although most layering combines elements of reprecipitation during slow cooling and Ostwald ripening. Evidence more than one process, the contribution of each mechfor compaction is found in deformed plagioclase laths and a relative anism can usually be recognized from its distinctive form deficiency of incompatible elements in rocks formed on the floor. and setting. Layering related to compaction becomes sharper with increasing The distinguishing features of dynamic magmatic layheight in the Layered Series until it suddenly disappears above the ering have been described in Part II of this series (McBirtrough horizon near the base of Upper Zone b. Mechanical sorting ney & Nicolas, 1997). They are best seen near the steep during compaction may have produced crude layering, but if it did margins of the Layered Series in what Wager & Brown the evidence has long since been destroyed by the superimposed effects (1968) referred to as the ‘cross-bedded zone’ where the of solution and reprecipitation when interstitial liquid rose through layering is disrupted by slumping and channeling and the overlying crystals and re-equilibrated with them. Numerical the rocks have a marked foliation and lineation. simulations illustrate how small differences of surface energy caused Layering not associated with magmatic flow takes a by variations of grain size, textural dependence of solubility, and variety of forms, but we can distinguish two general pressure solution can cause segregation of minerals into layers during end-members, each of which has a distinct origin and solution and reprecipitation. characteristics. The first results from varied rates of