Mechanisms of differentiation in shallow mafic alkaline intrusions, as illustrated in the Big Bend area, western Texas

Abstract

Syenitic bodies are a common feature in alkaline sills and laccoliths that range in composition from syenogabbroic to syenodioritic. The syenitic bodies are generally accepted to be the result of in-situ differentiation. Such bodies are usually called segregations, but relatively little discussion is given to the actual ways they may form. Subhorizontal sheets or layers are most common, although rounded ocelli and vertically elongated cylindroidal forms are also common. These are all systematically arranged in the intrusions. Such features are found in at least ten sills in the Big Bend area of Texas, and are reported as well in Montana, Utah, Australia, Sakhalin, Scotland and New Zealand. Detailed chemical and mineralogical analyses demonstrate crystal fractionation very convincingly for the generation of syenitic liquids that form the bodies. The analyses also allow the calculation of viscosities, cooling time and mineral settling rates in the sill studied most thoroughly. These factors indicate, or are compatible with, proposed mechanisms for the formation of syenite bodies. In many intrusions there is a bimodal distribution of syenite compositions; one type is relatively plagioclase rich and the other type is plagioclase poor. Mass balance calculations on several intrusions show that a residual liquid of plagioclase-rich composition will form after about 30% of the parent magma has crystallized, whereas a residual liquid of plagioclase-poor composition will form only after about 50% of the original magma has crystallized. The several mechanisms by which plagioclase-rich residual liquids are aggregated into the different forms of syenite bodies include localized crystal settling and sagging of a crystaline framework on a scale of tens of centimeters to a meter, within an upper solidification front; formation of cylindroidal columns of syenitic differentiate in a crystal mush within a lower solidification front, and diapiric rise of such masses. After the amount of magma crystallized reaches 50% or more, rupture of a rigid crystalline framework occurs through the processes of contraction upon cooling, shearing caused by deformation, and expansion of bubbles in the interstitial residual liquid, while plagioclase-poor differentiate flows into spaces created in these processes. Important for the effectiveness of these processes is the buildup of volatiles in the residual liquids, which results from crystallization of anhydrous minerals, that causes second boiling and lowering of the viscosity of the liquids.