Mechanisms of Formation of Igneous Layering

Abstract Layering is a common, almost ubiquitous, feature of gabbroic and syenitic intrusions. Individual layers, or layered sequences, however, vary greatly in such features as thickness and length, the nature of layer boundaries, internal vertical and lateral variations within layers, and the relationships to other nearby layers. Their modal proportions, grain-sizes, mineral compositions, whole-rock compositions, and textures present in layers and their surrounding host rock, are also quite varied. Given the wide range of these characteristics, it is unlikely that any single layer-forming mechanism can explain all or even most of the known occurrences of igneous layering. A wide variety of layer-forming mechanisms has been proposed. Some operate during the initial filling of a magma chamber, as a result of the settling of crystals carried in suspension, flow segregation during magma transport, magma chamber recharge, or magma mixing. Other proposed mechanisms operate in response to continuous, intermittent, or double-diffusive convection. Layering may also form as the result of mechanical processes, such as gravity settling, crystal sorting by magma currents, magmatic deformation, compaction, seismic shocks, or tectonic deformation. Variations of intensive parameters and kinetic factors, such as fluctuations of rates of nucleation and growth of crystals, oxygen fugacity, pressure, and rates of separation of immiscible liquids, may also be responsible for certain types of layering. During late-stage crystallization and cooling, layering may form in response to porous flow of interstitial liquids, metasomatism, constitutional zone refining, solidification contraction, Ostwald ripening, or contact metamorphism. The simple concept of a magma chamber undergoing differentiation as a result of early-formed crystals settling out of the magma and accumulating in layers on the floor of the chamber, has been discarded by most petrologists in favor of models involving in situ crystallization, in which magma chambers are thought to have the general form of a central mass of nearly crystal-free magma, that gradually loses heat and crystallizes inwards from its margins. The transition from crystal-free magma in the central part of the chamber to completely solidified rock in the outer parts is thought to occur through a marginal zone of crystal-liquid mush. As magmas crystallize and differentiate, components included in early-crystallizing minerals are depleted, while those excluded from these phases are enriched. It is unclear, however, how the latter are effectively transferred through the crystal mush zone, so that crystallization at margins results in differentiation of the body as a whole. It is also not clear what non-steady-state or non-equilibrium processes are responsible for the formation of layering during the crystallization process. Because these two problems are interrelated, an understanding of the formation of igneous layering should eventually lead to a better understanding of the processes responsible for igneous differentiation. The time scales and length scales involved in the formation of igneous layering preclude direct experimentation on silicate melts at magmatic temperatures, and as a result, the origin of these features must be largely deduced from field observations and theoretical considerations. The challenge for the igneous petrologist is to determine which features of igneous layering are diagnostic of a particular mechanism, which reflect subsequent compositional or textural modifications, and which can best discriminate between the plethora of possible mechanisms that have been proposed.