Texture Development in Cumulate Rocks

Abstract

In the past three decades, the cumulus terminology developed by Wager and co-workers has provided the framework for understanding texture development in crystal mushes. Much of the debate has concerned the conditions necessary for development of adcumulate rocks and has involved discussion of mechanisms of heat and mass transfer within mushes. In this article the historical development of ideas is reviewed and aspects of the nomenclature are discussed. The development of primary and secondary textures in mushes are then discussed, principally with respect to the relative roles of crystal overgrowth, compaction, and cementation. Most crystal accumulation in moderate- to large-sized layered intrusions occurs on the floor, where crystal mushes develop by either in situ crystallization or crystal sedimentation. Except where a preferred crystal shape orientation occurs as a result of directional growth from a substrate, there are no definitive textural criteria for distinguishing in situ crystallization from crystal sedimentation in the accumulation of mushes. Mushes inherit primary textural characteristics that influence the subsequent texture development within the crystal pile. Primary porosity and permeability are influenced by initial packing and clustering characteristics of crystals which are a function of the way in which crystals accumulate and any subsequent mechanical reorganization. Crystal growth, solution/replacement, cementation, compaction, and recrystallization are competing processes involved in the secondary texture development of the crystal pile. The densification of a crystal mush involves the reduction of primary porosity of the cumulus grains. This may be by overgrowth on the grains or compaction. Either process will be restricted by the nucleation and growth of poikilitic grains which cement the granular crystal framework. These processes are analogous to syntaxial overgrowth, compaction and cementation involved in sediment diagenesis. Whether crystals grow under near-isothermal conditions or during cooling depends upon whether the mush is open or closed to melt percolation but is independent of the mechanism of heat and mass transfer within the mush. Compaction, necessarily an open-system process, involves deformation (dislocation creep) or solution/reprecipitation of grains (diffusion creep) and usually results in an increase in the degree of local textural equilibration. However, recrystallization ( e.g. by thermal annealing) also results in textural equilibration. Growth, compaction, and recrystallization are all competing processes and it is commonly not possible to isolate their contribution to any given texture; all produce rocks with the textural characteristics of adcumulates. The extent of densification of a mush of cumulus grains depends critically on the timing of nucleation and growth of poikilitic cementing phases. In any given magma composition this is a function of the local phase relationships. A cyclicity will develop in the texture in a crystal mush that is a function of the balance of densification and poikilitic cementation. Repeated replenishment of a magma chamber may result in suppression of the cementation cycle and allow mushes to become highly densified. On the scale of an intrusion the texture which develops depends upon the interaction of fronts of densification and cementation and hence is dependent on intrusion geometry.