Quantitative Textural Analysis of Rocks in Layered Mafic Intrusions

Abstract

Quantification of rock textures (= microstructures), in the form of crystal sizes, shapes and orientations, complements chemical and isotopic approaches to the determination of the petrogenesis of rocks in layered mafic intrusions. Textures develop initially by nucleation and kinetic growth, but this process is commonly obscured in layered mafic intrusions by later textural modifications. Foliation in layered mafic intrusions is common but lineation of non-equant crystals is generally weak or absent. This makes it difficult to determine if simple shear (flow) or pure shear (compaction) is dominant. However, if pure shear is important then it must be accompanied by significant pressure-solution to produce the strength of the observed foliations. Correlation of fabric strength and overall crystal shape suggest that growth and shear were concurrent. But by far the most important textural process is equilibration (coarsening), testified by crystal size distributions. Almost all rocks in layered mafic intrusions have equilibrated, but the degree of progress towards equilibrium is very variable, even within layered mafic intrusions. It appears that the process is controlled by local variations in liquid content, mineral mixture and temperature.