Magmatic fabrics, structures and microstructures

Abstract

A magma is a two-phase material made of crystals immersed in a silicate melt, which displays a high viscosity contrast between the liquid and the solid fractions. A specific rheological behaviour is therefore expected from such a material, particularly as a function of the volume ratio between phases. Emplacement of magma to shallower levels of earth’s crust results in crystallization. As a consequence, crystal percentage increases and volume ratio between phases changes. Different structures at both the mesoscopic (field) and microscopic scales develop, which are characteristic of a particular crystal fraction. These aspects, and how shape preferred orientations (shape fabrics) develop in magmas, are discussed in this chapter. Rheological aspects of magma systems are presented, illustrated by significant microstructural features observed in granites. Our focus will then concern the construction mode of magmatic fabrics. Examples will demonstrate that, with the help of microstructures and sometimes of near-field gravity data distribution, emplacement modes of plutons are rather simple to analyse. Finally, mafic rocks will be considered at the end of chapter through case studies concerning, principally, the Skaergaard complex and gabbros from the oceanic crust.