Description of flow microstructure of a phenocrystal-bearing magmatic dyke swarm from Southern Mexico (Invited)

Flowing magma is an heterogeneous mixture of a liquid phase, suspended phenocrystals, and/or bubbles that may be considered a fluid suspension with complex dynamics. When fluid magma is subjected to shearing, as is the case in a dike, viscous forces take control of the dynamics among its phases including the solid phase constituted by phenocrystals. Specific flow alignment of phenocrystals and magnetic crystals as tilling or imbrication (magnetic and physical) have been noteworthy for the determination of the direction and sense of flow. However, the fabric formed by flow -the so called flow-microstructure- is due to viscous forces of the fluid phase, and includes not only the alignment of crystals but also more complex interactions such as different types of hydroclustering, produced by the hydrodynamic forces. If the magma cools rapidly then the actual arrangements of phenocrystal in basaltic dykes may record the flow microstructure in the last stages of the flow and give valuable information for interpreting the flow history. Detailed measurements of selected geometrical attributes of phenocrystals 2D outlines and its distribution have been used to describe flow microstructure in basaltic dykes in southern Mexico dike swarm. The methodology proposed the manipulation of shape, longest axis orientation, aspect ratio of phenocrystals outlines and the probability of contact with phenocrystal neighbours as a microstructure signature. The results reveal that flow microstructure can vary from one zone to other inside a dike, or from one dike to another in the same dike swarm.