Fabric of porphyritic magmatites inferred from the preferred orientation of feldspar phenocrysts
The determination of foliation and lineation in magmatic rocks can be often a difficult problem. Moreover, there are only a few sound kinematic criteria for determination of a shear sense during magmatic flow. Therefore we improved the method of optical goniometry that helps recognise the fabric of porphyritic rocks. This method enables: (1) determination of shape preferred orientation (SPO) of feldspar phenocrysts, (2) more precise definition of magmatic foliations and lineations, and (3) assessment of the kinematics of the combined coaxial and non-coaxial flow using asymmetries in feldspar subfabrics (Fernandez 1987). The application of the method is demonstrated on samples of the Třebíč durbachites (E of the Bohemian Massif) in which K-feldspar phenocrysts reveal strong magmatic preferred orientations gained during the emplacement. Asymmetry between more anisotropic and less anisotropic feldspar subfabrics in the XZ plane is compared with theoretical models for the fabric evolution of particles in a slowly flowing viscous matrix (Jeffery 1922).
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Policy: Open Access