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RESEARCH PRODUCT

Evolution of mica fish in mylonitic rocks

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Abstract Mineral fish are lozenge-shaped porphyroclasts, single crystals in a finer grained matrix, which occur in ductile shear zones and which are commonly used as shear sense indicators. Mineral fish of biotite, tourmaline, K-feldspar, garnet, hypersthene and quartz occur in mylonites but most common are white mica fish. These mica fish can be subdivided into six morphological groups that develop by different mechanisms determined by different initial shapes and orientations. The principal mechanisms of formation are intracrystalline deformation combined with rigid body rotation. Concomitant selective grain size reduction occurs by recrystallisation, cataclastic separation, pressure solution and diffusional mass transfer. Microboudinage has been proposed for the breakdown of large mica fish into smaller ones but many mica fish undergo shortening rather than extension along their long axes. Evidence is presented for an alternative process in which the tips of mica fish are isoclinally folded and then break off along the hinge of these microfolds. All the presented fish-shaped porphyroclasts or mineral fish have a specific shape-preferred orientation with their long axis at a small antithetic angle with respect to the foliation. They represent a special group of objects with a stable orientation due to flow partitioning into narrow shear bands.