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Deformation-controlled cation diffusion in compositionally zoned tourmaline

Rhodes University, Department of Geology, PO Box 94, Grahamstown 6140, South Africa

^* E-mail: s.buettner{at}ru.ac.za

The compositional zonation of both undeformed and plastically deformed tourmaline crystals from an amphibolite-facies mylonitic pegmatite from the Sierras Pampeanas (NW Argentina) has been investigated using electron microprobe analysis (EMPA). Undeformed tourmaline shows optical and compositional major and minor element growth zonation with a Ca- and Mg-rich rim zone and an Fe-rich core zone. The tourmaline population of the mylonite consists of crystals which appear undeformed at microscopic scale, and of weakly, moderately, and strongly deformed crystals. Depending on the intensity of plastic deformation, the optical zonation is blurred or absent, and the compositional zonation is less pronounced or destroyed. Plastic deformation mobilizes small cations (Fe²⁺, Mg²⁺) more efficiently and at lower deformation intensity than larger cations (Na⁺, Ca²⁺). In addition to intra-crystal homogenization, plastic deformation caused variable but generally minor Fe, Mg, Si, Al, Ca, and Na exchange between deformed tourmaline domains and co-existing fluid or solid phases. Dislocation creep is interpreted as the dominant deformation mechanism leading to the homogenization of the initial tourmaline growth zonation. The composition and the degree of homogeneity of deformed tourmaline domains depend on the initial composition of the growth zones, their initial volume ratio, the intensity and homogeneity of plastic deformation, and the size of the mobilized cation. Consequently, the composition of and the element distribution within plastically deformed crystals is not entirely controlled by intensive variables (P-T-X), and therefore not suitable for petrogenetic interpretation.

KEYWORDS: cation diffusion, compositional zonation, crystal-matrix exchange, homogenization, plastic deformation

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