Mechanisms of myrmekite formation: case study from the Weinsberg granite, Moldanubian zone, Upper Austria

Original Paper

Abstract

Myrmekites have attracted the attention of petrographers over more than a century, and several genetic models have been proposed. We report on myrmekites from the Weinsberg granite of the Moldanubian zone of Upper Austria. Based on petrographic evidence, fluid-mediated replacement of alkali feldspar by myrmekite during the sub-solidus evolution of the granite is inferred. The replacement was metasomatic on the scale of the myrmekite domains requiring addition of sodium and calcium and removal of potassium from the reaction site. In contrast, silica and aluminum were conserved across the reaction front. Myrmekite formation appears to have been synchronous with and related to the hydration of orthopyroxene and concomitant replacement of primary magmatic plagioclase by biotite at around 500 °C. The evolution of the myrmekite microstructure and a peculiar composition zoning of the plagioclase constituting the myrmekite matrix is qualitatively explained by a model for discontinuous precipitation, which accounts for chemical segregation by diffusion within the reaction front and the propagation of the reaction front with finite mobility as potentially rate limiting processes. Constraints on the underlying reaction rates are derived from the preserved microstructure and chemical pattern. Crystal orientation imaging by electron backscatter diffraction reveals grain-internal deformation, which is primarily concentrated in the quartz and less pronounced in the plagioclase matrix of the myrmekite. This is interpreted as a growth feature related to different transformation strain at the segments of the myrmekite reaction front, where quartz and plagioclase are formed.

Keywords

Myrmekites Open system replacement Discontinuous precipitation Microstructure evolution Reaction rates 

Notes

Acknowledgments

We are indebted to F. Koller for helping with field work and fruitful discussions of the subject and to D. Topa for helping with high-resolution element mapping. Comments from two anonymous reviewers helped to improve the manuscript. Financial support by the Austrian science foundation project I-474 N19 in the framework of the FWF-DFG DACH research group FOR 741 is gratefully acknowledged.

Supplementary material

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Supplementary material 1 (pdf 3562 KB)

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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Department of Lithospheric ResearchUniversity of ViennaViennaAustria

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