Mass-balance analysis of bulk-rock chemical changes during mylonitization of a megacryst-bearing granitoid,Cheongsan shear zone,Korea

Deformation of a megacryst-bearing granitoid pluton has produced a ～100 m-thick shear zone (viz., Cheongsan shear zone of South Korea) from protomylonite to muscovite-rich ultramylonite, showing variations in microstructural and mineralogical characteristics along the pluton margin. Petrographic observations within the shear zone suggest evidence for various syntectonic K-feldspar and plagioclase (feldspars hereafter) breakdown and muscovite formation through chemical reactions during mylonitization. Megacrysts up to 15 cm long make it difficult to have representative samples for chemical analysis. Here, we have used a mass-balance analysis (i.e., ‘best-fit’ Al–Ti–Fe isocon) for inferring the changes in major element chemistry during the mylonitization of megacryst-bearing granite. The results show variations in alkalies and silica, reflecting concentration changes (loss or gain) by fluid-assisted mass transfer and the role of SiO₂-rich fluids during progressive development from protolith to ultramylonite of the Cheongsan shear zone. Various bulk-rock volume-changes from 8% loss to 13% gain are calculated relative to the ‘best-fit’ Al–Ti–Fe isocon. Although comparison of the calculated protolith and observed mylonite compositions using ‘best-fit’ isocon analysis may result in uncertainties in interpretation, the present results are consistent with petrographic observations, indicating the element mobility, fluid-assisted mass transfer and the role of SiO₂-rich fluids during progressive development of the Cheongsan shear zone.