State of strain in mylonites from the western Blue Ridge province, southern Appalachians: the role of volume loss

Abstract

This study examines the state of finite strain in quartzo-feldspathic mylonites from two localities in the western Blue Ridge province of the southern Appalachians. Mineral shape fabric and whole-rock chemical and modal data suggest that the mylonites deformed by inhomogeneous shortening normal to the foliation and progressive shear parallel to the foliation. The deformation involved substantial bulk-rock volume loss. Strain was partitioned between solution transfer processes in feldspar and dislocation creep processes in quartz and micas at temperatures of between 300 and 450°C. Initially feldspars underwent grain-size reduction by cataclasis but at advanced stages of mylonitization alkali feldspar grains display subrounded oblate shapes (0 < k < 0.5) which are attributed to dissolution processes normal to the foliation. Quartz deformed by crystal-plastic processes and quartz grains and ribbons display a range of k values (0.2 < k < 0.9) reflecting heterogeneous deformation. Whole-rock mylonite samples are enriched by a factor of 3 relative to their gneissic protolith in ‘immobile’ trace elements such as Ti, Zr, Y, P and V, and the REE which is attributed to the concentration of residual phases such as zircon, apatite, epidote and ilmenite observed in the mylonites. The trace element enrichments are interpreted as due to large (>60%) bulk volume loss. Density changes were minor. On the basis of feldspar grain shapes and modal and whole-rock chemical data, volume loss was largely accommodated by fluid infiltration and incongruent dissolution of alkali feldspar (producing muscovite) and involved the loss of alkalis and silica to the fluid phase. The mylonites may have nucleated as solution zones which subsequently underwent strain softening and displacement parallel to the zone boundaries.