Use of accessory zircon for the quantification of volume changes in ductile shear zones cutting plutonic rocks

Abstract

Abstract The behaviour of accessory zircon during deformation has been investigated for a deep-crustal ductile shear zone in the western Tauern Window (Eastern Alps, Austria). Within the about 1 m wide transition zone, a high-Si metatonalite is transformed to a subsilicic garnet-chlorite-biotite schist. Damage of zircon crystals in moderately to highly deformed rocks is limited to rounding, corrosion, and fracture, while metamorphic growth could not be recognized. The quantitative zircon analysis compares the number of zircon crystals per unit volume between wall rock and shear zone rocks. An estimation of possible volume changes is based on the assumption that within volume-loss shear zones zircons are passively enriched, i.e. increased in number per unit volume, and vice versa. This enables the use of zircon crystals as passive markers for volume changes during deformation. Respective results of our study indicate that volume loss of the garnet-chlorite-biotite schist reached 45 ± 16% with respect to the metatonalite. Geochemical profiles across the shear zone display a dramatic decrease of the elements Si, Na, and Sr with rising deformation, while the concentrations of Al, Fe, Mg, Rb, and Zr are subject to a significant increase. Related mass balance calculations also indicate a significant volume loss of 32 ± 10% and therefore correlate very well with the quantitative zircon analyses. For deformation zones within granitoid rocks the quantitative investigation of zircon crystal seems to be a promising tool for assessing possible volume changes during deformation.