Original paper

Volume balancing in ductile shear zones by the quantification of accessory zircon in oriented thin sections

Sturm, Robert


For studying element transfers in ductile shear zones, the exact knowledge of possible volume and mass changes between wall rock and related mylonite is indispensable. Until now, this essential information has been only obtained from a geochemical approach using the concept of 'immobile' elements, i.e. elements which are not subject to any wide-range transfer during deformation. Due to considerable uncertainties with this method, an alternative technique is introduced in this contribution that is solely based on the systematic quantification of accessory zircon in oriented thin sections. Zircon crystals are marked by high resistance to mechanical and chemical attacks preferentially occurring during medium- to high-grade deformation and therefore may act as passive markers in ductile shear zones cutting rocks of plutonic origin. The number of zircon crystals per reference volume in the mylonite is increased with respect to that of the wall rock, if shearing took place under volume loss, and vice versa. First application of this method is described for an amphibolite-facies shear zone in the south-western Bohemian Massif. While the zircon quantification method indicates a volume loss of 28.2 ± 11.5 % between wall rock and mylonite, with the geochemical approach, a volume loss of 17.3 ± 5 % could be estimated. Advantages and drawbacks of the new method are discussed in detail.


zirconshear zonemylonitedeformationmass balancevolume balance