Stability of zircon in a low-grade ultramylonite and its utility for chemical mass balancing: the shear zone at Miéville, Switzerland

At Miéville, Switzerland, granitic basement rocks of the Aiguilles Rouges Massif are cut by a subvertical shear zone. The Variscan shearing event produced a low-grade ultramylonite zone, which is up to 50 m wide and several kilometers long. Investigations of accessory zircons from the undeformed wall rock and from the most highly deformed ultramylonite show only minor alterations and mechanical damage of zircon crystals even in an extreme state of deformation. This outstanding stability of zircon allows one to consider the element Zr as immobile and therefore to use it as a passive marker for calculations of mass and volume changes during deformation processes. Application of this method confirms that the deformation at Miéville proceeded under simple shear, constant volume, and isochemical conditions. This is in agreement with the results of classical geochemical mass balancing. In addition, uncertainties inherent in the geochemical approach can be minimized by the new method.