Mylonites of the South Armorican Shear Zone: Insights for crustal-scale fluid flow and water–rock interaction processes

Abstract

Mylonites display petrographical and geochemical characteristics that can be related to syn-deformation fluid circulation. The South Armorican Shear Zone, a major structural feature of the Armorican Massif (France), is outlined by the presence of mylonitic rocks cropping out mostly in open quarries. These mylonites were essentially formed at the expense of peraluminous granitic bodies. Deformation occurred from ductile conditions in the biotite stability field (>400°C) down to lower greenschist cataclasis and brecciation, where carbonation developed. U–Pb analyses on zircon and monazite define a minimum duration of 15Ma for the deformation and hydrothermal history, between 315Ma and 300Ma. Fluid circulations are well documented, by way of petrographic observation (chlorite and carbonate crystallization), mineralogical composition analysis (muscovite chemistry), erratic mobility behavior of some elements (As, Sn, U for instance), and stable isotope composition analysis of the infiltrated rocks. High temperature deformation is not accompanied by alteration of the O isotope system, which implies either low fluid/rock ratio and/or the involvement of δ18O crustal fluids with a composition similar to that of the rocks. On the other hand, some low temperature mylonites show a drastic decrease in the δ18O values, which has to be related to the influx of surface derived waters. The heat source necessary for this crustal scale downward infiltration of fluids followed by upward motion was likely provided by the exhumation of lower crustal units in the South Armorican domain.