Syn-shearing mobility of major elements in ductile shear zones: state of the art for felsic deformed protoliths

Abstract

The mineral assemblages of ductile shear zones, acting as open systems, may be strongly influenced by the occurrence of mass transfer processes induced by channeling H2O-rich fluids and mobilizing major elements. In this respect, the mineralogy of ductilely sheared felsic rocks under metamorphic conditions is believed to be controlled mainly by the following variables: the infiltrated fluid fluxes, the thermobaric deformation conditions of felsic rocks, the tectonic context and the likely fluid source. To investigate the presence of common features regarding the gain and loss of mobilized major elements with respect to these variables, a review of literature case studies dealing with felsic sheared protoliths has been undertaken. Qualitative results suggest that: (i) the mobilization of preferred major elements seems not related to fixed values of time-integrated fluid flux; (ii) no general relations exist between the mobility of major elements and the tectonic context for shearing, with some exception; (iii) H2O, K, Ca, Na, Mg, Si and Fe exhibit the greatest mobility whatever P-T metamorphic conditions; (iv) fluids coming from far-field and near-field sources induced a some preferred mobility of major elements within ductile shear zones; (v) the chemical selection of major elements seems greatly related to the fluid chemistry, while the amounts of fluids seems only to drive at completion the metamorphic fluid-rock reactions within ductile shear zones.