Element mobility during Mylonitization of the San Cristóbal granite, South-east of the Sierra de Velasco, Sierras Pampeanas, Argentina

Acta geológica lilloana 26 (2): 123–132, 2014

Abstract

“Element mobility during Mylonitization of the San Cristóbal granite, South-east of the Sierra de Velasco, Sierras Pampeanas, Argentina”. At southeastern tip of theSierra de Velasco, western of La Rioja, Argentina, Granitic rocks of San Cristobal pluton aredeformed into mylonites, developing a ductile shear zone, called Faja Milonítica Sur, undergreen schists facies conditions. The mobilization of chemical elements in this shear zoneduring mylonitization is evaluated in this paper. Using the Nb-Ti-Zr isocon, a volume loss of30% is calculated. Zr, Ti, Fe, Mn and Nb are relatively immobile, SiO2, K, Na, Rb, Ba, Sr,Mg, Y and Ni are lost, and Ca, Cr, and P are enriched. This is consistent with petrographicstudies that show K-feldspar hydrolysis, plagioclase retrogradation to clinozoisite and albite,muscovite neoformation, biotite incremented and modal variations of accessory minerals.