Textural variations and chemical mobility during mylonitization: The El Tigre granitoid shear zone, Sierra de Pie de Palo, Western Sierras Pampeanas, San Juan, Argentina

Abstract

A high-strain ductile shear zone trending NEE with southeasterly dipping mylonitic foliation, has been recognized affecting a Mesoproterozoic (ca. 1105 Ma) peraluminous garnet-bearing two mica granitoid (El Tigre granitoid: 31º31’30”S- 68º15’12”W) which is part of the crystalline basement of the Sierra de Pie de Palo, Western Sierras Pampeanas. Kinematic analysis indicates a main strike-slip component and provides evidence that the relative movement within the shear zone have dextral sense. Relict igneous and peak amphibolite facies metamorphic mineral assemblages and textures are preserved in the granitoid protolith although, within the shear zone, deformation obliterates those generating typical mylonitic fabrics. Mylonitization operated under open-system conditions, provoking mobilitization (either enrichment or depletion) of almost all major and trace elements, including rare earth elements and Rb/Sr and Sm/Nd isotopes. Observed chemical variations are mostly controlled by syntectonic fluid-transport processes and decrease in the garnet, biotite and accessory minerals amounts during mylonitization, and the neoformation of white-mica in the fine-grained mylonite matrix. Moreover, the different isotopic signatures between the protolith and the mylonites could be a consequence of mechanisms of deformation-driven processes assisted by fluid flow with different fluid-host rock interaction ratios. Textural, modal and chemical changes between the El Tigre granitoid protolith and its mylonites, allow reconstructing its tectono-metamorphic evolution and the metamorphic conditions achieved. Paragenetic associations and deformation textures on mylonites suggest that El Tigre granitoid shear zone was developed under low-T greenschist facies conditions, probably at temperatures below 400 °C. This deformational event took place at ca. 473 ± 10 Ma during the Famatinian orogeny.