Textural variations and chemical mobility during mylonitization: the El Tigre Granitoid shear zone, sierra de Pie de Palo, Western Sierras Pampeanas, San Juan

Resumen

A high-strain ductile shear zone trending NEE with southeasterly dipping mylonitic foliation, has been recognizedaffecting 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. Kinematicanalysis indicates a main strike-slip component and provides evidence that the relative movement within the shear zone havedextral sense. Relict igneous and peak amphibolite facies metamorphic mineral assemblages and textures are preserved in thegranitoid 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 almostall major and trace elements, including rare earth elements and Rb/Sr and Sm/Nd isotopes. Observed chemical variations aremostly controlled by syntectonic fluid-transport processes and decrease in the garnet, biotite and accessory minerals amountsduring mylonitization, and the neoformation of white-mica in the fine-grained mylonite matrix. Moreover, the different isotopicsignatures between the protolith and the mylonites could be a consequence of mechanisms of deformation-driven processesassisted by fluid flow with different fluid-host rock interaction ratios. Textural, modal and chemical changes betweenthe El Tigre granitoid protolith and its mylonites, allow reconstructing its tectono-metamorphic evolution and the metamorphicconditions achieved. Paragenetic associations and deformation textures on mylonites suggest that El Tigre granitoid shearzone was developed under low-T greenschist facies conditions, probably at temperatures below 400 °C. This deformationalevent took place at ca. 473 ± 10 Ma during the Famatinian orogeny.