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

CASTRO DE MACHUCA, BRÍGIDA; MORATA, DIEGO; PONTORIERO, SANDRA; PREVILEY, LORENA; ARANCIBIA, GLORIA

San Salvador de Jujuy

Congreso; XVII Congreso Geologico Argentino - Simposio de Tectonica Preandina; 2008

Asociacion Geologica Argentina

The Sierra de Pie de Palo (SPP), Western Sierras Pampeanas, considered to represent part of the Proterozoic basement of the Precordillera Terrane, is characterized by a complex polyphase tectono-metamorphic history. Granitoid rocks of diverse nature and age make up only a small proportion of the crystalline basement of the SPP. At the downstream of Quebrada del Tigre (31° 31´ 30´´ S-68° 15´ 12´´ W) on the southwest side of the SPP, outcrop the El Tigre Granitoid (ETG) and mylonites derived from it. The ETG, a Mesoproterozoic (1105 Ma) peraluminous garnet-bearing two mica granitoid, intruded concordantly the foliation of the host rocks of the Pie de Palo Complex. The ETG experienced deformation and metamorphism under amphibolite to greenschist facies retrogressive conditions. The last deformational and metamorphic event of the ETG took place at ca.473 Ma and was localized along narrow ductile shear zones under greenschist facies conditions. To study petrographic variations and the behavior of elements and isotopic signatures in these shear zones during mylonitization, sampling was developed from small-scale mylonites bands and neighboring granitoid protolith, covering the entire strain-gradient fabric from undeformed/slightly deformed protolith to ultramylonite. The coupled microstructural and geochemical (major, trace elements and Rb/Sr-Sm/Nd isotopes) investigations allow us to link specific geochemical and mineralogical changes to particular microstructural changes. Local ETG ductile shear zones, trending NE with steep easterly dips, range from few centimeters to a meter width. They are characterized by the presence of mylonites, stretching lineations and retrogression. Numerous microstructures, most of which are visible in the field as well as in thin sections have been recognized, amongst them are: mica-fish, pressure shadows, sigma-type porphyroclasts, asymmetrical microfolds and microsheared feldspar porphyroclasts. In some samples composite foliation planes (S-C planes) develop simultaneously within the shear zone. The well developed kinematic markers in these mylonites provide evidence that the relative movement within the shear zone have a dextral normal sense with dominant west-southwest transport direction (present coordinates). Other macroscopic- to mesoscopic structural observations suggest that these shear zones evolved in a transpressive tectonic regime. The ETG is progressively transformed and grade laterally into several textural/compositional categories from undeformed/slightly deformed protolith through protomylonite to fine-laminated mylonite-ultramylonite. Strain effects in the protolith are minor and only evident microscopically: undulose extinction in quartz, some bending in micas and limited subgrain formation without recrystallization. In the moderately deformed rocks boundaries between feldspars grains are serrated and irregular. Micas are easily bent and notorious kinking is developed, quartz behaves plastically by various dislocation/glide mechanisms as undulose extinction and deformation lamellae. Progressive subgrain development and incipient recrystallization especially at their margins resulted in an incipient grain size reduction and grain boundary bulging. Protomylonites have a foliated groundmass of dynamically recrystallized quartz, micas and epidote. Feldspars porphyroclasts behaved passively during shearing of the matrix so they developed ò-type structures. Deformation twins vary from straight, to kinked, to poorly conserve and some grains show undulose extinction. Large mica flakes develop characteristic mica-fish. Mylonites and ultramylonites are cohesive, strongly foliated and lineated rocks with feldspar porphyroclasts in a very fine-grained quartz-phyllosilicate-epidote matrix. Pronounced foliation and extensive grain-size reduction compared to the crystal size of the protolith, characterize the more-intense mylonitic deformation. The extreme plastic deformation of quartz results in extremely long and thin ribbons which are often partially replaced by a very fine-grained mass of new recrystallized micropolygonal-granoblastic quartz aggregate. Ribbon quartz usually shows partial to full recovery, but very rare original quartz grains elongated parallel to the shear zone fabric are still present. Brittle garnet porphyroclasts are completely dismembered in small grain trails, and are no longer recognizable in the ultramylonites. Regarding geochemical changes, 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. The element mobility is mainly a function of the amount of relict, recrystallized and neocrystallized minerals. Using the isocon method (Grant 1986), differential mobility of major and trace elements has been established, being SiO2 and Al2O3 almost immobile elements during mylonitization whereas FeOt, MgO, MnO, CaO and TiO2 evidenced the higher depletion with increasing deformation. A notable increase in LOI and K2O during mylonitization progress is also observed. Trace elements (including REE) are mostly depleted in higher mylonitized rocks except by an increasing in Ba. High mobility has been detected in the Rb/Sr system meanwhile the Sm/Nd system seems to have a more closed behavior. It is notably to remark the high 87Sr/86Sr isotopic ratio measured in the ultramylonite when compared with the undeformed ETG. The observed chemical variations are mostly controlled by syntectonic fluid-transport processes and decreasing in the garnet and biotite amounts during mylonitization, and the neoformation of white-mica in the fine-grained mylonite matrix. Moreover, the different isotopic signature observed between the undeformed and mylonitized granitoid could be a consequence of mechanisms of deformation-driven processes assisted by fluid flow under open system conditions with different fluid-host rock interaction ratios.