PROGRESSIVE MYLONITIZATION OF A PALEOZOIC GRANITE IN THE CORDILLERA ORIENTAL, NW ARGENTINA

A granitoid intrusion in Tacuil (province of Salta, NW Argentina) forms part of an Ordovician magmatic belt that was emplaced at the western margin of Gondwana between ~480 and 470 Ma. The granite and its Late Neoproterozoic-early Paleozoic host rocks were tectonically overprinted by a NW-SE trending deformation belt that likely is part of a system of linked anastomosing shear zones that accommodated deformation during the Ordovician Famatinian orogeny. Oriented samples were collected from a 5 m transect in order to document the microstructural variations and dynamic processes involved in the gradual mylonitization of the granite. Samples with very low degree of deformation consist of Kfs phenocrysts (up to 8 cm) in a groundmass mostly composed of Kfs, Qtz, Plag, Bt, and Mus grains (0.2 to 2 cm). The granite gradually transitions into protomylonite, mylonite and ultramylonite. The main textural and microstructural variations that we observe in samples with increasing degree of deformation are: 1- the progressive change from discontinuous anastomosing mm-thick biotitic seams that wrap around asymmetric Feldspar cm-scale porphyroclasts to a pervasive mm-thick planar ultramylonitic foliation defined by Bt-rich and quartzofeldspathic bands; 2- a decrease in grain-size of most minerals. Kfs porphyroclasts show a gradual size reduction by intense microfracturing. The resultant angular mm-scale fragments form mylonitic bands in the more strained samples; 3- strained Qtz grains and Qtz grain aggregates that show evidences of dynamic recrystallization change into quartz ribbons with increasing strain; 4- an increase in the degree of feldspar sericitization. Intracrystalline deformation and dynamic recrystallization of Qtz, in contrast to the lack of intracrystalline deformation in feldspar, and the stable mineral assemblage Kfs+Qtz+Plag+Bt+Mus are consistent with deformation under greenschist-facies conditions. In sections parallel to the dip-slip stretching lineation and perpendicular to the foliation, kinematic indicators are generally consistent with a top down to the SW normal displacement. Future work on strain analysis will constrain the significance of this shear zone in the Ordovician shear system and in the regional context of the Famatinian orogeny.