THE EMPLACEMENT OF THE LAS CHACRAS POTRERILLOS BATHOLITH (SIERRA DE SAN LUIS, ARGENTINA): CONSTRAINTS FROM FIELD OBSERVATIONS, MICROSTRUCTURES, MAGNETIC FABRICS AND AGE DETERMINATIONS

Within the southern part of the Sierra Pampeanas (Sierra de San Luis, Argentina), a series of extensive intrusive bodies are regarded to post-date the Famatinian cycle but were emplaced during the Achalian, a period of heterogeneous deformation along crustal scale fault lines. Those plutons either are in conformity with regional structures (e.g. La Escalerilla - Sims et al. 1997) or appear as discordant ellipsoidal batholiths (Renca, La Totora - López de Luchi 2002). The largest of those is the Las Chacras Potrerillos batholith that is situated at the northern end of the transpressive, sinistral Guzman shear zone. This composite pluton exhibits three sub-domains that comprise two granitoid sub-units each: The southern Potrerillos stock (MRG, BRG), the central (BPG, GPG) and northern domain (EP, PG) (López de Luchi et al. 2001). Ar-Ar age determinations on the mylonitic fault rocks of the Rio Guzmán shear zone, covering the range from 375 to 351 Ma (Sims et al. 1997), point to a syn-tectonic emplacement of the batholith (382±5 Ma zircon crystallisation age - this study). The pluton is interpreted to be positioned at the crossover of a secondary set of NNW trending sinistral shear zones with the Rio Guzmán shear zone. The origin of a NNE directed extensional setting in a transpressive regime seems to be related to the transfer of displacement along this secondary set of NNW trending faults. This model is based on the relative timing of the emplacement sequence and macroscopically visible planar fabrics in the field. Additionally, a limited number of 49 well-distributed samples were taken for magnetic anisotropy measurements in order to reveal the linear fabric of the granitoids. Difficulties arising from the high amount of ferromagnetic (s.l.) fabric carriers were overcome by biotite texture measurements and the subsequent calculation of a theoretical AMS tensor. Microstructural investigations in combination with the overall foliation pattern suggest that the obtained fabrics were archived after juxtaposition of the different sub-units prior to final solidification. Planar fabrics form interconnected ellipsoidal 'funnel' structures, suggesting an erosion level somewhere below the equatorial plane of the pluton. Magnetic lineations predominantly exhibit a NNE trend parallel to the long axes of the foliation 'funnels'.