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Dr. Aránzazu Piñán Llamas
Department of Geosciences
Indiana University – Purdue University Fort Wayne
The Cordillera Oriental in northwestern Argentina contains Late Neoproterozoic-Early Paleozoic rocks that were forming part of the proto-Andean margin of southwest Gondwana. These rocks (mainly metamorphosed marine sediments) were intruded between ~480 and 470 million years by granitic bodies and pegmatites that localized substantial deformation during the Ordovician Famatinian orogeny. As a result of the Famatinian deformation, N-S and NW-SE trending anastomosing highly deformed (shear) belts overprinted the igneous intrusions and their host rocks. The development of these shear zones was coeval with, and followed the thermal Ordovician peak. Famatinian metamorphism and deformation occurred at greenschist to granulite-facies conditions (garnet-cordierite-sillimanite and orthopyroxene) in various locations of the Cordillera Oriental. Oriented samples were collected from a sheared granite in Tacuil (province of Salta, NW Argentina) in order to compare microstructural and chemical compositions between protomylonitic, mylonitic and ultramylonitic rocks; the ultimate goal was to document mechanisms of deformation observed in samples with increasing degree of deformation.
The petrographic analysis of the samples shows that deformation occurred under greenschistfacies conditions. The relatively undeformed granite consists 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). In the deformed granite, metamorphic fabrics transpose magmatic fabrics and L-S fabrics are present. The main microstructural variations observed in samples with increasing degree of deformation are: 1- the progressive change from discontinuous anastomosing mm-thick biotitic seams 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; 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. In contrast to quartz grains, feldspar shows little evidence of dynamic recrystallization. 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.
Earth Sciences | Physical Sciences and Mathematics
Maassel, Daniel, "Progressive Mylonitization of a Paleozoic Granite in the Cordillera Oriental, NW Argentina" (2015). 2015 IPFW Student Research and Creative Endeavor Symposium. 17.