Metapelitic mylonites from the North Patagonian Andes, Argentina: ductile event P-T conditions estimated through geothermobarometry and deformation mechanisms in minerals

BIANCHI FRANCO DENIS; MASSONNE, HANS -JOACHIM; DELPINO SERGIO HUGO; MARTINEZ JUAN CRUZ; DRISTAS, JORGE A.,

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

Asociación Geológica Argentina

This contribution deals with detailed petrographical-microstructural and geothermobarometric studies, carried out on garnet-bearing metapelitic mylonites outcropping south of San Carlos de Bariloche in the western Ventana Range foothills, at Gutierrez and Mascardi lakes shores. Mesoscopically, these rocks show intense foliation highlighted by the alternation of light and dark-gray bands. Some leucocratic polycrystalline coarse-grained lenses show asymmetric tails with a -like pattern, clearly indicating the ductile shear imposed on these rocks. Under the microscope, the studied rocks are composed of Qtz-Pl-Ms-Grt-Bt and accessory minerals Ap-Zr-Mz-Ilm-Rt-Ti-Tur, all arranged in a well-developed anastomosed mylonitic foliation (Sm, Fig. 1). Qtz exclusively forms ribbons of up to 1 mm thick, which can have two different characteristics (Fig. 1). Those in which Qtz crystals are limited in their normal growth to Sm by pinning of micas or plagioclase that develop one-grain-wide ribbons with rectangular shape. Unrestricted growth of Qtz grains locally form windows (Fig. 1d), overstepping Ms-Pl folia causing coalescence of adjacent ribbons. Coalescense of Qtz ribbons give place to wider ones, characterized by large amoeboidal crystals contacting each other through sutured boundaries. Qtz ribbons wrap around relict lenticular Pl porphyroclasts and their envelope of recrystallized new-grains. Pseudopolygonal recrystallized grains of up to 50-70 m, were mainly preserved in extension sites originated by porphyroclast boudinage (Figs. 1a and 1b) or their pressure shadows. The growth and coalescence of Qtz ribbons, sometimes leaves relics of Pl crystals with very irregular shapes (Fig. 1e). Ms relict flakes of very dissimilar sizes are oriented parallel to Sm. Most of them are bent and sometimes kinked. Coarser-grained crystals are usually replaced at their borders and cleavage planes by fine red-brown Bt. Grt appears as small euhedral crystals included in Pl or distributed along Sm where is partially or entirely surrounded by Qtz ribbons, as also occurs with irregular fragments of relict Pl (Figs. 1d and 1e). Bt appears as very small plates replacing relict Ms crystals and along Sm planes (Fig. 1a). Larger crystals, randomly oriented, can be observed at extension sites and pressure shadows (Figs. 1a and 1e). Mineral equilibria calculations were performed with the software winTWQ v2.32 (Berman, 1991) in the system Na-Ca-K-Fe-Mn-Mg-Al-Si-Ti-H-O. P-T estimation of mylonitic event were obtained using Grt rim composition (n=4), coarse adjacent Bt (n=2), Pl (n=6), Ilm (n=5), plus Qtz, Rt, Ti and H2O (aH2O=1). All phases activities were set to 1.0, except for Ti, slightly reduced to 0.99 to improve intersection and considering its scarce mineral mode. Results are shown in Fig. 1f. The very good fit of the 10 reactions at 720 ºC and 11.8 kbar, supports the choice of phases considered in probable equilibrium during the ductile deformation event (Fig. 1f). Applied Grt-Bt geothermobarometer of Wu (2019) for the same garnet rim and biotite, yielded similar P-T conditions (12.00 kbar and 685 ºC). Consistent temperatures (691 ºC) were obtained with Ti-in-Bt geothermometers of Wu and Chen (2015) (with pressure obtained from Wu 2019) and (658 ºC) and Henry et al. (2005) (Fig. 1f). Temperatures range (650 ºC - 720 ºC) from thermobarometry agree well with observed deformation mechanisms in Qtz and Pl. Large amoeboidal Qtz crystal (>0.5 mm) with sutured grain boundaries, window structures and inclusion of other phases are consistent with high-temperature grain boundary migration recrystallization (GBM). According to Stipp et al. (2002) changeover from Qtz GBM I and GBM II fields, can be texturally recognizable by the change from impurity to non-impurity controled GBM, which they observed at ̴ 650 ºC. On the other hand, similar temperature conditions (>600 ºC) for starting of coalescence of Qtz ribbons and advanced coalescence (>700 ºC) were documented by Delpino et al. (2016) through geothermometry. Thus, geothermobarometry and deformation mechanisms in minerals supports upper-amphibolite metamorphic facies conditions for the development of the ductile deformation event recognized in the studied rocks. Further investigations are being carried out to better constrain the events and relations between metamorphism and deformation in the region.