Paleozoic terrane accretion in northern Patagonia evidenced by High-Pressure metamorphism and subsequent thermal relaxation.

MARTÍNEZ, J.C.; DRISTAS, J.A.; MASSONNE, H-J,

Foz de Iguazú, Brasil

Congreso; Meeting of the Americas 2010; 2010

American Geophysical Union (AGU)

Gondwanan basement is outcropping in North Patagonia along the eastern foothills of the Andes. Recently, García-Sansegundo et al. (2008) reported a high-pressure (HP) event that had affected the metamorphic rocks of an area south of San Carlos de Bariloche (latitude ~41°S), Argentina, probably in Carboniferous times. We have investigated mica-schists from this area by a careful study of the chemical zonation of garnet and potassic white mica with the electron microprobe. The P-T evolution of these rocks was deduced by constructing P-T pseudosections with the PERPLE_X software package (Connolly, 1990 and updates) in the system Na-Ca-K-Fe-Mn-Mg-Al-Si-Ti-H-O and contouring them with chemical parameters such as molar fractions of garnet components. In sample 03081109 relics of phengites with Si contents around 3.4 per formula unit (pfu) and the garnet core, rich in Mn and poor in Mg, were coexisting at 1.8 GPa and 440°C. Increasing pyrope and grossular contents in garnet and decreasing Si contents in phengite signalize increasing temperatures at falling pressures. Peak temperatures of 580°C were reached at pressures around 1.2 GPa. The garnet rim with 8 mol% pyrope and 22 mol% grossular finally equilibrated at 9 GPa and 570°C at the beginning of retrogression during which potassic white mica with Si contents of only 3.16 pfu coexisted with biotite. We interpret the reconstructed P-T path by a collisional event during which a microplate, now located west of the study area but entirely covered by young sediments and igneous rocks of the Andes and its western foreland, was thrust under the South American part of Gondwana. The HP event deduced for the studied metapelites at 1.8 GPa and 440°C, reflecting a very low geothermal gradient of 7°C/km, at the beginning of accretion of the microplate was possibly caused by subduction of sediments on top of oceanic crust in close contact to this plate. However, these sediments were not deeply subducted but trapped between underthrust microplate and the overlying South American plate. This tectonic event was accompanied by flattening resulting in a pervasive foliation of the studied rocks. Furthermore, this event caused an upwards directed, reverse thrusting during which the studied metapelites approached the temperature of the overlying Gondwana lower crust. As this tectonic process is poorly dated, it is not clear yet if the involved microplate is a southern portion of the previously suspected but more to the north recently confirmed Chilenia terrane or another microplate (Deseado terrane, recently established by Pankhurst et al., 2006) that was thrust under Gondwana significantly later than Chilenia.