Paleoproterozoic P-T-d-t evolution of a gneiss with quartz-sillimanite nodules from the Azul ranges, Rio de la Plata craton, Argentina

Abstract

The southernmost outcrops of the ~30 km long metamorphic belt of the Azul ranges in the Rio de la Plata craton were studied in detail with special emphasis on a gneiss with quartz-sillimanite nodules. Pressure (P) - temperature (T) and T-H2O pseudosections were calculated for such a gneiss and contoured with the PERPLE_X computer software package. Remnant oriented, K-white mica laths in K-feldspar of the gneiss matrix suggest an initial metamorphic P-T path at the onset of anatexis near ~660° at 0.43 GPa and water-saturation. Peak metamorphic conditions of ~680 °C and 0.40 GPa were reached excluding garnet growth. Back-reactions in the presence of a hydrous fluid and syntectonic coaxial deformational stress (D1) produces (1) the nodules by dealkalization of K-feldspar and (2) large, roughly oriented porphyroblasts of K-white mica after sillimanite at ~650 °C and 0.38 GPa. Homogeneous compositions of this mica (3.08–3.10 Si per formula unit) are the result of retrograde conditions in the range 450–580 °C at 0.15–0.35 GPa. A ductile deformation (D2) overprinted the exhumed rocks afterward. A set of 41 U-Th-Pb ages was determined with the electron microprobe from monazite in a leucocratic sillimanite-bearing vein, a quartz-sillimanite nodule, and the quartzofeldspathic matrix of the gneiss. Two age groups could be defined: (I) 2127 ± 10 (1σ) Ma and (II) 2039 ± 3 (1σ) Ma. Group II is assigned to the peak metamorphic event, whereas group I is related to the Encantadas accretionary orogeny, which provided the source material for the sedimentary protolith of the gneiss.