Exploring paleoproterozoic metamorphism in the western Río de la Plata craton from drill-core evidence

Rhyacian basement is not exposed in the western part of the Rio de la Plata craton, but deep borehole samples (depth of 1046-3340 m) provide information on its composition and metamorphic evolution. An amphibole schist sample from the Camilo Aldao borehole underwent two metamorphic stages: M1 and M2. The P-T conditions of M1 are not constrained whereas the M2 main metamorphic stage is characterized by conditions of 6.1-7.4 kbar and 575-605 degrees C. In contrast, an olivine gabbronorite sample from the Santiago Temple borehole records crystallization conditions of 1.7 +/- 0.6 kbar and 1180 +/- 10 degrees C based on clinopyroxene composition, and late magmatic to subsolidus low-pressure granulite facies metamorphism at 2.3 +/- 1.2 kbar and 749 +/- 85 degrees C based on multi-equilibrium thermobarometry. The Rhyacian evolution of the Rio de la Plata craton is charac-terized by at least three thermo-tectonic orogenic stages: T1 (ca. 2200-2100 Ma), T2 (ca. 2100-2040 Ma), and T3 (ca. 2040-2010 Ma) that correspond respectively to: 1) early oceanic subduction and juvenile continental arc magmatism; 2) subsequent continental collision (clockwise intermediate P/T paths of metamorphism), and 3) final low-pressure metamorphism coeval with post-orogenic magmatism. The Camilo Aldao amphibole schist is representative of the T2 orogenic stage while the Santiago Temple gabbronorite is representative of T3 magmatism and metamorphism.