THE ROLE OF PRE-EXISTING ANISOTROPIES AND PARTIAL MELT IN CONTROLLING STRAIN LOCALIZATION IN A PALEOZOIC LOWER CRUSTAL SUTURE ZONE, SIERRAS PAMPEANAS, WEST-CENTRAL ARGENTINA

The western Sierra Pampeanas of west-central Argentina preserves a tectonically imbricated middle to lower crustal suture zone that developed over tens of millions of years, from the Cambrian to Early Silurian. The suture zone juxtaposes mafic and clastic rocks in the west against an eastern arc complex thought to have formed on the Gondwana margin. A transect of the suture zone displays markedly different styles of deformation due to differences in timing of deformation, lithology, and pressure-temperature conditions. Deformation along the westernmost exposure of this crustal section, in the Sierra Pie de Palo, was localized along laterally continuous discrete shear zones. The mafic-ultramafic Pie de Palo complex is bounded above and below by significant low-angle amphibolite grade mylonitic high-strain zones. At its base, Ar/Ar hornblende ages of 511±5 Ma and 517±5 Ma (relative to FCs at 28.02 Ma) from the Las Pirquitas thrust suggest that imbrication of the crustal section had initiated by that time. Above, the Duraznos shear zone places high-grade metasedimentary rocks over the Pie de Palo complex. This shear zone formed at relatively high pressures, as shown by compositional zoning in syntectonic garnet and hornblende. The age of the Duraznos shear zone is currently constrained to be younger than the Las Pirquitas thrust and older than 417.4±1.9 Ma muscovite cooling ages in the footwall. Peak temperatures in the eastern part of the suture were not achieved until the Ordovician, when high-pressure migmatization localized deformation immediately west of the arc complex in the Loma de Las Chacras. Migmatite leucosomes record zircon ages of 461±7 Ma and 456±2 Ma and the abundant migmatization occurred in the presence of kyanite. Map-scale low permeability amphibolite layers arrested the upward migration of partial melt through migmatite and gneissic host rock. Lateral migration of leucocratic sills beneath the amphibolite horizons facilitated localization of deformation into numerous non-coaxial high strain zones that were subsequently folded. This study illustrates that construction of lower crustal sections can occur over very long time periods during which interplay of lithologic and rheologic strength contrasts, thermal gradients, and pre-existing anisotropy is fundamental in the localization of deformation.