EARLY PALEOZOIC ASSEMBLY OF WESTERN GONDWANA: EVIDENCE FROM THE SIERRA DE SAN LUIS, CENTRAL ARGENTINA

Deformational and metamorphic fabrics within the Sierra de San Luis, Eastern Sierras Pampeanas, Argentina provide evidence for multiple tectonic events related to the early Paleozoic development of Gondwana. The Eastern Sierras Pampeanas contain a suite of complexly folded and faulted, predominantly metasedimentary rocks, interpreted as an accretionary prism package that formed on the PaleoPacific margin of Gondwana in the late Precambrian.

Metasedimentary rocks of the Sierra de San Luis contain several superimposed deformational fabrics, the first of which is a N-S striking pressure solution banding evident within metapsammites. Early east-directed contraction was followed by regional metamorphism, presently observed as broad NNE-trending bands of chlorite-grade through 2nd sillimanite-grade rocks. Field observations and microstructures indicate smooth gradations between lower- and higher-grade metamorphic rocks, and provide multiple examples of small-scale syn- to post-peak metamorphism folding. This suggests that regional folding closely followed peak metamorphism. Monazite geochronology documents a disparity between Ordovician peak metamorphism in the Sierra de San Luis and Cambrian peak metamorphism in the Sierras de Córdoba, suggesting Ordovician or later accretion of the San Luis block to the Córdoba terrane.

Evidence for two distinct plutonic events that constrain the timing of regional folding can be seen within the Sierra de San Luis. NNE-trending, elongate granitic plutons from central and western regions yield Ordovician crystallization ages (U/Pb zircon); structural evidence indicates emplacement prior to regional folding. A second group of mostly discordant intrusions post-dates regional folding and yields Devonian U/Pb zircon crystallization ages. The Ordovician-Devonian regional deformation is broadly correlative with, and likely related to, accretion of the Laurentia-derived Precordillera terrane onto the Gondwana margin, west of the Eastern Sierras Pampeanas. Thus, deformational and metamorphic fabrics within the Sierra de San Luis document a sequence of emplaced terranes along the western margin of Gondwana, and likely provide further evidence for the early Paleozoic interaction between Gondwana and Laurentia.