Paleozoic deformation and metamorphism of the Sierra de San Luis, central Argentina: Evidence for the tectonic development of western Gondwana.

摘要

The western margin of the Gondwana supercontinent was substantially enlarged during the Paleozoic Era by multiple accretionary events. Of particular importance was the accretion of the Precordillera terrane, considered by many to have rifted from the Texas embayment region of the Laurentian continent. An improved understanding of the tectonic interrelationship between Gondwana and Laurentia is critical for constraining crustal development and global plate reconstructions. Accordingly, this study focuses on the Sierra de San Luis and the Sierras de Cordoba of central Argentina, where evidence of Paleozoic convergence along the western margin of Gondwana is well-exposed. Structural, petrologic and geochronologic analyses of metasedimentary and plutonic rocks of the Sierra de San Luis reveal a long sequence of Early to Middle Paleozoic deformation and metamorphism. NeoProterozoic- to Cambrian-aged metasedimentary rocks preserve an early pressure solution cleavage, which may be correlative with similar fabrics in the northern Sierras de Cordoba. Ordovician plutons in the Sierra de San Luis represent the southern extension of the Famatinian magmatic arc and produced peak metamorphism across the San Luis region. This strong Ordovician signature is lacking within the Sierras de Cordoba and indicates that juxtaposition of these two terranes could not have occurred prior to the Late Ordovician. Regional folding of the San Luis terrane produced NNE-trending bands of metasedimentary rocks that range from greenschist-facies in synformal cores through amphibolite-facies in antiformal cores. Extensive, NNE-trending ductile deformation zones truncate the fold pattern along decameters-thick zones in the western Sierra de San Luis, and along the kilometers-thick Tres Arboles fault zone on the western margin of the Sierras de Cordoba. Detailed analyses of these zones document early amphibolite-facies, east-over-west oblique movement, which is equated with the suture of the Precordillera terrane to the Gondwana margin, west of the Sierra de San Luis. Local reactivation of ductile faults produced fine-grained mylonitic and ultramylonitic fabrics, primarily through reaction-enhanced, greenschist-facies deformation. 40Ar/39Ar geochronology of reactivation fabrics indicates Middle Devonian to Early Carboniferous movement, which facilitated the final juxtaposition of the Sierra de San Luis against the Sierras de Cordoba during the culmination of Paleozoic convergence along the western margin of Gondwana.