Closing the ocean between the Precordillera terrane and Chilenia: Early Devonian ophiolite emplacement and deformation in the southwest Precordillera

Abstract

New geologic mapping, structural, and geochronologic analyses of the pre-Carboniferous ultramafic,mafic, and metasedimentary rocks exposed on the southwest margin of the Precordillera terrane, western Argentina, show that the ultramafic and mafic rocks belong to at least four distinct tectonic units, not one ophiolite pseudostratigraphy, as previously thought. One unit comprises gabbro, microgabbro, diabase, and minor plagiogranite that are interpreted as the mafic crustal section of an ophiolite pseudostratigraphy. Another unit consists of serpentinized peridotite, ultramafic cumulate, layered gabbro, and quartzofeldspathic gneiss that experienced granulite facies metamorphism in a deep continental crust environment. Other mafic rocks include highly altered basaltic flows interlayered with low-grade clastic metasedimentary rocks and diabase and microgabbro sills that intrude the metasedimentary rocks. The pre-Carboniferous rock units were juxtaposed along synmetamorphic, ductile, top-to-the-east shear zones in the Early to Middle Devonian. The intense deformational fabrics formed during the juxtaposition event were overprinted by strong Permian folding and top-to-the-west brittle thrust faulting, Late Permian to middle Mesozoic extension, and Late Tertiary west-vergent folding and top-to-the-west brittle thrust faulting. A speculative tectonic scenario for the juxtaposition of the pre-Carboniferous in the Early to Middle Devonian places the deep continental crust at the base of the upper plate (eastern Chilenia) of a west-dipping subduction zone, the mafic ophiolite crustal section in an ocean basin between the western Precordillera terrane and eastern Chilenia, and the mafic sills and flows on the extended western margin of the Precordillera terrane. Early to Middle Devonian closure of the ocean basin resulted in juxtaposition of the pre-Carboniferous rock units.