Uranium-lead dating of felsic magmatic cycles in the southern Sierras Pampeanas, Argentina: Implications for the tectonic development of the proto-Andean Gondwana margin

Regional deformation of the Sierra de San Luis, Argentina: Implications for the Paleozoic development of western Gondwana

“…Distinctive suites of Ordovician and Devonian granitoids intrude the metasedimentary rocks [Ortiz Suárez et al, 1992;Rapela et al, 1992;Stuart-Smith et al, 1999], and Tertiary volcanic rocks crop out through the center of the region [Brogioni, 1987[Brogioni, , 1990Ramos et al, 1991] (Figure 2). Metasedimentary lithologies are divided into three principal groups: a western complex of predominantly gneissic and migmatitic rocks (the ''Nogoli Metamorphic Complex'' of Sims et al [1997] and ''western basement complex'' of Von Gosen and ; see inset in Figure 3), a central region of slate-grade through sillimanite-grade pelitic and quartzo-feldspathic rocks (includes the ''San Luis Formation'' of Prozzi and Ramos [1988], ''Phyllite Group'' of Von Gosen and , ''Pringles Metamorphic Complex'' of Sims et al [1997], and ''Micaschist Group'' of Von Gosen and ; Figure 3), and an eastern region of biotite-grade through migmatitic quartzo-feldspathic schists, quartzites and minor pelites (the ''Conlara Metamorphic Complex'' of Sims et al [1997] and ''eastern basement complex'' of Von Gosen and ; Figure 3).…”

“…However, in several locations pluton margins are defined by ductile shear zones (e.g., the Rio Claro pluton and northern tip of the La Escalarilla pluton; RC, LE, Figure 2) with a foliation subparallel to the internal fabric, which suggests syntectonic crystallization. Crystallization ages obtained from this suite of pretectonic to syntectonic granitoids are constrained to Early to Middle Ordovician by U/Pb zircon geochronology from several plutons (490 ± 15 Ma to 468 ± 6 Ma [Sims et al, 1998;Stuart-Smith et al, 1999;Von Gosen et al, 2002]; Figure 5). In addition, recent U/Pb zircon ages from the northern lobe of the elongate La Escalarilla granite (507 ± 24 Ma [Von Gosen et al, 2002] and 482 ± 23 Ma (L. P. Gromet, Brown University, personal communication, 2002) suggest that this section of the La Escalarilla pluton is a member of the EarlyMiddle Ordovician suite of syntectonic granitoids.…”

“…In several locations, plutons crosscut or truncate metamorphic isograds, indicating that emplacement was postpeak metamorphism (Figure 3). Crystallization ages from the eastern, peraluminous granitoids range from 408 ± 25 Ma (U/Pb zircon [Brogioni, 1993]) for the Chacras batholith (CB, Figure 5) to 393 ± 5 Ma (U/Pb zircon [Stuart-Smith et al, 1999]) for the Renca pluton in the southeast (RP, Figure 5). A minimum age constraint on regional folding of 403 ± 6 Ma [Sims et al, 1998] is provided by a southern outcrop of the undeformed La Escalarilla pluton where it clearly intrudes folded metasedimentary country rocks (Figure 6a).…”

“…The Famatinian magmatic arc was initially recognized within the Sierra de Famatina, northwest of the Eastern Sierras Pampeanas [Aceñolaza and Toselli, 1976;Ramos, 1988] (SF, Figure 1) and consists of metaluminous to peraluminous granitic rocks [Rapela et al, 1992;Saavedra et al, 1992]. Southward extension of the arc includes the voluminous granitic rocks of the Sierras de Chepes Saavedra et al, 1998] and likely includes the elongate granitoids of the western Sierra de San Luis [Sims et al, 1997;Stuart-Smith et al, 1999] (Figure 9c). Development of the arc in the Sierras de Chepes and San Luis is constrained to the Early Ordovician by U/Pb and Rb/Sr granitoid crystallization ages of 490-468 Ma Sims et al, 1998;Stuart-Smith et al, 1999;Von Gosen et al, 2002].…”

“…Well-exposed plutonic and metamorphic rocks and zones of ductile deformation within the Eastern Sierras Pampeanas provide evidence for a succession of Early Paleozoic tectonic events, including the Cambrian Pampean Orogeny Gromet and Simpson, 1999], the Early to Middle Ordovician Famatinian Orogeny Sims et al, 1998], and possibly the Ocloyic and/or Achalian Orogenies (Late Ordovician through Middle Devonian [Sims et al, 1998;Stuart-Smith et al, 1999]). Most previous work has viewed the Sierras de Córdoba and the Sierra de San Luis as an interrelated unit throughout the early Paleozoic [Ramos, 1988;Prozzi, 1990;Von Gosen and Prozzi, 1998], and therefore subject to consistent styles of deformation and metamorphism.…”