Geoscience Frontiers 5 (2014) 43e52 Contents lists available at SciVerse ScienceDirect China University of Geosciences (Beijing) Geoscience Frontiers journal homepage: www.elsevier.com/locate/gsf Research paper The Rhyacian El Cortijo suture zone: Aeromagnetic signature and insights for the geodynamic evolution of the southwestern Rio de la Plata craton, Argentina Carlos J. Chernicoff a,b,*, Eduardo O. Zappettini b, Javier Peroni b a Council for Scientific and Technical Research (CONICET), Argentina b Argentine Geological-Mining Survey (SEGEMAR), Argentina article info abstract Article history: The amalgamation of the southern Río de la Plata craton involves two possibly coeval Rhyacian sutures Received 21 September 2012 associated with the Transamazonian orogeny, rather than a single one as previously envisaged, i.e. the El Received in revised form Cortijo suture zone and the Salado suture. We circumscribe the Tandilia terrane to the region between 20 March 2013 these two sutures. Accepted 15 April 2013 The El Cortijo suture zone runs along a roughly WNW oriented magnetic low aligned along the Available online 7 May 2013 southern boundary of the Tandilia terrane, i.e. boundary between the Tandilia and Balcarce terranes. This extensive magnetic low, ca. 300 km long, and ca. 90 km wide, would be caused by demagnetization Keywords: El Cortijo suture zone associated with shearing. At a more local scale, the trend of the El Cortijo suture zone often turns e Rhyacian toward the E W. At this scale, WNW trending tholeiitic dykes of Statherian age are seen to cut the Río de la Plata craton Rhyacian El Cortijo suture zone. Spatially associated with the El Cortijo suture zone, there are small Aeromagnetics magnetic highs interpreted to be related to unexposed basic bodies of ophiolitic nature related to those forming part of the El Cortijo Formation. We envisage the pre-Neoproterozoic evolution of the Tandilia belt to have been initiated by the extension of Neoarchean (w2650 Ma) crust occurred during Siderian times (2500e2300 Ma), causing the separation between the Balcarce, Tandilia and Buenos Aires terranes, and the development of narrow oceans at both north and south sides of the Tandilia terrane, accompanied by w2300e2200 Ma sedi- mentation over transitional econtinental to oceanice crust, and arc magmatism developed in the Tan- dilia terrane. The island arc represented by the El Cortijo Formation was also developed at this time. At late Rhyacian times, it occurred in both the closure of the narrow oceans developed previously, the entrapment of the El Cortijo island arc, as well as anatectic magmatism in the Balcarce terrane. Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. 1. Introduction Pankhurst et al., 2003; Cingolani, 2010, and references therein) to Paleoproterozoic (e.g. Cordani et al., 2000; Santos et al., 2003; Dalla The Rio de la Plata craton of southern South America encom- Salda et al., 2005; Rapela et al., 2007, and references therein) passes autochthonous Archean (e.g. Hartmann et al., 2002; Gondwana basement. Its type outcrop areas occur near the Atlantic margin of the continent, mainly in Uruguay, and also in the Buenos Aires Province of Argentina (Tandilia belt). A very small outcrop of * Corresponding author. Council for Scientific and Technical Research (CONICET), Argentina. Tel.: þ54 11 47748624. the craton also occurs in the Martín García island located in the Rio E-mail addresses: [email protected], [email protected] de la Plata estuary (see Fig. 1). (C.J. Chernicoff). Based on geological evidence, it has been proposed that the Tandilia belt comprises two main terranes eBuenos Aires and Peer-review under responsibility of China University of Geosciences (Beijing) Tandilia terranese amalgamated at ca. 2.1 Ga (Transamazonian event; Rhyacian; Ramos et al., 1990; Ramos, 1999). The boundary between the Tandilia terrane and the Buenos Aires terrane would run along the Salado suture indicated by Pángaro and Ramos Production and hosting by Elsevier (2012), as delineated by a curvilinear, roughly WNW trending 1674-9871/$ e see front matter Ó 2013, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gsf.2013.04.004 44 C.J. Chernicoff et al. / Geoscience Frontiers 5 (2014) 43e52 Figure 1. (A) Tectonostratigraphic terranes of the broader study region, with key structural lineaments, modified after Ghidella et al. (2001), Chernicoff and Zappettini (2004) and Pángaro and Ramos (2012). Salado basin after Pángaro and Ramos (2012), Claromecó and Colorado basins after Introcaso et al. (2008). (B) Magnetic map of the broader study region combines: (1) grid of total magnetic field of the province of Buenos Aires (SEGEMAR, 2012), and (2) global magnetic grid (EMAG2) (grids 1 and 2, separated by short white dashes). (C) Analytic signal C.J. Chernicoff et al. / Geoscience Frontiers 5 (2014) 43e52 45 Bouguer anomaly high associated with the Salado Mesozoic model ages of ca. 2.4e2.7 Ga of granitoids, reported by Hartmann episutural basin, in turn covered by Quaternary sediments. No et al. (2002) and Pankhurst et al. (2003). These Hf and Nd iso- actual basement exposures of the Tandilia or Buenos Aires terranes topic data unequivocally point to the occurrence of Neoarchean occur near the Salado suture. crust in the southwestern portion of the Rio de la Plata craton. About 200 km to the south of the Salado suture, the occurrence The early magmatic arc plutonic rocks would have been largely of an assemblage of metacherts, metawackes and metabasites of juvenile (mantle-derived), as indicated by the initial 87Sr/86Sr ratios Rhyacian age (El Cortijo Formation) near the city of Tandil has been between 0.702 and 0.706 of the tonalitic-granitic suite, also proposed to represent remnants of oceanic crust obducted during a consistent with the preliminary Hf isotope determinations referred collisional event (Teruggi et al., 1988; Ramos et al., 1990; Ramos, to above, in contrast with the late leucogranitic suite that yielded a 1999). high initial 87Sr/86Sr value of 0.7181 (Varela et al., 1988), reflecting In this article we analyze the geological, geochronological and large crustal recycling. geophysical information available for the Tandil area, reinterpreting In contrast to previous views, Massonne et al. (2012) have it as forming part of a larger suture zone eEl Cortijo suture zonee recently invoked the occurrence of underthrusting of oceanic crust coeval with the Salado suture. We herein present the aeromagnetic without eclogitization, as opposed to subduction leading to conti- data, plus local scale 3D modeling, that have guided us to delineate nental collision, for the Paleoproterozoic peak pressure meta- the location and extent of the El Cortijo suture zone. As part of the morphism in the Balcarce area. According to the latter authors, the broader revision of all the available geological information, we have heating generated by this process at mid crustal levels would have also reinterpreted the pre-Neoproterozoic evolution of the south- caused widespread anatexis at 2.07 Ga. western portion of the Rio de la Plata craton. The Buenos Aires Complex is highly deformed, and it is char- acterized by the occurrence of wide mylonitic belts developed in a 2. Geological framework transpressive tectonic setting (Dalla Salda, 1981a,b; Teruggi et al., 1988; Ramos, 1999); the mylonitic belts would have been formed The Precambrian basement exposed in the Tandilia region in connection with a collisional event. This dynamic event reached comprises an igneous-metamorphic assemblage referred to as amphibolite facies and locally the granulite facies, though some the Buenos Aires Complex assigned to the Transamazonian mylonitic rocks were partially retrograded to greenschist facies cycle ePaleoproterozoic agee (Marchese and Di Paola, 1975; Dalla (Frisicale et al., 2012). The predominant vergence of the contrac- Salda, 1999; Cingolani and Dalla Salda, 2000; Dalla Salda et al., tional component of the belts is toward the north-northwest. In 2006; Bossi and Cingolani, 2009; Cingolani, 2010, and references this context, the oceanic rocks of the El Cortijo Formation (Teruggi therein), covered by thin Neoproterozoic to lower Paleozozic et al., 1988) would be related to the partial obduction of a suture sedimentary units displaying sub-horizontal bedding (e.g. Iñiguez zone. Rodríguez, 1999; Ramos, 1999; Cingolani and Dalla Salda, 2000; The record of multiple events of deformation and intrusive Cingolani, 2010, and references therein). magmatism in Tandilia has led to suggest that the Transamazonian The Buenos Aires Complex mostly consists of gneisses, migma- orogeny involved continentecontinent collision (tonalitic-granitic tites, amphibolites and minor ultramafic rocks, intruded by tona- plutons) as well as post-collisional generation of leucogranites plus litic/granitic and leuco-monzogranitic plutons. There are also conspicuous transcurreent faulting (e.g. Dalla Salda et al., 2006; subordinate amounts of schists, marbles, metavolcanic units, and Cingolani, 2010, and references therein). dykes of acid and mafic compositions. Particular attention is herein The only available geochronological data for the highly radio- given to the presence of an association of low-grade metamorphic genic and typically post-collisional leucogranites in Tandilia come rocks comprised