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The Gondwana-South America Iapetus Margin Evolution As Recorded by Lower Paleozoic Units of Western Precordillera, Argentina: the Bonilla Complex, Uspallata

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The Gondwana-South America Iapetus Margin Evolution As Recorded by Lower Paleozoic Units of Western Precordillera, Argentina: the Bonilla Complex, Uspallata Serie Correlación Geológica, 29 (1): 21-80 TemasGREG deORI Correlación ET AL. Geológica III Tucumán, 2013 - ISSN 1514-4186 - ISSN en línea 1666-947921 The Gondwana-South America Iapetus margin evolution as recorded by Lower Paleozoic units of western Precordillera, Argentina: The Bonilla Complex, Uspallata Daniel A. GREGORI1*, Juan C. MARTINEZ2, and Leonardo BENEDINI1 Abstract: THE GONDWANA SOUTH AMERICA IAPETUS MARGIN EVOLUTION AS RECORDED BY LOWER PALEOZOIC UNITS OF WESTERN PRECORDILLERA, ARGENTINA. The Cambro-Ordovician belt of western Precordillera, that includes slightly metamorphosed sandstones and pelites with interbedded basalts and ultramafic bodies, were considered part of the allochthonous Cuyania Terrane, accreted to Gondwana South America during Ordovician times. The Bonilla Complex, which represents the southern tip of the Precordillera, is constituted of metasedimentary rocks of internal and external platform environments. Paleocurrents inferred from sedimentary structures indicate provenance from the northeast and southeast (actual coordinates). The limestones of this complex, located in the eastern part of the outcrops, suggest evolution toward a carbonate-dominated Late Neoproterozoic to Cambrian passive margin. Mafic volcanic rocks were emplaced coevally with sedimentation, whereas ultramafic rocks were later tectonically emplaced. Chemical evidence suggests that the protolith of the metasedimentary rocks was derived from an older exhumed felsic basement belonging to an upper continental crust. The most prominent population of detrital zircons (~500- 600 Ma) from the Bonilla Complex support the hypothesis that these rocks are equivalent to those of the Sierras Pampeanas and the northern Patagonia. The most proximal source of the Pampean zircons found in the Bonilla Complex is the Sierras Pampeanas, located immediately to the east (present coordinates). The Bonilla Complex was deposited in an open marine basin considerably earlier (~50 Ma) than the supposed detachment of the Cuyania Terrane from the Ouachita embayment in the Laurentia margin. It is therefore not necessary to invoke the presence of an allochthonous terrane between the Bonilla Complex and the Gondwana margin to explain the 1 Ga zircon populations. Silurian to Devonian deformation was characterized by metamorphism and imbrication within an accretionary prism, the consequence of eastward subduction in the western margin of Gondwana. Therefore, the Bonilla Complex, as well as equivalent units in western Precordillera, was originally deposited as sediments on a continental shelf at the southwestern margin of Gondwana, covering a basement that was already part of the Gondwana continent by Neoproterozoic-Cambrian times. Resumen: LA EVOLUCIÓN DEL MARGEN DEL OCEANO IAPETUS EN EL CONTINENTE GONDWANA-SUDAMERICA REGISTRADO EN UNIDADES DEL PALEOZOICO INFERIOR DE LA PRECORDILLERA DE ARGENTINA. La faja de rocas Cambro-Ordovícicas de la Precordillera occidental, que incluye areniscas ligeramente metamorfizadas y pelitas con basaltos interestratificados y cuerpos ultramáficos, fue considerado como parte del Terreno alóctono Cuyania, que se habría acrecionado a la parte Sudamericana del continente de Gondwana durante tiempos ordovícicos. El Complejo Bonilla, que representa el extremo sur de este cinturón, está constituido por rocas metasedimentarias depositadas en plataformas marinas internas y externas. Los datos de paleocorrientes deducidos a partir de estructuras sedimentarias indican procedencias del noreste y sureste, considerando coordenadas actuales. Las calizas de este complejo, situadas en la parte oriental de los afloramientos, sugieren una evolución hacia una plataforma marina dominantemente carbonática de margen pasivo durante el Neoproterozoico tardío al Cámbrico. Las rocas subvolcánicas máficas fueron emplazadas coetáneamente con la sedimentación, mientras que rocas ultramáficas fueron emplazadas tectónicamente más tarde. Los datos químicos sugieren que los protolitos de las rocas metasedimentarias se derivaron de un antiguo basamento félsico exhumado perteneciente a una corteza continental superior. La población más prominente de circones detríticos (~ 500-600 Ma) del Complejo Bonilla 1) INGEOSUR, Cátedra de Geología Argentina, Universidad Nacional del Sur and CONICET, San Juan 670, 8000 Bahía Blanca, Argentina. 2) INGEOSUR, Cátedra de Petrología, Universidad Nacional del Sur and CONICET, San Juan 670, 8000 Bahía Blanca, Argentina. 22 THE GONDWANA-SOUTH AMERICA IAPETUS MARGIN apoyan la hipótesis de que estas rocas son equivalentes a las de las Sierras Pampeanas y de la Patagonia Norte. La fuente más próxima de los circones de edad Pampeana que se encuentran en el Complejo Bonilla son las Sierras Pampeanas, situadas inmediatamente al este (coordenadas actuales). El Complejo Bonilla fue depositado en una cuenca marina abierta, considerablemente antes (~ 50 Ma) que la supuesta separación del Terreno Cuyania de la bahía Ouachita en el margen de Laurencia. Por lo tanto, no es necesario invocar la presencia de un Terreno alóctono entre el Complejo Bonilla y el margen de Gondwana para explicar las poblaciones de circones de 1 Ga. La deformación Silúrica-Devónica se caracterizó por metamorfismo y deformación dentro de un prisma de acreción, consecuente con procesos de subducción dirigidos hacia el este (siempre coordenadas actuales) en la margen occidental de Gondwana. Por lo tanto, el Complejo Bonilla, así como otras unidades equivalentes en Precordillera occidental, fueron originalmente depositadas como sedimentos sobre una plataforma marina en el margen suroeste de Gondwana, cubriendo un basamento que ya era parte del continente de Gondwana en el Neoproterozoico- Cámbrico. Keywords: Bonilla Complex, passive margin, western Gondwana, Gondwana provenance, Argentina. Palabras clave: Complejo Bonilla, margen pasivo, Gondwana occidental, procedencia gondwánica, Argentina. Introduction The Grenville orogeny (1.1–0.9 Ga) was produced by collisional events during the amalgamation of Rodinia (Murphy et al., 2010). The disconnection of Laurentia from western Gondwana at ca. 570 Ma (Cawood et al., 2001) is documented by the generation of rift structures and related magmatism formed along the Laurentian margin of Iapetus. The Neoproterozoic Brasiliano-Pan African cycle (850 - 500 Ma) describes the assembly of western Gondwana from the breakup of the Rodinia supercontinent to the closure of all oceanic basins, over a series of subduction and collisional events, up to its final agglutination. According to Unrug (1992, 1994) the paleomagnetic data, and age of suturing belts, permits distinction between eastern and western Gondwana with different Neoproterozoic histories. The former comprised the Kalahari-Grunehogna craton of Africa and East Antarctica, Madagascar, India, Antarctica, and Australia. Western Gondwana included the Archean- Paleoproterozoic East Sahara-Nile, West African, Amazonian, and Río de la Plata cratons. West of the Río de la Plata Craton, in the central part of Argentina, four geological regions appear which are predominantly composed of low to high-degree metamorphic and sedimentary rocks of the Neoproterozoic to Paleozoic age (Fig. 1). The easternmost area, known as Sierras Pampeanas, includes low and high-degree metamorphic rocks, as well as limestones and marbles. Radiometric dating by Varela and Dalla Salda (1992), Ramos et al. (1996), Varela et al. (1996), Pankhurst and Rapela (1998), and Söllner et al. (2000) identify ages between 500 and 600 Ma. The limestones and marbles form extensive outcrops in the Sierras Pampeanas de Córdoba, San Luis, San Juan, Catamarca, Tucuman and Salta Provinces in an area 1000 km long in the north-south direction (24° to 33° 15´S) and 360 km wide in the east-west direction (64° 18´to 68° 06´W). The calcareous rocks are interbedded in the Ediacaran-Cambrian metasedimentary rocks, assigned to the Puncoviscana and equivalent formations (Aceñolaza and Toselli, 1981). The age is strongly documented by means of detrital zircons and fossil remains. Outcrops of rocks ~1Ga were recognized in the Sierra de Pie de Palo and Sierra de Umango in the Western Sierras Pampeanas (Varela and Dalla Salda, 1992; Varela et al., 1996, 2011). Westwards of the Sierras Pampeanas is the Precordillera mountain range and its southern extension, the San Rafael Block. The range extend between 28º to 36º 30´ S, and is mainly characterized by Lower Paleozoic marine limestones. Grenvillian ages have been reported GREGORI ET AL. 23 Figure 1. Simplified map of Precordillera displaying the distribution of the Neoproterozoic-Ordovician carbonate marine platforms and the siliciclastic low-degree metamorphic facies. Location of the studied area (Fig. 2) is also shown. Cordillera Frontal, Sierras Pampeanas, northern Patagonia and localization of Puncoviscana Formation are indicated apart. Figura 1. Mapa simplificado de Precordillera mostrando la distribución de las plataformas marinas carbonáticas del Neoproterozoico-Ordovícico y la facies metamórficas de bajo grado siliciclásticas. También se muestra la ubicación de la zona estudiada (Fig. 2). La localización de la Cordillera Frontal, Sierras Pampeanas, norte de Patagonia y de la Formación Puncoviscana figuran aparte. 24 THE GONDWANA-SOUTH AMERICA IAPETUS MARGIN for detrital zircons in Devonian sandstone of the Precordillera (Loske, 1992). Scarce Tertiary volcanic rocks (Leveratto, 1968) carry basement xenoliths that were dated by Abbruzzi et al. (1993) and Kay et al. (1996) at 1188 ± 122
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