The belt of metagabbros of La Pampa: Lower Paleozoic back-arc magmatism in south-central Argentina |
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| Institution: | 1. Instituto de Geociencias Básicas, Aplicadas y Ambientales (IGEBA, UBA-CONICET) Departamento de Ciencias Geológicas, Pabellón II, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina;2. Instituto de Geología y Recursos Minerales, Servicio Geológico Minero Argentino, Avda, General Paz 5445, Edificio 25, B1650KNA Buenos Aires, Argentina;1. Instituto de Geocronología y Geología Isotópica (INGEIS), Pabellón INGEIS, Ciudad Universitaria, C1428EHA, Buenos Aires, Argentina;2. Instituto de Ciencias Básicas, Aplicadas y Ambientales de Buenos Aires (IGEBA), Department of Geological Sciences, FCEN, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón 2, C1428EHA, Buenos Aires, Argentina;3. Geoscience Centre of the University of Göttingen (GZG), Göttingen, Germany;4. Research School of Earth Sciences, The Australian National University, Mills Road, Canberra, ACT 2600, Australia;5. Instituto de Geociências, Universidade de São Paulo (USP), Rua do Lago 562, CEP-05508-900 São Paulo, Brazil |
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| Abstract: | Combined geological, geochronological, geochemical and geophysical studies have led to identification of a large (∼300 km long, ∼5 km wide) N–S trending belt of metagabbros in the province of La Pampa, south-central Argentina. This belt, though only poorly exposed in the localities of Valle Daza and Sierra de Lonco Vaca, stands out in the geophysical data (aeromagnetics and gravity). Modeling of the aeromagnetic data permits estimation of the geometry of the belt of metagabbros and surrounding rocks.The main rock type exposed is metagabbros with relict magmatic nucleii where layering is preserved. A counterclockwise P–T evolution affected these rocks, i.e., during the Middle Ordovician the protolith reached an initial granulite facies of metamorphism (M1), evolving to amphibolite facies (M2). During the Upper Devonian, a retrograde, greenschist facies metamorphism (M3) partially affected the metagabbros.The whole-rock Sm–Nd data suggest a juvenile source from a depleted mantle, with model ages ranging from 552 to 574 Ma, and positive Epsilon values of 6.51–6.82. A crystallization age of 480 Ma is based on geological considerations, i.e. geochronological data of the host rocks as well as comparisons with the Las Aguilas mafic–ultramafic belt of Sierra de San Luis (central Argentina).The geochemical studies indicate an enriched MORB and back-arc signature.The La Pampa metagabbros are interpreted to be originated as a result of the extension that took place in a back-arc setting coevally with the Famatinian magmatic arc (very poorly exposed in the western part of the study area). The extensional event was ´aborted´ by the collision of the Cuyania terrane with Pampia-Gondwana in the Middle Ordovician, causing deformation and metamorphism throughout the arc–back-arc region.The similarities between the La Pampa metagabbros and the mafic–ultramafic Las Aguilas belt of the Sierra de San Luis are very conspicuous, for example, the age (Lower Paleozoic), geochemical signature and timing of metamorphism (dated at ca. 465 Ma in the study area), which allow definition of a single, mafic back-arc belt in central Argentina, from San Luis to La Pampa. |
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