Lower Paleozoic Orogenies at the proto-andean margin of South America, Sierras Pampeanas, Argentina

El margen proto-andino de Gondwana ha sido el escenario de al menos dos orogénesis desde el desmembramiento del supercontinente Rodinia al final del Neoprotrozoico, hasta el reagrupamiento de las masas continentales en Pangea al final del Carbonifero. Ambas orogénesis van precedidas de un periodo de apertura oceánica y sedimentación en márgenes pasivos y culminan en subducción oceánica con desarrollo de arcos-magmáticos de tipo cordillerano y colision de tipo continente-continente. La primera, orogénesis Pampeana, tiene lugar en el Cámbrico, en un intervalo de tiempo relativamente pequeño (535-520 Ma: etapas de subducción-arco magmático y colisión), y culmina con la acreción ortogonal de un pequeño terreno continental (terreno Pampeano) de naturaleza semiautóctona. Por el contrario, la orogénesis Famatiniana, tiene lugar en un periodo de tiempo más dilatado, durante el Ordovícico y Silúrico (499-435 Ma). Durante esta orogénesis tuvo lugar la acreción de un terreno exótico a Gondwana, el terreno Precordillera (460 Ma). Este terreno está constituido por un basamento grenvilliano (aprox. 1.1Ga) y una cubierta sedimentaria de plataforma carbonatada de edad Cámbrico-Ordovícico. La acreción al margen de Gondwana fue probablemente oblicua, y el margen oriental del terreno Precordillera fue afectado por fuerte deformación y metamorfismo regional. El basamento de los cinturones andinos del Paleozoico Superior y Mesozoico situados al oeste de la Precordillera, parece estar constituido también por rocas metamórficas grenvillianas; con lo cual, gran parte de los Andes centrales entre los 26ºS y 34ºS se encuentra asentado sobre terrenos alóctonos. En cualquier caso, la paleogeografía de las masas continentales involucradas en la colisión de los terrenos exóticos durante el Paleozoico Inferior no se conoce bien todavía.The proto-Andean margin of Gondwana was the site of at least two orogenies between the break-up of the Rodinia supercontinent, at the end of the Neoproterozoic, and the new continental amalgamation of Pangea at the end of the Carboniferous. Both orogenies were preceded by a period of ocean opening and passive margin sedimentation and ended with ocean subduction, development of cordilleran-type magmatic arcs and continent-continent collision. The Pampean orogeny took place in a relatively short period of time in the early Cambrian (535- 520 Ma; subduction – magmatic arc stage and continental collision), and ended with the orthogonal accretion of a semiautochthonous microcontinental fragment - the Pampean Terrane- to the Gondwana margin. On the other hand, the Famatinian orogeny spanned a longer period of time in the Ordovician and Silurian (499-435 Ma). During this orogeny the exotic Precordillera Terrane was accreted to the margin of Gondwana (460 Ma). This terrane, probably of Laurentian provenance, consists of a Grenvillian basement (1.1 Ga) and a Lower Paleozoic sedimentary cover of the carbonate platform type. Accretion was probably oblique to the Gondwana margin, and the eastern margin of the Precordillera Terrane was thoroughly affected by Famatinian deformation and regional metamorphism. The basement to the Upper Paleozoic and Mesozoic Andean belts, situated to the west of the Precordillera, also appears to be formed by Grenvillian metamorphic rocks, so that the greater part of the Central Andes between 26ºS and 34ºS are established upon allocthonous terranes. The Lower Paleozoic paleogeography of continental masses involved in the collision is not still fully understood.Centro de Investigaciones Geológica

A history of Proterozoic terranes in southern South America : from Rodinia to Gondwana

The role played by Paleoproterozoic cratons in southern South America from the Mesoproterozoic to the Early Cambrian is reconsidered here. This period involved protracted continental amalgamation that led to formation of the supercontinent Rodinia, followed by Neoproterozoic continental break-up, with the consequent opening of Clymene and Iapetus oceans, and finally continental re-assembly as Gondwana through complex oblique collisions in the Late Neoproterozoic to Early Cambrian. The evidence for this is based mainly on a combination of precise U-Pb SHRMP dating and radiogenic isotope data for igneous and metamorphic rocks from a large area extending from the Rio de la Plata craton in the east to the Argentine Precordillera in the west and as far north as Arequipa in Peru. Our interpretation of the paleogeographical and geodynamic evolution invokes a hypothetical Paleoproterozoic block (MARA) embracing basement ultimately older than 1.7 Ga in the Western Sierras Pampeanas (Argentina), the Arequipa block (Peru), the Rio Apa block (Brazil), and probably also the Paraguaia block (Bolivia)

Sr, C and O isotope composition of marbles from the Sierra de Ancasti, Eastern Sierras Pampeanas, Argentina : age and constraints for the Neoproterozoic-Lower Paleozoic evolution of the proto-Gondwana margin

The Sierra Brava Complex on the eastern flank of the Sierra de Ancasti consists of marbles, metabasites, calc-silicate rocks, psammo-pelitic schists and gneisses. In the central part of this sierra a thick succession of banded schists (Ancasti Formation) crops out. Regional metamorphism of these rocks is attributed to the Famatinian orogeny (Ordovician), metamorphic grade increasing westwards and southwards and culminating in a migmatite complex on the western side of the Sierra. The meta-carbonate rocks are subdivided into a northeastern group (low-grade calcite marbles), and a southeastern group (high-grade calcite and calcite-dolomite marbles). Twenty-three marble samples were analysed for Sr isotope composition and Rb, Mn, Mg and Ca contents, and six for C and O isotope composition. An Ediacaran depositional age of 570 –590Ma is inferred by reference to the trend of 87Sr/86Sr in Neoproterozoic seawater. Thus the metacarbonates are probably slightly older than the Ancasti Formation (equivalent to the Puncoviscana Formation of northern Argentina), which has a maximum sedimentation age of ca. 570Ma. Ediacaran depositional ages have also been reported for metacarbonates elsewhere in Argentina, Uruguay and Brazil. We propose that the Sierra de Ancasti carbonates on one hand, and those in the Western Sierras Pampeanas (Difunta Correa Sequence) and –tentativelythe Corumbá Group of Brazil on the other, represent platforms on opposite margins of the extinct Clymene Ocean, whereas Neoproterozoic carbonate successions such as the Loma Negra Formation (Tandilia, southern Argentina) and the Arroyo del Soldado Group (Uruguay) were deposited on the eastern side (present coordinates) of the Rio de la Plata craton, which at the time occupied a position farther to the north

New SHRIMP U-Pb data from the Famatina Complex : constraining Early-Mid Ordovician Famatinian magmatism in the Sierras Pampeanas, Argentina

New SHRIMP U-Pb zircon ages are reported for igneous and sedimentary rocks of the Famatina Complex, constraining the age of the magmatism and the ensialic basins. Together with whole-rock and isotope geochemistry for the igneous rocks from the complex, these ages indicate that the voluminous parental magmas of metaluminous composition were derived by partial melting of an older lithosphere without significant asthenospheric contribution. This magmatism was initiated in the Early Ordovician (481 Ma). During the Mid-Late Ordovician, the magmatism ceased (463 Ma), resulting in a short-lived (no more than ~20 Ma) and relatively narrow (~100-150 km) magmatic belt, in contrast to the long-lived cordilleran magmatism of the Andes. The exhumation rate of the Famatina Complex was considerably high and the erosional stripping and deposition of Ordovician sediments occurred soon after of the emplacement of the igneous source rocks during the Early to mid-Ordovician. During the upper Mid Ordovician the clastic contribution was mainly derived from plutonic rocks. Magmatism was completely extinguished in the Mid Ordovician and the sedimentary basins closed in the early Late Ordovician

The San Blas Pluton: An example of Carboniferous plutonism in the Sierras Pampeanas, Argentina

New geochronological and geochemical data are reported for the San Blas Pluton (SBP), in the northwestern Sierra de Velasco, Sierras Pampeanas, which intrudes Ordovician granitoids developed during the Famatinian orogeny. A precise Carboniferous age of 340±3 Ma is established by U–Pb dating of zircon using a sensitive high-resolution ion microprobe (SHRIMP). The SBP illustrates several petrological and geochemical characteristics of previously reported Carboniferous granitoids in the Sierras Pampeanas. Their generation is consistent with a regional reheating of the crust at approximately 342 Ma, which resulted in the formation of relatively large amounts of granitic melts that were emplaced in higher crustal levels along master fractures (older master shear zones of Lower Paleozoic age). The SBP can be chemically defined as a typical A-type granitoid related to postcollisonal or postorogenic magmatism. Its high REE content and extraordinarily high U and Th concentrations may have economic significance. Many previously published Devonian and Carboniferous K–Ar dates are reset Ordovician ages, but the existence of other Carboniferous bodies in the Sierra de Velasco cannot be discounted until detailed mapping of the whole Sierra is completed

The evolution of a mid-crustal thermal aureole at Cerro Toro, Sierra de Famatina, NW Argentina

A more than 12 km wide sheeted tonalite complex in western Sierra de Famatina, NW Argentina, was emplaced at middle crust levels (ca. 5 kbar), coeval with regional metamorphism during an early phase of the Ordovician Famatinian orogeny (ca. 480 Ma). Advective heat from the tonalite complex caused a rise in the host regional temperatures (≤ 700 °C) by a maximum of ca. 100 °C, developing an aureole (~ 3 km wide) parallel to the igneous contact. This was accompanied by significant melting (ca. 40%) of the host rocks that hybridized to a variable extent with the tonalitic magmas. Three metamorphic zones were distinguished in a cross-section through the aureole: (1) an external zone consisting of metatexitic gneisses, amphibolites and minor tonalites, (2) an intermediate zone formed by screens of highly melted gneisses, amphibolites and metagabbros lying between tonalite and newly formed leucogranitoid and hybrid rock sheets, and (3) an internal zone formed almost exclusively of massive tonalite and minor hybrid rocks. Incongruent melting of biotite in gneisses of the intermediate zone produced peritectic cordierite and garnet. Hybrids resulting from variable mixing of anatectic granitoids and tonalite magma developed in the innermost part of the aureole at 750–800 °C. Increased water activity within this zone eventually promoted increased melting of plagioclase + quartz in the gneisses. Leucogranitoid magmas formed in part by extraction from the hybrid magmas led to heterogeneity of the Sr-isotope composition. The Cerro Toro contact aureole shows that assimilation of metasedimentary rocks through partial melting can play an important role during emplacement of tonalitic magmas at mid-crustal level

New SHRIMP U-Pb data from the Famatina Complex : constraining Early-Mid Ordovician Famatinian magmatism in the Sierras Pampeanas, Argentina

New SHRIMP U-Pb zircon ages are reported for igneous and sedimentary rocks of the Famatina Complex, constraining the age of the magmatism and the ensialic basins. Together with whole-rock and isotope geochemistry for the igneous rocks from the complex, these ages indicate that the voluminous parental magmas of metaluminous composition were derived by partial melting of an older lithosphere without significant asthenospheric contribution. This magmatism was initiated in the Early Ordovician (481 Ma). During the Mid-Late Ordovician, the magmatism ceased (463 Ma), resulting in a short-lived (no more than ~20 Ma) and relatively narrow (~100-150 km) magmatic belt, in contrast to the long-lived cordilleran magmatism of the Andes. The exhumation rate of the Famatina Complex was considerably high and the erosional stripping and deposition of Ordovician sediments occurred soon after of the emplacement of the igneous source rocks during the Early to mid-Ordovician. During the upper Mid Ordovician the clastic contribution was mainly derived from plutonic rocks. Magmatism was completely extinguished in the Mid Ordovician and the sedimentary basins closed in the early Late Ordovician