Estructuras nodulares en esquistos del Complejo Pichanas

En la región noroeste de las Sierras de Córdoba, afloran esquistos aluminosos caracterizados por la presencia poco común de nódulos ovoides de cordierita de hasta 10 cm de longitud (Figs.1a-c). Estas estructuras nodulares y de las dimensiones mencionadas no sonfrecuentes en la literatura lo que hace de este lugar un sitio de interés dentro de la geología de la provincia de Córdoba. Los esquistos nodulares fueron mencionados por Lyons et al. (1997) y descriptos en detalle por Verdecchia (2004), Baldo y Verdecchia (2004) y Martino et al. (2009). Están compuestos principalmente por cordierita (nódulos), andalucita, biotita, muscovita, plagioclasa y cuarzo. Estos nódulos de cordierita y la paragénesis mineral asociada son la principal evidencia que sustenta la existencia de un evento metamórfico de baja presión en este sector de las Sierras de Córdoba (Verdecchia y Baldo, 2004).Fil: Verdecchia, Sebastián O. Universidad Nacional de Córdoba. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Fil: Baldo, Edgardo G. Universidad Nacional de Córdoba. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Geologí

Metamorfismo de baja presión y alta temperatura en las unidades metasedimentarias ordovícicas del sector norte de la Sierra de Los Llanos, Sierras Pampeanas Orientales (Argentina)

La Sierra de Los Llanos (Sierras Pampeanas Orientales) se compone principalmente de plutones granodioríticos, considerados como la extensión norte del batolito Los Llanos-Ulapes del Ordovícico inferior (483-497 Ma, y en menor proporción por monzogranitos intruyendo a la granodiorita (Unidad Asperezas, de ca. 477 Ma;), gabros, dioritas formando mingling con la granodiorita, y granitos anatécticos de ca. 479 Ma; (Pankhurst et al., 1998, Pankhurst et al., 2000 referencias allí citadas). Dentro de esta sierra, se localizan septos de dimensiones kilométricas de rocas metasedimentarias que afloran de manera discontinua e intercalada con las unidades plutónicas.Uno de estos septos se localiza en el sector noroeste de esta sierra, en cercanías a la localidad de El Alto (Figura 1), compuesto por una sucesión metasedimentaria de esquistos micáceos y cuarzo-micáceos con andalucita-cordierita, gneises y migmatitas, con desarrollo localizado de corneanas en respuesta a la intrusión tardía de cuerpos básicos (Pascua et al., 1998; Pankhurst et al., 1998). En esta contribución se presenta la descripción de paragénesis no citadas en trabajos previos del área y un estudio preliminar de la evolución tectono-térmica y estimación de las condiciones P-T para el evento térmico principal utilizando cálculos multireaccionales.Fil: Verdecchia, Sebastián O. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Fil: Murra, Juan A. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Fil: Baldo, Edgardo G. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Fil: Colombo, Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Geologí

The continental assembly of SW Gondwana (Ediacaran to Cambrian): a synthesis

SW Gondwana resulted from complex interplay between continental amalgamation and dispersal between ~ 650 and 490 Ma. The main cratons involved were Laurentia, Amazonia– MARA (Proterozoic Maz–Arequipa–Rio Apa, Casquet et al., 2012), Kalahari, Rio de la Plata (RPC), Congo and East Antarctica (Mawson block). Several collisional orogenic belts resulted, notably the East Africa–Antarctica, Brasiliano–Panafrican, Pampean–Saldania, and Ross– Delamerian orogens. East-Antarctica broke away from the western margin of Laurentia in Rodinia. After a long drift and counter-clockwise rotation (Dalziel, 2013) it collided with Congo and Kalahari to produce the southern part of the left-lateral transpressional East Africa–Antarctica orogen between 580 and 550 Ma, completing the amalgamation of East Gondwana. The Trans-Antarctic margin became an active one in the Ediacaran and subduction of the Pacific Ocean lithosphere occurred throughout the Paleozoic, forming a tract of the Terra Australis orogen. NW–SE directed compression in Late Cryogenian and Early Ediacaran times promoted closure of the Adamastor Ocean, resulting in the left-lateral transpressional Brasiliano–Pan African orogeny between 650 and 570 Ma. The Pampean orogenic belt to the west of the RPC resulted from right-lateral collision between Laurentia and its eastern extension MARA on the one hand and Kalahari–RPC on the other. Ocean opening started at ~ 630 Ma and subduction and further collision took place between 540 and 520 Ma, coeval with the northward drift of Laurentia (~ 540 Ma) away from MARA and the consequent formation of the proto-Andean margin of Gondwana. The margins of the intervening Puncoviscana ocean were covered by Laurentia-derived siliciclastic sediments and carbonates on the MARA side between 630 and ~ 540 Ma (Rapela et al, 2014; this symposium), and by the marine siliciclastic Puncoviscana Formation on the other. The latter formation, deposited between a 570 and ~530 Ma, received input from large alluvial fans descending from juvenile Mesoproterozoic and Neproterozoic sources (new Hf isotope evidence) largely located in the southern East Africa–Antarctica orogen. The Pampean orogen extended into the Saldania–Gariep orogen of southern South Africa (545–520 Ma) and was apparently discordant to the earlier Brasiliano–Pan African orogen. In late-Early to late Cambrian times the Pampean–Saldania realm evolved into a passive margin with siliciclastic platform sedimentation. The Pampean-Saldania realm was separated from the active Trans-Antarctic margin of East Antarctica by an inferred transform fault in Ediacaran to Cambrian times. Regional NW–SW shortening in the Ediacaran became N–S directed in the Cambrian, suggesting a major plate reorganization at this time.Peer reviewe

Geocronología de las Sierras de Córdoba : revisión y comentarios

Fil: Baldo, Edgardo G.. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Centro de Investigaciones en Ciencia de La Tierra; CórdobaFil: Rapela, Carlos Washington. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Pankhurst, Robert J.. British Geological Survey; Reino UnidoFil: Galindo, Carmen. Universidad Complutense. Instituto de Geociencias. Departamento de Petrología y Geoquímica; EspañaFil: Casquet, César. Universidad Complutense. Instituto de Geociencias. Departamento de Petrología y Geoquímica; EspañaFil: Verdecchia, Sebastián O.. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Centro de Investigaciones en Ciencia de La Tierra; CórdobaFil: Murra, Juan A.. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Centro de Investigaciones en Ciencia de La Tierra; Córdob

Review of the Cambrian Pampean orogeny of Argentina; a displaced orogen formerly attached to the Saldania Belt of South Africa?

The Pampean orogeny of northern Argentina resulted from Early Cambrian oblique collision of the Paleoproterozoic-Mesoproterozoic MARA block, formerly attached to Laurentia, with the Gondwanan Kalahari and Rio de la Plata cratons. The orogen is partially preserved because it is bounded by the younger Córdoba Fault on the east and by the Los Túneles-Guacha Corral Ordovician shear zone on the west. In this review we correlate the Pampean Belt with the Saldania orogenic belt of South Africa and argue that both formed at an active continental margin fed with sediments coming mainly from the erosion of the Brasiliano–Pan-African and East African–Antarctica orogens between ca. 570 and 537 Ma (Puncoviscana Formation) and between 557 and 552 Ma (Malmesbury Group) respectively. Magmatic arcs (I-type and S-type granitoids) formed at the margin between ca. 552 and 530 Ma. Further right-lateral oblique collision of MARA between ca. 530 and 520 Ma produced a westward verging thickened belt. This involved an upper plate with high P/T metamorphism and a lower plate with high-grade intermediate to high P/T metamorphism probably resulting from crustal delamination or root foundering. The Neoproterozoic to Early Cambrian sedimentary cover of MARA that was part of the lower plate is only recognized in the high-grade domain along with a dismembered mafic–ultramafic ophiolite probably obducted in the early stages of collision. Uplift was fast in the upper plate and slower in the lower plate. Eventually the Saldania and Pampean belts detached from each other along the right-lateral Córdoba Fault, juxtaposing the Rio de la Plata craton against the internal high-grade zone of the Pampean belt

Edad U-Pb y análisis isotópico de Hf en circones del plutón Guasayán del Cámbrico temprano, Sierras Pampeanas, Argentina: implicancias para el límite noroccidental del arco pampeano

An Early Cambrian pluton, known as the Guasayán pluton, has been identified in the central area of Sierra de Guasayán, northwestern Argentina. A U-Pb zircon Concordia age of 533±4 Ma was obtained by LA-MC-ICP-MS and represents the first report of robustly dated Early Cambrian magmatism for the northwestern Sierras Pampeanas. The pluton was emplaced in low-grade metasedimentary rocks and its magmatic assemblage consists of K-feldspar (phenocrysts)+plagioclase+quartz+biotite, with zircon, apatite, ilmenite, magnetite and monazite as accessory minerals. Geochemically, the granitic rock is a metaluminous subalkaline felsic granodiorite with SiO2=69.24%, Na2O+K2O=7.08%, CaO=2.45%, Na2O/ K2O=0.71 and FeO/MgO=3.58%. Rare earth element patterns show moderate slope (LaN/YbN=8.05) with a slightly negative Eu anomalies (Eu/Eu*=0.76). We report the first in situ Hf isotopes data (εHft=-0.12 to-4.76) from crystallized zircons in the Early Cambrian granites of the Sierras Pampeanas, helping to constrain the magma source and enabling comparison with other Pampean granites. The Guasayán pluton might provide a link between Early Cambrian magmatism of the central Sierras Pampeanas and that of the Eastern Cordillera, contributing to define the western boundary of the Pampean paleo-arc.Un plutón de edad Cámbrica temprana, conocido como plutón Guasayán, ha sido identificado en el área central de la sierra de Guasayán, noroeste de Argentina. Una edad U-Pb en concordia de 533±4 Ma fue obtenida en circones mediante LA-MC-ICP-MS. Esta edad representa el primer reporte de magmatismo Cámbrico temprano para el noroeste de las Sierras Pampeanas. El mismo está emplazado en rocas metasedimentarias de bajo grado y se caracteriza por una asociación magmática de K-feldespato fenocristales)+plagioclasa+cuarzo+biotita, con circón, apatita, ilmenita, magnetita y monacita como minerales accesorios. Geoquímicamente, la roca granítica se clasifica como una granodiorita félsica metaluminosa subalcalina con contenidos de SiO2=69,24%, Na2O+K2O=7,08%, CaO=2,45% y relaciones de Na2O/K2O=0,71 y FeO/MgO=3,58%. Los patrones de elementos de tierras rara muestran una pendiente moderada (LaN/YbN=8,05) con una ligera anomalía negativa de Eu (Eu/Eu*=0,76). Nosotros reportamos los primeros datos in situ de isótopos de Hf (εHft=-0,12 a -4,76) para circones cristalizados en granitos del Cámbrico temprano de Sierras Pampeanas, lo que aporta información crítica sobre la fuente de los magmas, permitiendo la comparación con otros granitos pampeanos. El plutón Guasayán podría proveer el enlace entre el magmatismo del Cámbrico temprano del sector central de las Sierras Pampeanas y aquel de la cordillera Oriental, contribuyendio a definir el límite occidental del paleoarco Pampeano.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica

Geoquímica y procedencia de los metasedimentos ordovícicos del complejo metamórfico La Cébila, provincia de La Rioja, Argentina

The Famatinian orogenic belt of Early Palaeozoic age (Late Cambrian to Middle Ordovician) represents part of the Gondwana southwestern paleo-margin evolution within the pre-Andean basement of central-western Argentina. This belt is composed of igneous rocks with arc signature, a sedimentary succession of marine and volcaniclastic origin and low- to medium-pressure metamorphic units. One of such units is the La Cébila metamorphic complex (LCMC, Early to Middle Ordovician), which includes low- to high-grade metasedimentary successions composed mainly of phyllosilicate-rich (phyllite, mica schists and migmatites) and quartz-feldspar-rich (metapsammite, quartz-mica schists and quartzites) rock types. The current geochemical signal of metasediments from LCMC allows comparing them with at least three primary compositions referring to pelitic, psammites and quartz-psammites lithology types. This suggests that the protolith composition was not substantially altered during the metamorphic event. Low La/Th ratios (2.4�4.5; n = 7) and Hf contents (3.5�11.0; n = 7) , in addition to the absence of detrital zircon ages from the Famatinian volcanic arc (located to the west), suggest a provenance from a source area of felsic composition, consistent with the Pampean basement (Early Cambrian to Middle Cambrian) located to the east (current coordinates). The metasediments from the LCMC are geochemically equivalent to those of the Puncoviscana Formation (Late Neoproterozoic - Early Cambrian), showing minimal differences only in the content of some immobile (e.g., Zr and Ti) and mobile (e.g., K, Sr and Rb) elements. Consequently, major and trace elements are not reliable to differentiate between these two metasedimentary successions. Key words: Geochemistry, metasediments, retro-arc basin, Famatinian orogeny, La Cébila metamorphic complex, Argentina.Dentro del basamento pre-Andino del centro oeste de Argentina, la faja orogénica Famatiniana del Paleozoico temprano (Cámbrico Tardío a Ordovícico Medio) representa parte de la evolución del paleomargen suroccidental de Gondwana. Esta faja está compuesta por rocas ígneas con signatura de magmatismo de arco, sucesiones sedimentarias marinas y volcaniclásticas y unidades metamórficas de baja y media presión. Una de estas unidades corresponde al complejo metamórfico La Cébila (CMLC) del Ordovícico Temprano a Medio, el cual se compone de sucesiones metasedimentarias de bajo a alto grado metamórfico constituidas principalmente por variedades litológicas enriquecidas en filosilicatos (filitas, esquistos micáceos y migmatitas) y cuarzo-feldespáticas (metapsamitas, esquistos cuarzo-micáceos y cuarcitas). La señal geoquímica actual de los metasedimentos del CMLC permite equipararlos con al menos tres composiciones primarias referentes a tipos litológicos pelíticos, psamíticos y psamíticos cuarzosos. Esto sugiere que los procesos metamórficos sobreimpuestos a estos sedimentos no modificaron sustancialmente la composición del protolito. Las bajas relaciones de La/Th (2.4�4.5; n=7) y el contenido de Hf (3.5�11.0; n=7), sumado a la ausencia de circones detríticos de edades famatinianas provenientes del arco volcánico (localizado al oeste), sugieren una procedencia desde un área fuente de composición félsica, consistente con el basamento pampeano (Cámbrico Temprano a Cámbrico Medio) localizado al este de las coordenadas actuales. Los metasedimentos de CMLC resultan geoquímicamente equivalentes a los pertenecientes a la Formación Puncoviscana (Neoproterozoico tardío � Cámbrico Temprano), mostrando sólo escasas diferencias en el contenido de algunos elementos inmóviles (e.g., Zr y Ti) y móviles (e.g., K, Sr y Rb). En consecuencia, los elementos mayores y traza no son confiables para diferenciar estas dos sucesiones metasedimentarias

Polyphase white mica growth in low-grade metapelites from La Cébila Metamorphic Complex (Famatinian Belt, Argentina): evidence from microstructural and XRD investigations Crecimiento polifásico de mica blanca en metapelitas de bajo grado del Complejo Metamórfico La Cébila (Faja Famatiniana, Argentina): evidencias a partir de investigaciones microestructurales y de DRX

Two tectono-thermal metamorphic events, M1-D1 (S1, with associated white mica and chlorite: WM1-Chl1) and M2-D2 (S2, with development of WM2-Chl2), are established from polyphase white mica growth for low-grade units from the Ordovician metasedimentary successions of La Cébila Metamorphic Complex in the Famatinian belt (western-central Argentina). The thermobarometric characterization of the M1 main event was carried out by means of clay-mineral analysis and crystallo-chemical parameter measurements. Epizonal (temperatures between 300 and 400°C) and low-pressure conditions are suggested for M1 event, based in Kübler index values ranging from 0.23 to 0.17 A°20, white mica b parameter values between 9.004 and 9.022 Á (mean of 9.014 Á, n=16) and Si contents between 3.13-3.29 a.p.f.u. Temperatures of ~180-270°C are estimated for the M2 event, with Kübler index values ranging from 0.31 to 0.46 A°20. The M1-D1 event of La Cébila could be linked to high-strain heating tectono-metamorphic Ordovician regime recorded in others complexes from Famatinian foreland region of Sierras Pampeanas.<br>Dos eventos tectono-metamórficos fueron establecidos a partir de la blastesis superpuesta de mica blanca en sucesiones metasedimentarias ordovícicas de bajo grado del Complejo Metamórfico La Cébila, cinturón Famati-niano (centro oeste de Argentina): M1-D1 (S1, con blastesis asociada de mica blanca y clorita: WM1-Chl1) y M2-D2 (S2, con desarrollo de WM2-Chl2). La caracterización termobarométrica del evento principal M1 fue llevada a cabo a través del análisis de minerales de arcilla y de la medición de parámetros cristaloquímicos. Las condiciones de epizona con temperaturas entre 300 y 400°C, y baja presión fueron estimadas para el evento M1 sobre la base de valores de índice de Kübler de 0,23 a 0,17 A°20, parámetro b de la mica blanca entre 9,004 y 9,022 Á (valor medio de 9,014 Á, n=16) y contenidos de Si entre 3,13-3,29 a.p.f.u. Se estimaron temperaturas de ~180-270°C para el evento M2, con valores de índice de Kübler entre 0,31 y 0,46 A°20. El evento M1-D1 registrado en La Cébila podría ser vinculado al evento tectono-metamórfico ordovícico con calentamiento bajo un régimen de alta deformación ocurrido en otros complejos de la región del antepaís famatiniano de las Sierras Pampenas

Late Famatinian (440–410 Ma) overprint of Grenvillian metamorphism in Grt‐St schists from the Sierra de Maz (Argentina): Phase equilibrium modelling, geochronology, and tectonic significance

The analysis of major and trace elements in zoned minerals is useful for deciphering parts of the tectonothermal evolution of polymetamorphic tarrain. We applied this approach to the Maz Metasedimentary Series in Western Sierras Pampeanas of Argentina, where polymetamorphism resulted in the overprinting of a Grenvillian basement (the Maz Complex) during the pervasive Rinconada tectonic phase of the Famatinian orogeny. The older metamorphism (M1) is assigned to the youngest Grenvillian metamorphic event recognized in this basement at c. 1035 Ma, whereas the Rinconada metamorphism (M2) was Silurian to early Devonian, essentially between 440 and 410 Ma. The latter resulted from oceanward migration of the orogenic front relative to earlier late Cambrian to Ordovician (490–470 Ma) tectonic phases of the Famatinian orogeny. The M1 and M2 metamorphic events have been recognized in a staurolite-garnet schist from the Maz Metasedimentary Series. Most metamorphic minerals from this rock were formed during the M2 event which was of the Barrovian type (±kyanite). Part of the metamorphic P–T evolution is recorded in the complex compositional zoning of garnet porphyroblasts. Three types of garnet were identified based on texture and chemistry, including trace elements (REEs). Phase equilibrium analysis, compositional isopleth, and multi-equilibrium thermobarometry were applied in order to establish the P–T history. M1 is represented by preservation of Grt1 ± Kfs ± Sil, with peak P–T condition of 790°C and 5.2 kbar, that is, granulite facies. This early metamorphic event was related to a deformational D1 episode represented by a relict S1 foliation. The latter is preserved as aligned inclusions in staurolite porphyroblasts and as relics of an older crenulated foliation in microlithons from the matrix. M2 followed a clockwise P–T path with three mineral growth stages. The earliest occurred at ~585°C and ~8.7 kbar and is represented by Grt2 ± St1 ± Bt1 + Qz. Grt2 was partially coeval with growth of St1, which was stable at ~625°C and 9.0 kbar. Grt2 + St1 are syn-kinematic to the main S2 foliation (D2 episode). Subsequently, decompression (D3) started as St2 (+ Bt2 + Ms1 + Qz + Pl) crystallized, and garnet was partially consumed at ~612–620°C and ~7.3–7.7 kbar. St3 + Grt3 crystallized at ~608°C and ~6.8 kbar at the end of D3. Increasing P–T conditions during the earlier M2 growth stage suggest burial of the Maz Metasedimentary Series, probably linked to tectonic thickening by underthrusting (tectonic phase D2). Peak metamorphic conditions were attained during thrust stacking. The tectonic phase (D2) was responsible for the main S2 foliation, which was penetrative at all scales. The nearly isothermal decompression path is compatible with exhumation, probably resulting from extension along discrete shear zones (tectonic phase D3) that produced a mylonitic foliation S3

A Cambrian mixed carbonate–siliciclastic platform in SW Gondwana: evidence from the Western Sierras Pampeanas (Argentina) and implications for the early Paleozoic paleogeography of the proto-Andean margin

The Western Sierras Pampeanas (WSP) of Argentina record a protracted geological history from the Mesoproterozoic assembly of the Rodinia supercontinent to the early Paleozoic tectonic evolution of SW Gondwana. Two well-known orogenies took place at the proto-Andean margin of Gondwana in the Cambrian and the Ordovician, i.e., the Pampean (545–520 Ma) and Famatinian (490–440 Ma) orogenies, respectively. Between them, an extensive continental platform was developed, where mixed carbonate–siliciclastic sedimentation occurred. This platform was later involved in the Famatinian orogeny when it underwent penetrative deformation and metamorphism. The platform apparently extended from Patagonia to northwestern Argentina and the Eastern Sierras Pampeanas, and has probable equivalents in SW Africa, Peru, and Bolivia. The WSP record the outer (deepest) part of the platform, where carbonates were deposited in addition to siliciclastic sediments. Detrital zircon U–Pb SHRIMP ages from clastic metasedimentary successions and Sr-isotope compositions of marbles from the WSP suggest depositional ages between ca. 525 and 490 Ma. The detrital zircon age patterns further suggest that clastic sedimentation took place in two stages. The first was sourced mainly from re-working of the underlying Neoproterozoic metasedimentary rocks and the uplifted core of the early Cambrian Pampean orogen, without input from the Paleoproterozoic Río de la Plata craton. Sediments of the second stage resulted from the erosion of the still emerged Pampean belt and the Neoproterozoic Brasiliano orogen in the NE with some contribution from the Río de la Plata craton. An important conclusion is that the WSP basement was already part of SW Gondwana in the early Cambrian, and not part of the exotic Precordillera/Cuyania terrane, as was previously thought