Crystallochemical indexes and geothermobarometric calculations as a multiproxy approach to P-T condition of the low-grade metamorphism: The case of the San Luis Formation, Eastern Sierras Pampeanas of Argentina

In this work, crystallochemical indexes and geothermobarometric calculations are used together to estimate the P-T condition in low-grade metamorphic units of the San Luis Formation from Sierra de San Luis (Eastern Sierras Pampeanas of Argentina). Phyllites and slates from this formation record at least two episodes of white mica (Wm) and chlorite (Chl) growth associated with main D1-S1 (Wm2-Chl1) and local D2-S2 (Wm3-Chl2) stages. Mean KICIS values of 0.30 (Western Belt) and 0.23 Δ°2θ (Eastern Belt) were determined for the <2 μm fraction. Moreover, since the average grain size of the Wm2 exceeds 10 μm, KICIS values were also measured for the 10–20 μm fraction ranging between 0.22 and 0.24 Δ°2θ for both belts. For those fractions, the KICIS values indicate epizone conditions, associated with temperatures higher than 300 °C. These values are comparable to temperatures obtained from chlorite thermometers, 316°–416 °C in Western Belt and 261°-403 °C in the Eastern Belt. Pressure values for D1 stage constrained from chlorite – white mica – quartz – H2O equilibrium are 2.5–5.1 kbar in the Western Belt, and at 3.3–4.7 kbar in Eastern Belt. The pressures are consistent with white mica b parameter values measured in samples from Western Belt (9.006 to 9.025 Å) and Eastern Belt (9.003 to 9.020 Å), linked to intermediate thermal gradients (25–35 °C/km). The identification of increasing P-T condition paths in low-grade metasedimentary successions is not easy in the absence of higher grade rocks; in these situations, a systematic sampling and analysis of dozens of samples must be carried out. The very good correlation between crystallochemical index values (Kübler index and white mica b parameter) and thermobarometric calculations (empirical and semi-empirical chlorite thermometers and thermobarometry for chlorite – white mica – quartz – H2O equilibrium) presented in this work argues in favor of the development of a multiproxy study strategy in this type of low-grade metasedimentary successions that involve: (1) a systematic sampling of metapelite levels for X-ray diffraction study (mineralogical and crystallochemical indexes with a rapid implementation allowing a dense systematic sampling); (2) the recognition of variations in the P-T conditions from KI and b parameter values and the consequent selection of metapelitic samples for analyzing mineral chemistry and thermobarometric calculations. In order to facilitate these calculations, a user-friendly spreadsheet for crystallochemical indexes and chlorite thermometry calculations is also provided.Fil: Verdecchia, Sebastián Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Collo, Gilda. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Zandomeni, Priscila Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Wunderlin, Cecilia Ayelén. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Fehrmann, Mariano. Universidad Nacional de Córdoba; Facultad de Ciencias Exactas, Físicas y Naturales; Argentin

Mid- to Late Cambrian docking of the Río de la Plata craton to southwestern Gondwana: Age constraints from U-Pb SHRIMP detrital zircon ages from Sierras de Ambato and Velasco (Sierras Pampeanas, Argentina)

The Early Palaeozoic stratigraphy and tectonic history of the Eastern Sierras Pampeanas of central Argentina are complicated by metamorphism and deformation resulting from the Pampean (545-510 Ma) and Famatinian (490-440 Ma) orogenies. We report U-Pb sensitive high-resolution ion microprobe dating of detrital zircons in two metasedimentary successions exposed at Quebrada de La Cébila (c. 28°45'S, 66°25'W): the Ambato and the La Cébila metamorphic complexes. The Ambato zircons record age peaks corresponding to Pampean (530 ± 10 Ma), Brasiliano (c. 570 and c. 640 Ma), Grenville (c. 950 to c. 1025 Ma) and minor Neoarchaean ages. Similar peaks are also apparent in the La Cébila sample but it additionally contains Palaeoproterozoic zircons (c. 2.1 Ga) corresponding to the age of the Rio de la Plata craton, from which they are considered to have been sourced. Our interpretation is that the protolith of the Ambato complex was deposited prior to juxtaposition with the craton and is older than the Early Ordovician La Cébila metamorphic complex. We infer that the craton reached its current relative position in the Mid- to Late Cambrian, after the main Pampean tectonothermal event (530-520 Ma) and before deposition of the La Cébila protolith and the Achavil Formation (Sierra de Famatina), which contain comparable detrital zircon populations.Centro de Investigaciones Geológica

Early Cambrian U-Pb zircon age and Hf-isotope data from the Guasayán pluton, Sierras Pampeanas, Argentina: implications for the northwestern boundary of the Pampean arc

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.Fil: Dahlquist, Juan Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Verdecchia, Sebastián Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Baldo, Edgardo Gaspar Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Basei, Miguel A. S.. Universidade Do Brasilia. Instituto de Geociencias; BrasilFil: Alasino, Pablo Horacio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Secretaría de Industria y Minería. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Provincia de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; ArgentinaFil: Uran, Gimena Mariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Secretaría de Industria y Minería. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Provincia de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; ArgentinaFil: Rapela, Carlos Washington. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: da Costa Campos Neto, Mario. Universidade de Sao Paulo; BrasilFil: Zandomeni, Priscila Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentin

El Escorial calcitic marbles (Cushamen Metamorphic Complex, Macizo Norpatagónico): 87Sr/86Sr isotopic characterization and age of sedimentation

Los mármoles de El Escorial (Complejo Metamórfico Cushamen) se encuentran asociados a anfibolitas que forman septos metamórficos dentro de los granitoides pérmicos de la Formación Mamil Choique (261-286 Ma). El metamorfismo de los septos, determinado en gneises migmáticos en facies de granulita aflorantes 120 km al suroeste de El Escorial, ocurre a los 311±27 Ma (método CHIME en monacita). Los mármoles son de composición calcítica (calcita >95%, R.I.: 0,5 a 2,5%) y presentan razones de 87Sr/86Sr entre 0,70768 y 0,70825 (n=10). Los datos aportados en este trabajo, sumado a contribuciones previas, permiten acotar la edad de sedimentación de las sucesiones silícico-carbonáticas del Complejo Metamórfico Cushamen entre los ca. 385 y 335 Ma. Esto sugiere la existencia de una plataforma mixta (carbonática-siliciclástica) al menos en la porción más austral del suroeste de Gondwana entre el Devónico Medio y el Carbonífero inferior (Misisipiano Medio).The El Escorial marbles (Cushamen Metamorphic Complex) along with amphibolites form metamorphic septa within the permian granitoids of the Mamil Choique Formation (261-286 Ma). The metamorphism, determined in granulite facies migmatic gneisses septa cropping out 120 km southwest of El Escorial, occurs at 311±27 Ma (CHIME method in monazite). The marbles are calcitic (calcite >95%, R.I.: 0.5 to 2.5%) and show87 Sr/86 Sr ratios between 0.70768 and 0.70825 (n=10). The data provided in this work, added to previous contributions, allow to constraints the sedimentation age of the silicic-carbonate successions of the Cushamen Metamorphic Complex between ca. 385 and 335 Ma. This suggests the existence of a mixed carbonate-siliciclastic platform at least in the southernmost portion of southwestern Gondwana between Middle Devonian and early Carboniferous (Middle Mississippian).Fil: Murra, Juan Alberto Félix. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Baldo, Edgardo Gaspar Agustin. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Verdecchia, Sebastián Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Ramacciotti, Carlos Dino. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Galindo, Carmen. Universidad Complutense de Madrid; Españ

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.Centro de Investigaciones Geológica

Geochemistry and provenance of the Ordovician metasediments from La Cébila metamorphic complex, La Rioja province, 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.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.Fil: Verdecchia, Sebastián Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Baldo, Edgardo Gaspar Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentin

Polyphase white mica growth in low-grade metapelites from La Cébila Metamorphic Complex (Famatinian Belt, Argentina): evidence from microstructural and XRD investigations

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 (tempe-ratures between 300 and 400ºC) and low-pressure conditions are suggested for M1 event, based in Kübler index values ranging from 0.23 to 0.17 Δº2θ, white mica b parameter values between 9.004 and 9.022 Å (mean of 9.014 Å, n=16) and Si contents between 3.13-3.29 a.p.f.u. Temperatures of ~180-270ºC are estimated for the M2 event, with Kübler index values ranging from 0.31 to 0.46 Δº2θ. 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.Fil: Verdecchia, Sebastián Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Collo, Gilda. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Baldo, Edgardo Gaspar Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentin

P–T–t path of the Boroujerd Complex, north-west Sanandaj–Sirjan Zone, western Iran: Insights from phase equilibrium modelling and thermobarometry

Variations in the pressures and temperatures of metamorphism reflect the complexity of collisional orogenic systems. In the north-west Sanandaj–Sirjan Zone of western Iran, migmatites in the Boroujerd area record earlier dynamothermal metamorphism followed by the development of a contact metamorphic aureole induced by the Jurassic Boroujerd Batholith. Integrated micro-textural observations, mineral compositions, and the results of phase equilibrium modelling indicate that peak metamorphic temperatures reached 640–660°C in andalusite schists and ~700°C in cordierite–sillimanite migmatites. The pressure does not exceed 3.5 kbar, implying a maximum depth of ~13 km. A hairpin clockwise P–T path in the Boroujerd region was inferred from mineral relations (e.g., replacement of andalusite1 by sillimanite in the migmatites in the heating stage, which was followed by a retrograde episode with growth of andalusite2) and P–T pseudosections. Metamorphism of the Boroujerd area was coeval with voluminous intermediate–mafic magmatism, which provided the heat necessary for partial melting. This study demonstrates that metamorphic sequences of the north-west Sanandaj–Sirjan Zone developed under similar metamorphic conditions in the Lower to Middle Jurassic and record an extensional back-arc setting related to the subduction of Neo-Tethys.Fil: Rahmani Javanmard, Somayeh. Institution of Higher Education; IránFil: Verdecchia, Sebastián Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Yakymchuk, Chris. University of Waterloo; CanadáFil: Broekmans Maarten A. T. M.. Geological Survey of Norway; Norueg

Staurolite porphyroblast controls on local bulk compositional and microstructural changes during decompression of a St–Bt–Grt–Crd–And schist (Ancasti metamorphic complex, Sierras Pampeanas, W Argentina)

The staurolite–biotite–garnet–cordierite–andalusite–plagioclase–muscovite–quartz metapelitic mineral assemblage has been frequently interpreted in the literature as a result of superimposition of various metamorphic events, for example, in polymetamorphic sequences. The assemblage was identified in schists from the Ancasti metamorphic complex (Sierras Pampeanas of Argentina) where previous authors have favoured the polymetamorphic genetic interpretation. A pseudosection in the MnNCKFMASH system for the analysed XRF bulk composition predicts the stability of the sub-assemblage staurolite–biotite–garnet–plagioclase–muscovite–quartz, and the compositional isopleths also agree with measured mineral compositions. Nevertheless, the XRF pseudosection does not predict any field with staurolite, andalusite and cordierite being stable together. As a result of more detailed modelling making use of the effective bulk composition concept, our interpretation is that the staurolite–biotite–garnet– plagioclase–muscovite–quartz sub-assemblage was present at peak metamorphic conditions, 590 C and 5.2 kbar, but that andalusite and cordierite grew later along a continuous P–T path. These minerals are not in mutual contact and are observed in separate microstructural domains with different proportions of staurolite. These domains are explained as a result of local reaction equilibrium subsystems developed during decompression and influenced by the previous peak crystal size and local modal distribution of staurolite porphyroblasts that have remained metastable. Thus, andalusite and cordierite grew synchronously, although in separate microdomains, and represent the decompression stage at 565 C and 3.5 kbar.Fil: Verdecchia, Sebastián Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Reche, J.. Universitat Autònoma de Barcelona; EspañaFil: Baldo, Edgardo Gaspar Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Segovia Díaz, E.. Consejo Superior de Investigaciones Científicas; España. Universidad Complutense de Madrid; EspañaFil: Martínez, F. J.. Universitat Autònoma de Barcelona; Españ

Weathering assessment in the Achala Batholith of the Sierra de Comechingones, Córdoba, central Argentina. I: Granite–regolith fractionation

Weathering assessment in a weathering-limited environment is a difficult task because a significant proportion of the regolith has been removed by tectonically-induced denudation. In such an environment, the remnants of the altered mineral debris must be studied with care in order to attain a meaningful picture of the rate and intensity of ongoing weathering. A monolithologic (i.e., granite), small (~ 2 km2), and mountainous (~ 1500 m elevation) drainage basin in the Sierras de Comechingones (31°54′07″S 64°45′28″W–31°53′11″S 64°44′16″W, Córdoba, central Argentina) was selected as a pilot area to survey the nature of weathering in a weathering-limited erosional setting, and a semiarid climatic regime. A relatively thin-layered, coarse-grained regolith and scattered sediment (i.e., fine-grained regolith) that had accumulated in valleys and topographic depressions were analyzed. The most abundantly identified clay mineral in the regolith is illite followed by kaolinite > smectite. Smectite seems to be the only clay mineral clearly associated with weathering. Petrographic observations and geochemical analyses, supported by statistical tests, define chemical weathering as incipient. Petrography indicates that plagioclase and biotite are the main mineral phases affected by alteration, which is more intense in the fine-grained regolith. Coarse- and fine-grained regoliths are chemically similar, among them and with the country rock, with statistically significant losses in the fine fraction of MgO, MnO and P2O5. Depletions of trace elements and REE, which are best explained by sorting than by actual rock alteration, are not statistically significant. Ternary diagrams reveal that the masses of Al2O3, CaO, Na2O and K2O were not significantly altered during weathering and transportation, and that regolith samples correspond to a coarse residue relatively enriched in feldspars (and quartz), where the mud-fraction (with high clay mineral contents) has been removed from the drainage basin by high-energy processes.Fil: Campodonico, Verena Agustina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Martínez, Jorge Oscar. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Verdecchia, Sebastián Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Pasquini, Andrea Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Depetris Gallino, Pedro Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentin