Geoquímica de series metasedimentarias del Macizo Ibérico: contexto dinámico de la transición Ediacarense- Cámbrico

El Macizo Ibérico contiene una excelente representación de la estratigrafía del Paleozoico Inferior, así como de la transición con el Neoproterozoico Superior. La geoquímica de las series siliciclásticas (elementos mayores y trazas) y sus fuentes isotópicas (Sm-Nd) han sido muy poco investigadas hasta tiempos recientes, a pesar de que estos aspectos son importantes a la hora de conocer el contexto dinámico de las cuencas y su ubicación en el paleomargen de Gondwana. En esta tesis se presenta un estudio basado en estas metodologías y tiene como objetivo principal la investigación de las series estratigráficas que definen la transición entre el Cámbrico Inferior y el Ediacarense. Las series estudiadas corresponden a las Unidades Alóctonas Superiores y a las Unidades Alóctonas Basales del NW, al Cámbrico Inferior y Alcudiense del sector meridional de la Zona Centroibérica y a las Unidades Alóctonas Basales del SW. La labor realizada se ha sintetizado en varios trabajos ya publicados, que se han incluido en esta memoria y representan su contenido fundamental. La memoria se ha completado además con unos apartados introductorios de contenido metodológico y un capítulo final que presenta una síntesis de resultados a escala del Macizo Ibérico.Los resultados obtenidos indican como conclusión general, un cambio significativo en la influencia de un arco volcánico Cadomiense sobre los procesos sedimentarios que tenían lugar en el margen de Gondwana y por consiguiente, sobre la composición química de las secuencias sedimentarias siliciclásticas ediacarenses y cámbricas. Este cambio supuso una evolución progresiva del contexto deposicional, probablemente ligado a la apertura y ensanchamiento de una extensa cuenca back-arc. Esta apertura ocasionó durante el Paleozoico más inferior un progresivo alejamiento de las cuencas sedimentarias del foco principal de la actividad magmática, cuya actividad era también aparentemente decreciente, y una evolución muy lenta pero continua hacia un margen pasivo durante la mayor parte del Cámbrico. La transición es especialmente evidente en la geoquímica (elementos mayores y trazas) de las secuencias sedimentarias del Cámbrico Inferior y del Alcudiense (Ediacarense) del sector meridional de la Zona Centroibérica. También es patente en la composición de las secuencias detríticas de las Unidades Alóctonas Basales del NW de Iberia. La investigación de las fuentes isotópicas (Sm-Nd) refleja la misma evolución geodinámica de los terrenos peri-Gondwánicos entre el Ediacarense y el Cámbrico. Es particularmente visible en el caso de las secuencias metasedimentarias de la Zona Centroibérica, donde la evolución de los valores de Nd indican una menor influencia del arco volcánico durante la sedimentación de las series cámbricas. Las TDM más antiguas registradas en estas secuencias del Cámbrico Inferior (1444 - 1657 Ma), son compatibles con un incremento de los aportes desde áreas fuente gondwánicas antiguas y una disminución de los aportes juveniles procedentes del arco volcánico..

Age and petrogenesis of the beryl-bearing granitic magmatism of the Velasco Pegmatite District, Pampeana Province, NW Argentina

Two porphyritic granitoids (Huaco and Sanagasta) in the Velasco district of the Pampeana Pegmatite Province in Northwestern Argentina are recognized. They are considered the fertile granites of the beryl-bearing pegmatites and can be described as post-orogenic and peraluminous A-type granites formed in an intraplate tectonic setting during an extensive regime, whose magma source is predominantly of cortical origin and to a lesser extent, mantle-derived. The pegmatites are classified as Rare Elements of the beryl type and beryl-columbite-phosphate subtype, while the entire district shows characteristics related to the NYF (Nb-Y-F) petrogenetic family. From rocks and cogenetic minerals of an individual ‘Ismiango’ pegmatite of the Velasco district, two Rb/Sr isochrons have been constructed. They define an age of 330.3 ± 8.3 and 331.7 ± 2.3 Ma and fall in the Lower Carboniferous period, consistent with the age of the parental and host-rock, the Huaco granite. As the Ismiango pegmatite has a similar composition and structure to the other beryl mineralized pegmatites of the Velasco district, the obtained age is attributed extensively for the entire district. According to the initial 87Sr/86Sr value obtained of 0.713, the pegmatite-magmatism of the Velasco District might be mainly derived from the crust with some minor participation of mantle materials.Fil: Sardi, Fernando Guillermo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; ArgentinaFil: Marangone, Silvana Elizabeth. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; ArgentinaFil: Fuenlabrada, José Manuel. Universidad Complutense de Madrid; Españ

High-precision Sr and Nd isotope characterization of BHVO-2 reference material by thermal ionization mass spectrometry

Rationale: Control over accuracy and precision in isotope geochemical analysis is highly necessary for reliable results. Use of reference materials in Sr and Nd isotope geochemistry provides a means of validating optimal sample preparation and instrument configurations. Methods: A comprehensive data set of Sr and Nd isotope analyses of BHVO-2 reference material was obtained by thermal ionization mass spectrometry (TIMS). The intention is to assist other researchers in evaluating isotopic analyses of geological samples with similar chemical matrix. Results: 87Sr/86Sr isotope ratios analyzed in BHVO-2 reference material over 88 replicas from 2015 to the present provide an average value of 0.703485 ± 0.000024. In turn, 143Nd/144Nd isotope ratios in 62 replicas show an average value of 0.512978 ± 0.000012. The high number of analyses here presented ensures a high reproducibility over time in a non-certified material for isotopic analysis of these two elements. Conclusions: This high precision and reproducibility of Sr and Nd analysis by TIMS provide a basis for validation of analytical processes in the absence of certified reference materials and support the correlation of isotopic results between laboratories and the overall improvement in the interpretation of isotope results for geological, environmental and archeological samples.Unidad de Geocronología (CAI de Ciencias de la Tierra y Arqueometría - UCM)Unidad de GeocronologíaCAI Ciencias de la Tierra y ArqueometríaTRUEpu

A peri-Gondwanan arc in NW Iberia. II: Assessment of the intra-arc tectonothermal evolution through U–Pb SHRIMP dating of mafic dykes

The arc-derived upper units of the Órdenes Complex, NW Iberia, are emplaced above the Variscan suture and contain a low-grade metasedimentary uppermost section, with a maximum depositional age of 510–530 Ma, intruded by a number of mafic dykes. Three deformational events affect the metasedimentary section. The youngest deformation event (D3) is of undifferentiated Variscan age and consists of metre- to decametre-scale, close upright folds with axes plunging gently towards N20°E. The most important D2 structure is a regional S2 foliation axial planar of minor foldswith dextral asymmetry. The presence of a stretching lineation parallel to the D2 fold axes is related to a top-to-the-north sense of shearing in a context of regional extension. The oldestmeterscale D1 folds are developed in suitable greywacke–pelite alternations and consist of tight folds with chevron and similarmorphologies, axes plunging gently toward N20°E, and a continuous S1 axial planar foliation. The essential characteristic of the D1 deformation event is depicted by a set of west-vergent folds with reverse limbs less than 2 kmin wavelength, that are affected in their lower part by the generalised presence of the regional S2 foliation. The age of D2 and D1 structures is notwell constrained. The diabase dykes intruding the low-grade turbidites cut the D1 folds and their field relationships suggest that they were emplaced at the end of the D2 shearing event and prior to the upright D3 Variscan folds. Zircon grains obtained from one of the diabase dykes were analysed for U–Pb at the SHRIMP-RG facility at Stanford University. An age of c. 510 Ma, based on the analysis of 31 individual zircon grains, is interpreted to date the crystallization of the Ares dyke. The tectonic and magmatic evolution of the top turbiditic series of the Órdenes Complex is tentatively related to the dynamics of a peri- Gondwanan arc developed during active subduction beneath Gondwana and suggests: (1) accretion beneath the arc during west-vergent (present coordinates) nappe development (D1); (2) extension of the arc during top-tothe- north shearing (D2); and (3) final intrusion of the diabasic dykes into an intra-arc turbiditic series. This evolution spans the end of volcanic arc activity and the onset of the opening of the Rheic Ocean

Geochemistry in earth sciences: a brief overview

In this contribution we present a succinct overview of the role of Geochemistry in the Earth Sciences. Our idea is to offer a “bird’s eye” perspective of the advent of Geochemistry and its historical development and the advances and milestones in analytical techniques that led Geochemistry to its actual place in the context of Earth Sciences. Finally, we offer a summary of some relevant applications of Geochemistry in the study of a variety of geological problems

Sulphur, oxygen and strontium isotope compositions of Middle Miocene (Badenian) calcium sulphates from the Carpathian Foredeep, Poland: Palaeoenvironmental implications

Sulphur, oxygen and strontium isotope compositions have been measured in sulphate (gypsum and anhydrite) samples from the Badenian evaporite complex in the Carpathian Foredeep Basin (Poland) to determine the origin of brines from which these sulphates were formed. Studied samples display the δ values from +22.68 to +24.91‰ CDT for sulphur (ten samples) and from +12.26 to + 13.63‰ SMOW for oxygen (ten samples), and 87Sr/86Sr ratios from 0.708915 to 0.716329 (six samples). Most samples show isotopic values (both δ 34S and 87Sr/86Sr) higher than contemporaneous (Badenian) seawater, and thus suggest that these sulphates were formed (i) from brines with a significant component of non-marine waters, (ii) in a restricted system, where an important role was played by meteoric water inputs as well as by bacterial sulphate reduction. The results of this study show clear differences in the isotopic signatures between sedimentary (gypsum) and diagenetic (anhydrite) lithofacies. While sedimentary gypsum displays sulphur, oxygen and strontium isotope ratios close to contemporaneous (Badenian) seawater, diagenetic anhydrite values are largely elevated. The higher isotopic values (δ34S, δ18O and 87Sr/86Sr) for anhydrite when compared to gypsum are interpreted as reflecting different hydrological provenances of sulphate in the Badenian basin. Gypsum was formed from brines marine in origin that were subject to an important inflow of continental waters and a local bacterial sulphate reduction in a restricted, and therefore sulphate-limited basin, which is consistent with earlier interpretations based on sedimentological and geochemical studies. Our study shows that brines from which anhydrite was formed had a highly-radiogenic non-marine (riverine, groundwater) strontium component, which is a new contribution to the knowledge on the Badenian sulphate formation. The results reflect a lateral compositional evolution of parent waters during sulphate deposition and diagenesis in the Carpathian Foredeep Basin.Polish Committee for Scientific ResearchDepto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

A peri-Gondwanan arc in NW Iberia I: Isotopic and geochemical constraints on the origin of the arc—A sedimentary approach

The arc-derived upper terrane in the NW Iberia Variscan belt contains a 3000 m thick turbiditic formation at its structural top. Most of the sandstones are feldspathic greywackes with a framework of quartz and weakly altered plagioclase. Rock fragments of vitric and microgranular texture are common in polymictic conglomerates and coarse-grained greywackes, together with slates, cherts and bipyramidal volcanic quartz fragments. Although recrystallization under greenschists facies conditions (chlorite and biotite zones) and the presence of two cleavages hinder detailed textural analysis, the sandstones appear to be typically immature, first-cycle sandstones. The metagreywackes have average major and trace element compositions similar to PAAS (Post Archean Australian Shale), which is considered to reflect the composition of the upper continental crust. Their trace element composition is very consistent and records deposition within a convergent tectonic setting, probably in an intra-arc basin located in a volcanic arc built on thinned continental margin. Detrital zircon populations suggest a Middle Cambrian maximum depositional age (530–500 Ma) and a Gondwanan provenance located at the periphery of the West African Craton. Nd isotope data suggest mixing Ediacaran and Paleoproterozoic sources for the provenance of the greywackes, with TDM ranging between 720 and 1215 Ma with an average of 995 Ma (n=20)—an age range unrepresented in the detrital zircon population. The Nd model ages are similar to those exhibited by West Avalonia, Florida or the Carolina terrane, but younger than those of Cambrian and Ordovician sandstones and shales from the autochthonous realm. These data suggest a westernmost location along the Gondwanan margin for the upper terrane of NW Iberia relative to other terranes located in the footwall of the Variscan suture, consistent with several previously proposed paleogeographic models for the NW Iberia terranes