A reactive assimilation model for regional-scale cordierite-bearing granitoids: geochemical evidence from the Late Variscan granites of the Central Iberian Zone, Spain

Regional scale biotite and cordierite-bearing granites (s.l.) in the Variscan of the Central Iberian Zone (CIZ) are spatially closely associated with cordierite-rich nebulites and cordierite-bearing two-mica granites, and with cordierite-rich high grade hornfelses and cordieritites (>60% cordierite) that are relatively common in the aureoles of these granites. Building on published field evidence, petrological data are presented which, combined with new chemical and isotopic (Sr-Nd) modelling, indicate that the cordierite-bearing granites cannot be derived by simple anatexis of regional sedimenatry protoliths; but the data are consistent with a process of reactive assimilation that involves the interaction of biotite granite magma with high-grade host rocks ranging from cordierite nebulites to andalusite-bearing cordieritites. The contribution of the postulated cordierite-rich contaminants to the diversity of cordierite granite compositions is modelled using the compositions of regional Lower Cambrian-Upper Neoproterozoic metasedimentary rocks that are generally chemically mature (CaO very rarely exceeds 1.4%). These rocks include specific horizons in which extreme chemical alteration is attributable to sediment reworking during eustatic falls in sea level. Such compositions may account for the presence of the high concentrations in Al that later produced cordieritites. Fractional crystallisation is also important, particularly in generating the more evolved cordierite granite and cordierite biotite muscovite granite compositions. Although assimilation in situ is normally regarded as a minor contributor volumetrically to evolving plutons, in this instance the emplacement of large volumes of granite magma into a high-T-low-P environment significantly increased the potential for reactive assimilation

The Upper Neoproterozoic-Lower Cambrian of the Central Iberian Zone, Spain: Chemical and isotopic (Sm-Nd) evidence that the sedimentary succession records an inverted stratigraphy of its source

<P>The Upper Neoproterozoic-Lower Cambrian sedimentary succession in the Central Iberian Zone has recently been divided into 12 sedimentary units (I to XII). Units I to IV are of Late Neoproterozoic age, and units V to XII are Early Cambrian.Throughout the stratigraphic section, shales with similar ranges of SiO<SUB>2</SUB> and Al<SUB>2</SUB>O<SUB>3</SUB> have TiO<SUB>2</SUB> and Zr contents that are coupled and vary gradually from the bottom (unit I, TiO<SUB>2</SUB> = 1.0-1.1%, Zr = 245-287 ppm) to the top (unit XII, TiO<SUB>2</SUB> = 0.74-0.86%, Zr = 141-192 ppm). These two parameters clearly distinguish Upper Neoproterozoic from Lower Cambrian compositions, as do certain elemental ratios (Al<SUB>2</SUB>O<SUB>3</SUB>/TiO<SUB>2</SUB>, Rb/Zr, Nb/Ti), which vary from low values at the bottom of the section to high at the top. However, other elements (the rare earth elements [REEs], Y) do not fit this general trend, with heavy REEs and Y showing extreme concentrations in some Lower Cambrian units, which also have negative Ce anomalies.Units I to III and units XI to XII have uniform T<SUB>DM</SUB> neodymium model ages (~1.1 and 1.7 Ga, respectively), but the units between these two groups have erratic values between 1.4 Ga (unit VII) and 3.4 Ga (unit VIII). Thus, the bottom and top units have neodymium isotope ratios that indicate the presence of two source compositions: a composition that records a juvenile contribution younger than 1.1 Ga and a composition mainly derived from an old basement. Some samples from Lower Cambrian units have disturbed REE abundances and neodymium isotope ratios that are attributed to extensive chemical alteration that resulted in REE and Y redistribution. This alteration probably affected the material now preserved as unit IV during a period of relative sea level fall at the end of the Late Neoproterozoic. Both elemental and isotopic results support the suggestion that the Upper Neoproterozoic-Lower Cambrian stratigraphic section records an inverse stratigraphy of a source hinterland composed of a cover sequence of relatively juvenile crustal materials underlain by an older basement. Through time, the deeper crust gradually increased its contribution to the sediments in the basin from Late Neoproterozoic to Lower Cambrian times. These results are compatible with data from many other European zones.</P><br /&gt

Neuere statistische Methoden fuer die Versuchsplanung bei industriellen Fertigungsprozessen

SIGLETIB: RO 7722(238) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Oxygen, carbon and strontium isotope records of Ediacaran carbonates, in Central Iberia (Spain)

Carbon, oxygen and strontium isotope ratios were determined in the uppermost Neoproterozoic carbonate rocks at Pastores (SW Salamanca, Central Iberia). The low δ18O V-SMOW values (15–19‰) indicate an alteration of the whole stratigraphic series. However, the alteration process does not seem to have affected most of the δ13C and 87Sr/86Sr data. These data reveal that only one particular level of the carbonate section shows simultaneous relative increases in Mn/Sr and 87Sr/86Sr ratios, decreases in the δ18O and δ13C values, and covariations between 87Sr/86Sr and the other parameters. Despite this, most of the carbonate section lacks these covariations and its δ13C (−4.6 to −0.5‰) and 87Sr/86Sr (0.70845–0.70853) ratios probably reflect the seawater isotopic features at the time of sedimentation. The data for C and Sr isotopes at the Pastores carbonate section and the presence of Cloudina in platform carbonates together with an overlying major sub-Cambrian unconformity following a relative sea level fall are features comparable to those frequently shown by Proterozoic–Cambrian boundary series across the world. Thus, our results consistently support the notion that the carbonates at Pastores are equivalent to other carbonates deposited close to the Ediacaran/Cambrian boundary

Provenance of Upper Precambrian-Lower Cambrian shales in the Central Iberian Zone, Spain: evidence from a chemical and isotopic study

Geochemical and isotopic (Sr, Nd, O) data are presented for Upper Precambrian and Lower Cambrian shale sequences from the Central Iberian Zone, Spain, in order to define geochemical discriminants between them, and to assess the nature of the source regions supplying these basins. Elemental data, especially Th/Sc ratios, coupled with ε_Nd^t values of ∼ −2.5 suggest that the Upper Precambrian rocks contain a greater proportion of a much younger, more mafic component than the Lower Cambrian sequence which has high Th/Sc and ε_Nd^t of ∼ −7.3. Coherent trends between elemental and O isotope data imply a homogeneous, recycled source as the dominant provenance for the Upper Precambrian shales, whereas a lack of such coherence for the Lower Cambrian sediments suggests a more heterogeneous provenance. The sedimentological, geochemical and isotopic data are consistent with the Upper Precambrian succession being derived ultimately from a polycyclic mixture of young (pre-Cadomian), mafic sources, and older (> 2 Ga) crust. In contrast, the Lower Cambrian sequence was mainly derived from old, more felsic crust which would have contributed increasingly greater proportions of material as the younger sources were progressively removed by erosion

The Ediacaran–Cambrian transition in the Cantabrian Zone (northern Spain): sub-Cambrian weathering, K-metasomatism and provenance of detrital series

Sampling and analytical technique

Variscan Magmatism

This chapter aims to identify, characterize and locate the main facts/events related to orogenesis in the Iberian Peninsula. Its succession in space and time determines the geodynamic environment of the broader geological phenomenon corresponding to the Variscan cycle. In this sense, this section comprises two parts: I—The Iberian orogenic magmatism seen through a space-time approach of its westernmost region—focus on the enormous complexity of the inherited basement, its nature, age and distribution in space. Establishes a space-time sequence of geodynamic environments correlated with the obtained data and tries to identify the agents responsible for its genesis. Some case studies are presented to illustrated significant regional aspects of the magmatic process and II—An overview of the petrogenesis of the great batholiths and of the basic, intermediate and mantle-related rocks—identify and analyze a great amount of these rocks intruding and extruded from 400 to 280 Ma and to better understanding the large-scale process involving the whole lithosphere during Variscan cycle.publishe