| 1. |
- Kara, Jaakko, et al.
(author)
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1.90-1.88 Ga arcmagmatism of central Fennoscandia: geochemistry, U-Pb geochronology, Sm-Nd andLu-Hf isotope systematics of plutonic-volcanic rocks from southern Finland.
- 2018
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In: Geologica Acta. - 1695-6133 .- 1696-5728. ; 16:1, s. 1-23
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Journal article (peer-reviewed)abstract
- The earliest Svecofennian magmatism in southern Finland has been dated to 1.90-1.88Ga. As an example of this, the Orijärvi (ca. 1.89Ga) and Enklinge (ca. 1.88Ga) volcanic centres comprise bimodal plutonic batholiths surrounded by volcanic rocks of comparable ages and chemical compositions. The rock types range from gabbros to granites and indicate a subduction-related continental margin setting. The zircons from the Orijärvi granodiorite define an age of 1892±4Ma whereas the Enklinge granodiorite yields an age of 1882±6Ma. Several inherited ages of 2.25-1.95Ga as well as younger metamorphic ages of 1.86-1.80Ga were found in the Enklinge granodiorite. The initial εNd values of the mafic rocks from both locations fall in the range +1.1 to +2.9, whereas the felsic rocks exhibit initial εNd values of -0.4 to +1.2. The magmatic zircons from the Orijärvi and Enklinge granodiorites show average initial εHf values of -1.1 (at 1892Ma) and zero (at 1882Ma), respectively, both with a spread of about 7 ε-units. The initial εHf values for the inherited zircons from Enklinge range from +3.5 to +7.6 with increasing age. The Sm-Nd data indicate that the mafic rocks were derived from a “mildly depleted” mantle source while the felsi rocks show some crustal contribution. Also, the variation in ε Hf values indicates minor mixing between mildly depleted mantle-derived magmas and crustal sources. U-Pb ages and Hf isotopes for inherited zircons from the Enklinge granodiorite suggest the presence of juvenile Svecofennian “proto-crust” at depth
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| 2. |
- Soler, Josep, et al.
(author)
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Modelling of the LTDE-SD radionuclide diffusion experiment in crystalline rock at the Aspo Hard Rock Laboratory (Sweden)
- 2022
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In: Geologica Acta. - : Universitat Autònoma de Barcelona. - 1695-6133 .- 1696-5728. ; 20, s. 1-32
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Journal article (peer-reviewed)abstract
- This study shows a comparison and analysis of results from a modelling exercise concerning a field experiment involving the transport and retention of different radionuclide tracers in crystalline rock. This exercise was performed within the Swedish Nuclear Fuel and Waste Management Company (SKB) Task Force on Modelling of Groundwater Flow and Transport of Solutes (Task Force GWFTS). Task 9B of the Task Force GWFTS was the second subtask within Task 9 and focused on the modelling of experimental results from the Long Term Sorption Diffusion Experiment in situ tracer test. The test had been performed at a depth of about 410m in the Aspo Hard Rock Laboratory. Synthetic groundwater containing a cocktail of radionuclide tracers was circulated for 198 days on the natural surface of a fracture and in a narrow slim hole drilled in unaltered rock matrix. Overcoring of the rock after the end of the test allowed for the measurement of tracer distribution profiles in the rock from the fracture surface (A cores) and also from the slim hole (D cores). The measured tracer activities in the rock samples showed long profiles (several cm) for non-or weakly-sorbing tracers (Cl-36, Na-22), but also for many of the more strongly-sorbing radionuclides. The understanding of this unexpected feature was one of the main motivations for this modelling exercise. However, re-evaluation and revision of the data during the course of Task 9B provided evidence that the anomalous long tails at low activities for strongly sorbing tracers were artefacts due to cross-contamination during rock sample preparation. A few data points remained for Cs-137, Ba-133, Ni-63 and Cd-109, but most measurements at long distances from the tracer source (>10mm) were now below the reported detection limits. Ten different modelling teams provided results for this exercise, using different concepts and codes. The tracers that were finally considered were Na-22, Cl-36, Co-57, Ni-63, Ba-133, Cs-137, Cd-109, Ra-226 and Np-237. Three main types of models were used: i) analytical solutions to the transport-retention equations, ii) continuum -porous-medium numerical models, and iii) microstructure-based models accounting for small-scale heterogeneity (i.e. mineral grains, porosities and/or microfracture distributions) and potential centimetre-scale fractures. The modelling by the different teams led to some important conclusions, concerning for instance the presence of a disturbed zone (a few mm in thickness) next to the fracture surface and to the wall of the slim hole and the role of micro-fractures and cm-scale fractures in the transport of weakly sorbing tracers. These conclusions could be reached after the re-evaluation and revision of the experimental data (tracer profiles in the rock) and the analysis of the different sets of model results provided by the different teams.
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| 3. |
- Vaisanen, M., et al.
(author)
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Palaeoproterozoic adakite- and TTG-like magmatism in the Svecofennian orogen, SW Finland
- 2012
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In: Geologica Acta. - 1695-6133 .- 1696-5728. ; 10:4, s. 351-371
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Journal article (peer-reviewed)abstract
- The Palaeoproterozoic Svecofennian orogen in the Fennoscandian shield is an arc accretionary orogen that was formed at c. 1.92-1.86Ga. Arc accretion, magmatism and the subsequent continent-continent collision thickened the crust up to c. 70km, forming one of the thickest Palaeoproterozic orogens. At the end stage of accretionary tectonics, voluminous synorogenic magmatism occurred in southwestern Finland leading to the intrusion of intermediate to felsic plutonic rocks. Ion microprobe single zircon dating of one diorite sample yielded an age of 1872 +/- 3Ma (epsilon(Nd)=+2.2) and the trondhjemite sample an age of 1867 +/- 4Ma (epsilon(Nd)=+2.6). Inherited 2667-1965Ma cores and 1842 +/- 5Ma metamorphic rims were also found in zircons from the trondhjemite. The dioritic magmatism is mantle-derived and is slightly enriched by subduction-related processes. The felsic magmatism shows elevated Sr/Y and La/Yb ratios, which are typical for adakite- and TTG-like magmas. Their low Mg#, Ni and Cr contents argue against slab-melting and mantle-wedge contamination. We infer that the felsic magmatism was generated through crustal melting of the lower part of the previously generated volcanic-arc type crust. Based on published melting experiments and the Sr and Y contents of the felsic rocks we suggest that the melts were generated at a minimum pressure of 10kbar, with evidence of a 15kbar pressure for the highest Sr/Y trondhjemites. It is proposed that arc accretion combined with magmatic intrusions thickened the crust so that melting of the lower crust yielded adakite- and TTG-like compositions. The mafic magmatism is considered to be the heat source.
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| 4. |
- Vidal-Royo, O., et al.
(author)
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Structural evolution of Pico del Aguila anticline (External Sierras, southern Pyrenees) derived from sandbox, numerical and 3D structural modelling techniques
- 2013
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In: Geologica Acta. - 1695-6133 .- 1696-5728. ; 11:1, s. 1-26
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Journal article (peer-reviewed)abstract
- This paper reports on the integration of different modelling techniques to construct a unified conceptual model of structural evolution of the Pico del Aguila anticline (External Sierras, southern Pyrenees, Spain). The structure is a well-known example of a detachment fold, which exhibits a N-S structural trend, perpendicular to the general structural trend of the southern Pyrenees (mainly E-W). Based on field observations of an unevenly distributed Triassic decollement, analogue modelling shows how to generate orogen-perpendicular structures which may result in transverse anticlines. The models show how contrasts between high and low friction patches in the basal decollement led to the formation of structures at high angle, centered over the high friction areas. Numerical models investigate the effect of a complex mechanical stratigraphy, characterized by an interlayering of competent and incompetent layers, plus syn-kinematic sedimentation in the fold growth. Based on field data and seismic interpretations, a 3D reconstruction and sequential geomechanical restoration of the Pico del Aguila anticline suggests the coexistence of multiple folding mechanisms occurring simultaneously in different units and structural domains of the fold, leading to a complex strain pattern that can not be assessed by simplistic kinematic 2D approaches. By integrating the models with previous data in the region, the benefits and drawbacks of each modelling technique are discussed and an integrated model of structural evolution for the Pico del Aguila anticline is presented. This enables a better comprehension of the structure as well as of the processes that drove the evolution of the N-S detachment anticlines in the External Sierras of the southern Pyrenees.
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| 5. |
- Zaccarini, Frederica, et al.
(author)
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Chromite and platinum group elements mineralization in the Santa Elena Ultramafic Nappe (Costa Rica) : Geodynamic implications
- 2011
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In: Geologica Acta. - 1695-6133 .- 1696-5728. ; 9:3-4, s. 407-423
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Journal article (peer-reviewed)abstract
- Chromitites associated with strongly altered peridotite from six distinct localities in the Santa Elena ultramaficnappe (Costa Rica) have been investigated for the first time. Santa Elena chromitites commonly display acompositional variation from extremely chromiferous (Cr/(Cr+Al)=0.81) to intermediate and aluminous(Cr/(Cr+Al)=0.54). This composition varies along a continuous trend, corresponding to calculated parental liquidswhich may have been derived from the differentiation of a single batch of boninitic magma with Cr-rich and(Al, Ti)-poor initial composition. Fractional precipitation of chromite probably occurred during differentiation ofthe boninitic melt and progressive metasomatic reaction with mantle peridotite. The distribution of platinum groupelements (PGE) displays the high (Os+Ir+Ru)/(Rh+Pt+Pd) ratio typical of ophiolitic chromitites and, consistently,the platinum group minerals (PGM) encountered are mainly Ru-Os-Ir sulfides and arsenides. Textural relations ofmost of the platinum group elements suggest crystallization at magmatic temperatures, possibly under relativelyhigh sulfur fugacity as indicated by the apparent lack of primary Os-Ir-Ru alloys.The chemical and mineralogical characteristics of chromitites from the Santa Elena ultramafic nappe have astrong affinity to podiform chromitites in the mantle section of supra-subduction-zone ophiolites. Calculatedparental melts of the chromitites are consistent with the differentiation of arc-related magmas, and do not supportthe oceanic spreading center geodynamic setting previously proposed by some authors.
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| 6. |
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| 7. |
- Egger, H., et al.
(author)
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From shelf to abyss: Record of the Paleocene/Eocene-boundary in the Eastern Alps (Austria)
- 2009
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In: Geologica Acta. - 1695-6133. ; 7:1-2, s. 215-227
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Journal article (peer-reviewed)abstract
- In the Eastern Alps (Austria) several marine successions, which were deposited ranging from shallow shelf to bathyal slope and abyssal basin, provide detailed records across the Paleocene/Eocene-boundary. These records indicate a two-step event starting with a prominent sea-level fall and followed by climatic changes. At the northern and southern shelves that fringed the Penninic Basin, the shallow-water sedimentary records are incomplete across the Paleocene/Eocene transition. Erosional surfaces indicate a major sea-level drop, which was terminated by an early Eocene (Ypresian) transgression within calcareous nannoplankton Zone NP12. As a proxy for the onset of this sea-level fall a strong increase in the terrestrially-derived input into the Penninic Basin can be used. The abyssal Anthering section from the northern part of the basin comprises a complete succession from NP9 to the upper part of NP10 (upper Thanetian-lower Ypresian). The thickest turbidite beds of this 250 m thick succession appear just before the carbon isotope event in the upper part of zone NP9, which is used to recognize the Paleocene/Eocene-boundary. A major lithological change from a sandstone-dominated facies to a claystone-dominated facies occurs at the onset of the carbon isotope event. This might be the result of a climatic change, resulting in increased intra-annual humidity gradients and increased physical erosion of the hinterland. Consequently, mainly fine-grained suspended material would have come into the basin and caused an increase in hemipelagic sedimentation rates by about a factor of 6. A similar value has been calculated for the bathyal Untersberg section, which was deposited on the southern slope of the basin, where an increased input of siliciclastic material is associated with a carbonate dissolution event during the carbon isotope event. At the southern shelf, a stratigraphic gap within the Gosau Group in the Krappfeld area (Carinthia) comprises the Maastrichtian and Paleocene. After a sea-level rise nummulitic marlstone and limestone were deposited in the lower part of zone NP12. Since the northern and southern shelves of the Penninic Basin belonged to different tectonic domains, with different potentials of crustal subsidence, the temporal similarity of sea-level changes on both shelves in the latest Paleocene and earliest Eocene suggests that these sea level fluctuations were mainly eustatic in origin.
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| 8. |
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| 9. |
- Pujalte, V., et al.
(author)
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Correlation of the Thanetian-Ilerdian turnover of larger foraminifera and the Paleocene-Eocene thermal maximum: confirming evidence from the Campo area (Pyrenees, Spain)
- 2009
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In: Geologica Acta. - 1695-6133. ; 7:1-2, s. 161-175
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Journal article (peer-reviewed)abstract
- Abstract in UndeterminedIt has long been known that a major larger foraminifera turnover (LFT) occurred at the boundary between the Thanetian and Ilerdian stages, but its possible correlation with the Paleocene-Eocene thermal maximum (PETM) was unsuspected until the work of Baceta (1996), and has been controversial ever since. After summarizing the history of this controversy, we present information from three new sections that conclusively resolve the issue, all of them placed less than 2 km to the east of the classical Campo section in the southern Pyrenees. In these three sections, an up to 7 meter-thick intercalation of continental deposits rich in pedogenic carbonate nodules is sandwiched between uppermost Thanetian and lowermost Ilerdian shallow marine carbonates. The delta(13)C composition of 42 pedogenic nodules collected from two of these sections (San Martin and La Cinglera) ranges between -11.4 and -14.3% and averages -12.9%, values that conclusively represent the PETM and for the first time are recorded in sections where the LFT is clearly represented. Further, a high-resolution lithological correlation between Campo and the three new sections across the P-E interval unquestionably demonstrates that the lowermost marine beds with autochthonous specimens of Alveolina vredenburgi (a tell-tale of the LFT) are laterally interfingered -and are therefore coeval- with the nodule-bearing PETM continental deposits. On the basis of the new evidence, the temporal coincidence of the PETM and the LFT can no longer be doubted.
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| 10. |
- Pujalte, V., et al.
(author)
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Redefinition of the Ilerdian Stage (early Eocene)
- 2009
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In: Geologica Acta. - 1695-6133. ; 7:1-2, s. 177-194
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Journal article (peer-reviewed)abstract
- The Ilerdian Stage was created by Hottinger and Schaub in 1960 to accommodate a significant phase in the evolution of larger foraminifera not recorded in the northern European basins, and has since been adopted by most researchers working on shallow marine early Paleogene deposits of the Tethys domain. One of the defining criteria of the stage is a major turnover of larger foraminifera, marked by the FO's of Alveolina vredenburgi (formerly A. cucumiformis) and Nummulites fraasi. There is now conclusive evidence that this turnover was coeval with the onset of the Carbon Isotope Excursion (CIE) and, consequently, with the Paleocene-Eocene (P-E) boundary, a temporal correspondence that reinforces the usefulness of the Ilerdian as a chronostratigraphic subdivision of the early Eocene in a regional context. However, in addition to the paleontological criteria, the definition of the Ilerdian was also based on the designation of two reference sections in the southern Pyrenees: Tremp (stratotype) and Campo (parastratotype). In both sections, the base of the stage was placed at the lowest marine bed containing A. vredenburgi specimens. Using the CIE as a correlation tool we demonstrate that these two marine beds occur at different chronological levels, being older in Campo than in Tremp. Further, we show that both beds are in turn younger than the lowest strata with Ilerdian larger foraminifera at the deep-water Ermua section in the Basque Basin (western Pyrenees). Since the age of stage boundaries must be the same everywhere, the choice of these stratotype sections was misleading, since in practice it resulted in the Ilerdian being used as a facies term rather than as a chronostratigraphic unit. To eliminate that conflict, and yet be respectful with established tradition, we propose to redefine the Ilerdian Stage following a procedure similar to the one used by the International Commission on Stratigraphy to establish global chronostratigraphic standards, namely: by using a "silver spike" to be placed in the Tremp section at the base of the Claret Conglomerate, a widespread lithological unit that in the Tremp-Graus Basin coincides with the onset of the CIE. The redefined regional Ilerdian Stage becomes thus directly correlatable to the lower part of the global Ypresian Stage, as currently defined by the International Commission on Stratigraphy.
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