Carboniferous and Permian magmatism in Scotland

Extensional tectonics to the north of the Variscan Front during the Early Carboniferous generated fault-controlled basins across the British Isles, with accompanying basaltic magmatism. In Scotland Dinantian magmatism was dominantly mildly alkaline-transitional in composition. Tournaisian activity was followed by widespread Visean eruptions largely concentrated within the Scottish Midland Valley where the lava successions, dominantly of basaltic-hawaiitic composition, attained thicknesses of up to 1000 m. Changing stress fields in the late Visean coincided with a change in the nature of the igneous activity; subsequently, wholly basic magmatism persisted into the Silesian. As sedimentary basin fills increased, sill intrusion tended to dominate over lava extrusion. In the Late Carboniferous (Stephanian) a major melting episode, producing large volumes of tholeiitic magma, gave rise to a major dyke swarm and sills across northern England and Scotland. Alkali basaltic magmatism persisted into the Permian, possibly until as late as 250 Ma in Orkney. Geochemical data suggest that the Carboniferous-Permian magmas were dominantly of asthenospheric origin, derived from variable degrees of partial melting of a heterogeneous mantle source; varying degrees of interaction with the lithosphere are indicated. Peridotite, pyroxenite and granulite-facies basic meta-igneous rocks entrained as xenoliths within the most primitive magmas provide evidence for metasomatism of the lithospheric mantle and high-pressure crystal fractionation

A joint geochemical–geophysical record of time-dependent mantle convection south of Iceland

The North Atlantic V-Shaped Ridges (VSRs) provide a spatially extensive and clear record of unsteady mantle convective circulation over >40 My>40 My. VSRs are diachronous ridges of thick crust formed with a periodicity of ∼5 My∼5 My along the Mid Atlantic Ridge, south of Iceland. We present data from a set of dredged basalt samples that shows chemical variation associated with two complete VSR crustal thickness cycles where they intersect the Mid Atlantic Ridge. The new dataset also records chemical variation associated with a VSR crustal thickness cycle along a plate spreading flow-line. Inverse correlations between crustal thickness and both incompatible trace element concentrations and incompatible element ratios such as Nb/Y and La/Sm are observed. Geochemical and crustal thickness observations can be matched using a time-dependent mid-ocean ridge melting model with a basal boundary condition of sinusoidally varying potential temperature. Our observations and models suggest that VSRs are generated when hot patches are carried up the plume stem beneath SE Iceland and spread radially outward within the asthenosphere. These patches are then drawn upward into the melting region when passing beneath the Mid Atlantic Ridge. The geometry of the VSRs and the size of the dynamically supported swell suggest that the Iceland Plume is the strongest plume in the Earth at present, with a volume flux of View the MathML source49±14 km3yr−1

Una Visión General del Sistema Financiero Colombiano

Desde finales de los 80 el sistema financiero colombiano ha experimentado cambios sensibles. En efecto, la liberalización financiera, el fortalecimiento de la regulación prudencial, la conversión de un número importante de sociedades en establecimientos de crédito, el aumento en los requisitos de capital, etc. han determinado un cambio de perfil en el sistema. Adicionalmente, en el pasado reciente las autoridades han tomado medidas en cuanto a la estructura de los encajes, aumentos en los requisitos de capital, el acceso al crédito externo, etc. que afectan de manera importante a las entidades financieras. A raíz de estas medidas ha surgido un debate acerca del tipo de sistema financiero que resulta más deseable para Colombia. La discusión es de vital importancia puesto que la estructura de encajes, las formas de intervención del Banco de la República en los mercados cambiario y monetario, la supervisión y todo el aparato regulatorio deben ser consistentes con el tipo de sistema que se desee. Con el fin de contribuir al debate, en este documento se presenta una breve descripción del estado actual del sistema financiero y su evolución reciente, se plantea una reflexión normativa acerca del tipo de sistema financiero que puede resultar más deseable y, finalmente,se presentan algunas recomendaciones.

Triple oxygen isotopic composition of the high-³He/⁴He mantle

Measurements of Xe isotope ratios in ocean island basalts (OIB) suggest that Earth’s mantle accreted heterogeneously, and that compositional remnants of accretion are sampled by modern, high-3He/4He OIB associated with the Icelandic and Samoan plumes. If so, the high-3He/4He source may also have a distinct oxygen isotopic composition from the rest of the mantle. Here, we test if the major elements of the high-3He/4He source preserve any evidence of heterogeneous accretion using measurements of three oxygen isotopes on olivine from a variety of high-3He/4He OIB locations. To high precision, the Δ17O value of high-3He/4He olivines from Hawaii, Pitcairn, Baffin Island and Samoa, are indistinguishable from bulk mantle olivine (Δ17OBulk Mantle − Δ17OHigh 3He/4He olivine = −0.002 ± 0.004 (2 × SEM)‰). Thus, there is no resolvable oxygen isotope evidence for heterogeneous accretion in the high-3He/4He source. Modelling of mixing processes indicates that if an early-forming, oxygen-isotope distinct mantle did exist, either the anomaly was extremely small, or the anomaly was homogenised away by later mantle convection. The δ18O values of olivine with the highest 3He/4He ratios from a variety of OIB locations have a relatively uniform composition (∼5‰). This composition is intermediate to values associated with the depleted MORB mantle and the average mantle. Similarly, δ18O values of olivine from high-3He/4He OIB correlate with radiogenic isotope ratios of He, Sr, and Nd. Combined, this suggests that magmatic oxygen is sourced from the same mantle as other, more incompatible elements and that the intermediate δ18O value is a feature of the high-3He/4He mantle source. The processes responsible for the δ18O signature of high-3He/4He mantle are not certain, but δ18O–87Sr/86Sr correlations indicate that it may be connected to a predominance of a HIMU-like (high U/Pb) component or other moderate δ18O components recycled into the high-3He/4He source

Geochemical evolution of Dalradian metavolcanic rocks : implications for the break-up of the Rodinia supercontinent

Neoproterozoic basaltic magmatism in the Dalradian Supergroup of Scotland and Ireland was associated with the break-up of the Rodinia supercontinent. Magmas were erupted in rift-related basins along a strike length of at least 700 km and during a time period of c. 80 Ma. New major and trace element analyses of metabasalts from several formations are presented to trace the variations in magma compositions in time and space. The primary magmas resulted from variable degrees of mixing of melts derived from mantle sources similar to those of normal and enriched mid-ocean ridge basalts; some younger lavas also show evidence of contamination with continental crust. In contrast to speculations about magmatism elsewhere in Rodinia, the evidence here suggests that there was no involvement of a mantle plume in basalt generation. For example, the Scottish promontory of Laurentia drifted rapidly southwards through c. 25° over the duration of the magmatism, with no evidence of significant elevation above sea level, as might be expected from involvement of a plume. Generation of the primary magmas might have taken place predominantly through decompression melting in depleted upper mantle containing enriched streaks and blobs. Both the Dalradian lithostratigraphy and the metabasaltic compositions are consistent with extreme lithospheric stretching and possibly rupture during the earliest phase of magmatism, whereas generation of later magmatism appears to have been associated with major fault systems, possibly on a foundering continental margin. Supplementary material: Chemical analyses of Dalradian metavolcanic rocks (major elements recalculated to 100%, anhydrous) are available at www.geolsoc.org.uk/SUP18468

Petrogenesis of late stage magmatism at Hold with Hope, East Greenland

The Myggbukta caldera complex and a swarm of basic dykes constitute the latest Tertiary magmatism in the Hold with Hope region, East Greenland. The Sr and Nd isotope ratios of these rocks show coherent variations which extend to high 87Sr/86Sr and low 143Nd/144Nd values and require a contribution from continental lithosphere. Broad correlations with major element differentiation indices suggest that the continental component was incorporated during magmatic differentiation thereby favouring a crustal contamination process. Trace element concentrations are strongly correlated with isotopic compositions but display ranges for many incompatible elements which extend beyond likely crustal contaminant compositions. This is readily modelled by AFC processes in which the dominant cause of trace element enrichment is the concentration effect of fractional crystallisation rather than the composition of the contaminant. The simplest such models still require unrealistically high degrees of fractional crystallisation to explain the ten-fold enrichment of some trace elements. This can be overcome if the primary magmas entering the crust already had highly variable trace element compositions. Such variability is readily achieved if melts from different parts of the melting column escape without thorough homogenization. An AFC model which incorporates variability in parental magma composition is then able to simulate the range of compositions observed at Hold with Hope. This carries the implication that the variations observed are more readily attributed to changes in uncontaminated parental magma than to variations in the composition or amount of contaminant