The Variscan gabbros from the Spanish Central System: A case for crustal recycling in the sub-continental lithospheric mantle?

The gabbroic intrusions that crop out along the Spanish Central System (SCS) are geochemically heterogeneous, including primitive and evolved rocks. Differentiation is mainly related to fractionation of Cr-spinel and olivine, but mixing with coeval granitic magmas or crustal assimilation may have also played a role in the evolution of the most differentiated rocks. The most primitive uncontaminated gabbros show arclike trace element chondrite and primitive-mantle normalised patterns, characterised by large ion lithophile elements (LILE)-light rare earth elements (LREE) enrichment, Sr and Pb positive and Nb–Ta–Ti negative anomalies. However, paleogeographic constraints suggest that the SCS was located far from subduction zones, so these geochemical signatures could be better explained by a recycling of continental crustal components within the mantle. The most primitive SCS gabbros expand the Sr–Nd isotopic compositional range of the Variscan basic magmatism in the Central Iberian Zone to more depleted values. This reflects a heterogeneous sub-continental lithospheric mantle under central Spain ranging from a depleted mantle (εNd=+3.1, 87Sr/86Sr=0.704) towards an isotopically enriched component (εNd=−1.6, 87Sr/ 86Sr=0.706). Geochemical modelling suggests that mantle enrichment could be explained by minor lower crustal metapelitic granulite contamination (~2%). Additionally, the Sr–Nd–Pb isotopic ratios of the most primitive gabbros match the composition of the European subcontinental lithospheric mantle recorded in ultramafic xenoliths from western and central Europe

Early Proterozoic zircons in Variscan gabbros from central Spain: Evidence of an Icartian magmatic event at mantle depths?

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Electron microprobe monazite geochronology of granitic intrusions from the Montes de Toledo batholith (central Spain)

U–Th–Pb monazite dating by electron microprobe has been applied to three peraluminous granitic intrusions of the western Montes de Toledo batholith (MTB). Back scattered electron images of monazite crystals reveal a variety of internal textures: patchy zoning, overgrowths around older cores and unzoned crystals. On the basis of their zoning pattern and chemical composition, two monazite domains can be distinguished: (1) corroded cores and crystals with patchy zoning, exhibiting relatively constant Th/U ratios and broadly older ages, and (2) unzoned grains and monazite rims, with variable Th/U ratios and younger ages. The first monazite group represents inherited domains from metamorphic sources, which accounts for pre-magmatic monazite growth events. Two average ages from Torrico and Belvís de Monroy granites (33318 and 3335 Ma, respectively) relate these cores to a Viséan extensional deformation phase. The second group represents igneous monazites which have provided the following crystallization ages for the host granite: 29811 Ma (Villar del Pedroso), 3036Ma (Torrico) and 3143Ma (Belvís de Monroy). Two main magmatic pulses, the first about 314Ma and the second at the end of the Carboniferous (303–298Ma), might be envisaged in the western MTB. While Belvís de Monroy leucogranite is likely a syn- to late-tectonic intrusion, the Villar del Pedroso and Torrico plutons represent post-tectonic magmas with emplacement ages similar to those of equivalent intrusions from nearby Variscan magmatic sectors

Hercynian gabbroic intrusions from the Spanish Central System: Constraints on mantle composition under central Spain

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

"Geolodía 13 Madrid": excursión al plutón granítico de La Cabrera

El pasado 11 y 12 de mayo se celebró el Geolodía 2013 con diferentes actividades, normalmente excursiones abiertas a todo tipo de público, para impulsar la divulgación de la geología, dar a conocer el variado patrimonio geológico y concienciar a la población de la necesidad de protegerlo

Fomento de la empleabilidad y el emprendimiento entre los alumnos de Grado en Geología

En la actualidad, el acceso al mercado laboral constituye un reto importante para un buen número de titulados universitarios. Esto se hace especialmente patente en aquellos sectores que, como la obra pública, el medio ambiente, la docencia o la investigación, han resultado fuertemente afectados por la crisis económica. Es el caso de los egresados del Grado en Geología, un colectivo que tradicionalmente encontraba acomodo profesional en algunos de estos campos y que se ha visto sumido en una situación de baja empleabilidad en España a lo largo de los últimos años. Esto ha redundado en que muchos de los geólogos de reciente titulación se han visto forzados a emigrar al extranjero. En tiempos recientes se ha observado una incipiente recuperación del mercado laboral nacional, lo que se viene traduciendo en más oportunidades de empleo para los geólogos. Sin embargo, entre los titulados a menudo se observa la ausencia de determinadas destrezas que resultan importantes a la hora de buscar trabajo, así como una cierta desorientación ante la realidad del mercado laboral. Este proyecto tiene por objeto contribuir a subsanar dichas carencias, fomentando con ello la empleabilidad de los egresados.Depto. de Mineralogía y PetrologíaDepto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasFALSEsubmitte

Contrasting crustal sources for peraluminous granites of the segmented Montes de Toledo Batholith (Iberian Variscan Belt)

The Variscan Montes de Toledo Batholith (MTB) is an E–W linear array of peraluminous granite plutons which is chemically segmented. The study is focused on the western segment of the MTB (W-MTB), mainly composed of granites with slightly lower CaO and higher P2O5 contents than associated eastern plutonic units and nearby S-type granites, giving them a more pronounced peraluminous nature. The chemical contrast is also observed in isotopic composition, especially in radiogenic Nd and Pb ratios. The W-MTB granites have higher initial εNd (–5.0 to –5.9) and lower 206Pb/204Pb and 208Pb/204Pb ratios than peraluminous types from the E-MTB segment. A mixed pelitic–greywackeous derivation from regional Neoproterozoic formations is suggested, whereas lower crustal and meta-igneous sources were involved in the origin of the easternmost MTB granites. The presence of igneous muscovite together with coexisting andalusite and sillimanite in some of the studied granites suggests that solidus was reached at 650–700 ºC and depth corresponding to the pressure of 2–3 kbar

Material audiovisual para el aprendizaje de métodos y conceptos básicos en Geología

El proyecto se centra en la elaboración de vídeos en los que se resuelven problemas prácticos relacionados con conceptos básicos de Geología General, principalmente relacionados con mapas topográficos y mapas geológicos. El objetivo es que estos vídeo-tutoriales sirvan de apoyo a los alumnos de los primeros cursos de las titulaciones de Grado en Geología y Grado en Ingeniería Geológica, para el estudio de los contenidos y ejercicios prácticos

Geochemistry and geochronology of mafic rocks from the Spanish Central System: Constraints on the mantle evolution beneath central Spain

The Spanish Central System (SCS) contains several suites of Palaeozoic mafic igneous intrusions with contrasting geochemical affinity: Ordovician tholeiitic metabasites, Variscan calc-alkaline gabbros (Gb1) and microdiorites (Gb2), shoshonitic monzogabbros (Gb3) and alkaline diabases and lamprophyres (Gb4). Not all of these rocks are accurately dated, and several aspects of their genesis are still poorly understood. We present new whole-rock geochemical data (major and trace elements, and Sr–Nd isotopes), U–Pb and Lu–Hf isotopic ratios on magmatic zircons and 40Ar/39Ar amphibole geochronology results in order to establish a precise chronology for the successive events of magmatism in the SCS, and discuss the nature of their mantle sources. Accurate ages have been determined for the Variscan gabbros (305–294 ​Ma), the microdiorites (299 ​Ma) and the accompanying felsic porphyries (292 ​Ma), the shoshonitic monzogabbros (285 ​Ma), and the alkaline diabases (274 ​Ma) and monzosyenites (271–264 ​Ma). According to this information, the Variscan mafic magmatism would be mainly concentrated in the range of 305–294 ​Ma, with a final manifestation represented by the minor shoshonitic dykes. The alkaline magmatism proved to be slightly older than previously thought and yielded at least two distinct pulses: diabases and lamprophyres–monzosyenites. Zircon Hf isotopes evidence the involvement of depleted and slightly enriched mantle sources. The bulk of the εHf values are in the broad range of −8 to +11, indicative of melting both depleted and enriched mantle regions. The high within-sample Hf isotope variation (up to ~11 epsilon units) shown by samples from the Variscan series (gabbros, microdiorites and monzogabbros) could be explained mainly by hybridisation of magmas derived from heterogeneous lithospheric mantle sources. Pressure estimates indicate that the Variscan mafic magmas were extracted from the lithosphere. The Nd–Hf isotopic composition of these suites of rocks suggests the recycling of pelitic sediments during the Cadomian orogeny. Deeper (asthenospheric) mantle levels were involved in the generation of the alkaline suite, whose anomalous negative εHf values (moderately decoupled with respect to radiogenic Nd) could be associated with subducted oceanic components raised by mantle upwelling associated with lithosphere thinning and extension during the Permian