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

Pyroxenites and Megacrysts From Alkaline Melts of the Calatrava Volcanic Field (Central Spain): Inferences From Trace Element Geochemistry and Sr-Nd Isotope Composition

Alkaline volcanic rocks from explosive monogenetic centers often carry an unusual cargo of crystals and rock fragments, which may provide valuable constraints on magma source, ascent and eruption. One of such examples is the Cenozoic Calatrava Volcanic Field in central Spain, a still poorly explored area to address these issues. Clinopyroxene, amphibole and phlogopite appear either as megacryst/phenocrysts or forming fine-grained cumulates (pyroxenite enclaves s.l.) in some eruptive centers of this volcanic field. They have previously been interpreted as cogenetic high-P minerals formed within the upper lithospheric mantle. The presence of Fe-Na-rich green and Mg-Cr-rich colorless clinopyroxene types as phenocryst cores or as oscillatory zoned crystals in pyroxenite enclaves points to a complex evolution of mineral fractionates from petrogenetically related magmas. In trace element chemistry all studied clinopyroxene types show parallel rare earth element patterns irrespective of whether they are megacrysts, colorless or green core phenocrysts, or zoned crystals within pyroxenite cumulates. This similarity indicates a genetic relationship between all the fractionated minerals. This is in agreement with the overlapping of initial 143Nd/144Nd and 87Sr/86Sr ratios of pyroxenite enclaves (0.512793–0.512885 and 0.703268–0.703778) that is within the chemical field of the host magmas and the Calatrava volcanics. The initial 143Nd/144Nd and 87Sr/86Sr ratios of megacrystic clinopyroxene, amphibole and phlogopite show a more restricted range (0.512832–0.512890 and 0.703217–0.703466), also falling within the isotopic composition of the Calatrava volcanic rocks. Deep magmatic systems beneath monogenetic volcanic fields involve several stages of melt accumulation, fractionation and contamination at variable depths. Trace element and isotope mineral chemistry are powerful tools to understand the history of ascent and stagnation of alkaline basaltic magmas and discriminate between magma mixing, wall-rock contamination and closed magmatic system evolution. In our study, we establish a cogenetic origin for green and colorless clinopyroxene as high-pressure precipitates from liquids of different fractionation degrees (up to 80%, for the highly evolved melts equilibrated with the green clinopyroxene), originated from a highly solidified front of silica-undersaturated alkaline magmas at mantle reservoirs

Mineral chemistry of megacrysts and associated clinopyroxenite enclaves in the Calatrava volcanic field: crystallization processes in mantle magma chambers

Megacristales de clinopiroxeno, anfíbol y flogopita aparecen en los depósitos piroclásticos ricos en cristales, enclaves y xenolitos, de los volcanes de El Aprisco y Cerro Pelado (campo volcánico de Calatrava). Estos megacristales muestran una composición química similar a los cristales que forman los enclaves clinopiroxeníticos asociados, incluyendo los poco comunes enclaves ricos en flogopita (glimmeritas). El magma volcánico es de textura porfídica, mostrando una compleja población de fenocristales y macrocristales máficos, con núcleos residuales, que sugieren formen una suite cogenética con aquellos. Las estimaciones geobarométricas indican que los megacristales, así como los núcleos de fenocristales y los enclaves clinopiroxeníticos representan acumulados de alta presión, formados entre 12–16 kbar, en el manto litosférico superior (de 35 a 55 km). La variabilidad composicional de estos minerales máficos apunta a un proceso de diferenciación controlado por la cristalización de olivino, clinopiroxeno, anfíbol y flogopita. La cristalización de minerales máficos hidratados en el manto facilitaría la exsolución de CO2 y la subsecuente ebullición del fundido, posibilitando la fragmentación de los márgenes semicristalinos de la cámara magmática y la excavación de la roca mantélica encajante. Esta fragmentación profunda explicaría también la compleja variedad de cristales, enclaves y xenolitos atrapados por los magmas volcánicos. Se han encontrado dos tipos de clinopiroxenos (verde e incoloro) que aparecen como antecristales (núcleos de macrocristales/fenocristales) y también en los enclaves piroxeníticos. La coexistencia de ambos tipos de clinopiroxeno en los zonados cristalinos de las clinopyroxenitas sugiere que deben ser cogenéticos, representando precipitados de fundidos de distinto grado evolutivo, pero posiblemente de un mismo magma fraccionante. Este estudio propone un modelo de ascenso y origen de magmas ricos en cristales y xenolitos que puede ser útil para explicar otros tipos volcánicos que transportan complejos cargamentos de cristales profundos, como ocurre frecuentemente en la provincia volcánica circum-Mediterránea

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

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

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

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

Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Age and geological setting of the basic Ordovician magmatism from the Spanish Central System

Nuevos datos geoquímicos (elementos mayores, traza e isótopos) y geocronológicos obtenidos en metabasitas intrusivas en las series metasedimentarias y metaígneas pre-Arenigienses del Sistema Central Español confirman la heterogeneidad composicional de estos materiales. Su evolución es de afinidad toleítica, pero con unos contenidos en elementos incompatibles relativamente elevados, lo que las diferencia de magmas tipo N-MORB. Se pueden distinguir dos grupos: 1) uno isotópicamente empobrecido y más diferenciado, y 2) otro isotópicamente más enriquecido, pero que representa líquidos más primitivos y con mayores contenidos de metales (Cr, Ni) y LILE. Estos datos apuntan a la participación de varias fuentes de manto, incluyendo sectores litosféricos con una impronta cortical. Dos muestras han aportado edades U-Pb en circones entre 473 y 453 Ma, lo que las relaciona con el contexto de rifting asociado a la apertura del océano Rheico. En la Zona Centro-Ibérica no hay magmatismo básico Cambro-Ordovícico anterior al que se describe en este trabajo, lo cual podría indicar que el rifting sería un fenómeno más tardío en este sector, en comparación con lo que se observa en otros terrenos del Macizo Ibérico.New geochemical (major, trace and isotopic) and geochronological data obtained on metabasites intrusive into pre-Arenig metasedimentary and metaigneous rocks of the Spanish Central System confirm their heterogeneous composition. They exhibit a tholeiitic affinity, but show relatively high incompatible element contents, contrary to NMORB magmas. Two groups can be distinguished: 1) one isotopically depleted and more evolved and 2) one isotopically enriched, representing more primitive liquids with higher metal (Cr, Ni) and LILE contents. These data point to the involvement of several mantle sources, including lithospheric sections with a crustal imprint. Two samples have provided U-Pb zircon ages in the range 473- 453 Ma, thus related to the rifting context associated with the Rheic ocean opening. The absence of basic magmatism in the Central-Iberian Zone prior to that described in this work, might indicate that rifting was a late event in this region when compared to other terranes from the Iberian Massif.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de Economía y Competitividad (MINECO)pu