Mineral chemistry of late Variscan gabbros from central Spain: constraints on crystallisation processes and nature of the parental magmas

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Petrogenesis of highly fractionated I-type peraluminous granites: La Pedriza pluton (Spanish Central System)

The La Pedriza pluton stands out as the most extensively fractionated granite (Rb lt; 629; Sr lt; 2 and Ba lt; 2 ppm) of the Spanish Central System Batholith. These granites show a strong enrichment in some rare metal contents (Nb = 44, Y = 136, Yb = 10.7, U = 17, Ta = 7, Sc = 15 ppm). The petrography and geochemistry (including Sr- Nd isotopes) reveal that the pluton is composed of at least four units. These are classified as I-type peraluminous leucogranites (A/CNK=1.03-1.17), P-poor (P2O5lt;0.2 wt%) and Na2O-rich (lt; 4.24 wt%) exhibiting differences in their HFSE and REE contents and εNd compositions. Moreover, the units of the La Pedriza granite display different trends of fractional crystallization. REE spectra of the two most fractionated units suggest the involvement of a fluorine-rich melt in the last stages favouring the crystallization of xenotime and niobotantalates. Intermediate meta-igneous granulite protoliths are proposed as source rocks. The most evolved units of the La Pedriza pluton show chemical features convergent to A-type granites; these are explained by extensive fractional crystallization of a P-poor, I-type granite magma

Sobre el stock adamellítico de La Granja y el sector metamórfico encajante (Guadarrama Central)

[Resumen] El stock adamellítico de La Granja (Guadarrama Central) intruye en materiales metamórficos de alto grado (con cordierita-silnmanita-ortosa), provocando neoformación de andalucita. Se describen asímismo, algunas facies litológicas de «contacto» como son los gneises porfidoclásticos o los cinturones aplograníticos de borde del granitoide. Se describen los materiales metamórficos haciendo especial hincapié en los rasgos texturales de los mismos y en el fuerte control geoquímico que ejerce el medio petrológico a las diferentes blastesis minerales. El estudio geoquímico aunado al estructural nos define un carácter ortoderivado a las formaciones glandulares sálicas y leucogneísicas con las que forman Unidad. Además se apuntan pautas de variación geoquímica de los gneises porfidoclásticos de «contacto». El stock se define como adamellita de grano grueso porfídica de tendencias calco-alcalinas y como apéndice de los materiales de grano grueso leucocráticos de la vertiente meridional de la Sierra. Se describen las redes filonianas características del stock encontrando ciertas diferencias composicionales entre ambos materiales. Finalmente encontramos una variación geoquímica, clara en elementos menores, entre las facies de borde e interior del dique[Abstract] La Granja granitoid stock in Centrat Guadarrama (Spain) intrudes in high grade metamorphic rocks (cordierite-sillimanite-ortose) inducing neoformation of andalusite. There are lithological «contact» facies, such as porphyroclastic gneiss or aplogranitic belts of chilled margin.We describe here the metamorphic materials, in special the textural features and the geochemical control of mineral blastesis by the lithology. The geochemical and structural studies determine an orthogenesis for augen and leucocratic gneiss which belong to the same formation. We describe too, some geochemical features of the porphyroclastic gneiss. The porphyritic granitoid stock is adamellitic in composition (O-monzonite), coarse grained and has a calc-alcalic character. La Granja stock is the natural appendix of leucocratic coarse grained granites from the Sierra's southern side. In this paper we deal with the characteristic stock dike-swarm, finding compositional differences between both materials. Finally we find a clear geochemical variation, specially in minor elements, between margin and core in the dike

Igneous differentiation in porphyry dikes

[Resumen] Se aborda de manera teórica los principales procesos operativos de diferenciación ígnea en secciones transversales de los conductos filonianos, aplicándolo a los diques de pórfido granítico s. 1. del Sistema Central Español. Para estos magmas se deduce un régimen de flujo marcadamente laminar en el conducto (:~ 30 mts. potencia). Estos caracteres impiden tanto la efectividad de franccionamiento cristalino «in situ:\), como grados importantes de asimilación o contaminación en ruta hacia la superficie. Los diques simples, de flujos isoviscosos, van a diferenciarse por fenómenos, principalmente, de flujo ígneo, en ocasiones combinado con la presencia de varios magmas fluyendo a la vez en el mismo pulso (conductos gruesos o flujos veloces). Con estos mecanismos se tenderá a generar evoluciones del tipo félsico a máfico, de borde a centro del dique de pórfido. Los diques compuestos se formarán cuando los magmas son suficientemente contrastados en sus caracteres físicos (viscosidad, p. ej.) y químicos, o cuando son varíos pulsos magmáticos a través del mismo conducto. En estos casos podría haber fen6menos de mezcla magmática si la diferencia de viscosidad es alta y uno de esos magmas es poco viscoso (magmas básicos). En estos casos podrían generarse zonas básicas en el borde, o no, de los diques. de pórfido. En resumen la diferenciación de flujo, la mezcla magmática y la asociación de varios líquidos fluyendo en el mismo conducto (de manera simultánea o por pulsos magmáticos algo separados en el tiempo), parecen ser los mecanismos de diferenciación ígnea más efectivos de estos magmas graníticos que fluyen en angostos conductos filonianos[Abstract] We deal in theoretical way wíth the main differentiation proccesses taking place in dike conducts, applied to the porphyry dikes of the Spanish Central System. For these magmas we infer a strong laminar flow regime (~30 m dike witdh). This, prevent an efficient «in situ:\) cristal fractionation as well as large extent of assimilation-contamination processes during the ascent of the magmas «en route» to more surface levels. Simple dikes, with isoviscous flow, could differentiate mainley by igneous flow. Sometimes this mechanism is combinated with the possibility of severa! Magmas flowing togheter on the same pulse (thick dikes or fast flows). With these mechanisms felsic to mafic evolutions are favoured inward the porphyry dike. Composite dikes are formed when magmas are contrasted enough in their chemical or physical features (p. ej. viscosity), or when different magmatic pulses occur in the same channel. In these conditions commingling of magmas could exist if the viscosity ratio is high apd one of the magmas is very fluid (basic one). These processes could generate basic margins in the porphyry dikes. In short, flow differentiation, magmatic mixing and flowing of associated magmas (simultaneously or by successive pulses), seem to be the more efficient differentiation mechanisms for granitic magmas flowing in narrow dikes

Eclogite facies relics in metabasites from the Sierra de Guadarrama (Spanish Central System): P-T estimations and implications for the Hercynian evolution

Relics of HP-MT eclogitic assemblages related to the first metamorphic stage of the Hercynian orogeny in the Sierra de Guadarrama (Spanish Central System, SCS) are preserved as boudins of pre-Ordovician metabasites enclosed by felsic gneisses. Textures indicate a multi-stage metamorphic history starting in the MT eclogite facies (as deduced from the presence of omphacite and rutile included in garnet) and continuing through medium to low pressure granulite and retrograde amphibolite-greenschist facies. Thermobarometric calculations in the eclogitic paragenesis yield pressures of ~14 kbar for temperatures in the range 725 7758C. Thermobarometry for the subsequent granulitic stage indicates a significant drop in pressure (P < 10 kbar) for similar temperatures of ~7508C. Metabasites vary from gabbro to leucotonalites showing the typical Fe enrichment of the tholeiitic series. Chemical characteristics indicate a derivation from low-pressure crystallization of tholeiitic melts more enriched than typical MORB compositions. Their original location far from continental margins as evidenced by the absence of ophiolitic material in the area and their association with platform sediments suggests that eclogitization was related to intracontinental crustal subduction and thickening. The P-T conditions estimated in the metabasites for the first metamorphic stage are similar to ones deduced for the surrounding metasediments and suggest that the Hercynian crust could have reached a thickness of ~70 80 km, which is more than the double the present thickness

Xenolitos peridotíticos del volcán Morrón de Villamayor (Campo Volcánico de Calatrava)

The El Morrón de Villamayor (MVM) peridote xenoliths vary from orthopyroxene-poor lherzolite to wehrlite in modal composition. This compositional feature contrasts with other Calatrava (CVF) xenolith suites. The studied xenoliths equilibrated at lower temperatures (618-942 o C) and slightly shallower (8.8-13.6 kbar) conditions than other CVF peridotites. MVM peridotites show local intense interaction with the host leucitite displaying spongy rims around primary clinopyroxene and also reaction zones with K-rich minerals (e.g., sanidine, leucite and richterite) and widespread clinopyroxene, olivine and spinel neoblasts. Nevertheless, the orthopyroxene-poor character of MVM peridotites might be caused by some previous metasomatic eventLos xenolitos peridotíticos del volcán El Morrón de Villamayor (MVM) son lherzolitas pobres en ortopiroxeno y wehrlitas, en contraste con los datos previos de xenolitos de otros volcanes de Calatrava. Las peridotitas estudiadas están equilibradas a temperaturas más bajas (618-942 o C) y menor profundidad (8,8-13,6 kbar) que las estimadas en otras peridotitas del Campo Volcánico de Calatrava. Localmente hay una intensa interacción del fundido leucitítico con los minerales primarios de la peridotita, ya que se originan zonas de reacción con minerales ricos en K (p.ej., sanidina, leucita y richterita) y frecuentes neoblastos de clinopiroxeno, olivino y espinela. El carácter pobre en ortopiroxeno de las peridotitas MVM debe estar causado por algún evento metasomático previ

Diques lamprofíricos norteados con facies brechoidales eruptivas en la región de La Palamera de Ávila (Sistema Central español)

[Resumen] Según direcciones N-S afloran diques diabásicos y lamprofíricos en los sectores centrales del Sistema Central, estos últimos, muy localmente (Sierra de La Paramera), presentan facies brechoidales eruptivas que podrían explicarse como fenómenos de tipo diatrema, generados en un proceso de fluidizaci6n del magma lamprofírico. Los lamprófiros pueden considerarse términos alcalinos, en cuyas facies brechoidales engloban gran cantidad de xenolitos, tanto de las rocas encajantes (granitos, pórfidos graníticos, apIopegmatitas, ...), como de rocas esquistosas y granul1ticas varias (charnockitas, granoblastitas de granate y rutilo, ... ), de niveles más profundos[Abstract] In the central part oE the Sistema Central thelamprophyre dikes of N-S direction locally show (Sierra de La Paramera) an eruptive breccia facies that ve explain by a fluidization process similar to diatreme types. This lamprophyres, alkaline in composition, contain a lot oE xenoliths in that breccia facies. They are not also wall rocks xenoliths (granites, microgranites, aplopegmatites, ... ) but schistose and granulitic types (foliated or massives with charnockitic character, granoblastites with garnet and rutile, ... ), from deeper level

Nature and Composition of the Lower Continental Crust in Central Spain and the Granulite–Granite Linkage: Inferences from Granulitic Xenoliths

Xenolith-bearing alkaline ultrabasic dykes were intruded into the of the Hercynian basement of the Spanish Central System in early Mesozoic times. The suite of lower-crustal xenoliths in the dykes divided into three groups: felsic peraluminous granulites, metapelitic granulites and charnockitic granulites. The felsic granulites form ~95% of the total volume of the xenoliths, whereas the charnockitic and metapelitic granulites are much less abundant (~0·01 5%, respectively). Thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850–950°C, 7–11 kbar; thus the xenoliths represent lower continental crustal material. Superimposed on this high-T high-P assemblage is a high-T low-P paragenesis represented mainly by kelyphitic coronas, reflecting re-equilibration during transport in the clearly restitic mineral assemblages, with up to 50% garnet and 37% sillimanite. Major and trace element modelling supports the idea that the late-Hercynian peraluminous granites of central Spain represent liquids in equilibrium with restitic material of similar composition to the studied lower-crustal xenoliths. 87Sr/86Sr and eNd of the felsic xenoliths, calculated at an average Hercynian age of 300 Ma, are in the range 0·706–0·712, and –1·4 to –8·2, respectively. These values match the isotopic composition of the outcropping late Hercynian granites. The Sr isotopic composition of the xenoliths is lower than that of the outcropping mid-crustal lithologies (orthogneisses, pelites). A major contribution from the lower crust to the source of Hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis. The felsic nature of the lower continental crust in central Spain contrasts with the more mafic lower-crustal composition estimated in other European Hercynian areas, suggesting a non underplated crust in this region of the Hercynian orogenic belt