Alkaline metasomatism in hercynian granitoids from the Sierra de Guadarrama (Spanish Central Range)

Se describen dos casos particulares de granitos decuarcificados y alcalinizados a favor de planos de fractura, Los afloramientos son pequeños lentejones (menos de 5 m de potencia) de escasa continuidad longitudinal (menos de 200 m) y tránsitos centimétricos con el granito sin transformar, En el plut6n de Villacasún estos tipos episieníticos aparecen en fracturas N50E, definiendo afloramientos con un complejo bandeado composicional cuyos tipos más transformados están constituidos por plagioclasa albitizada, clinopiroxeno ferroaugítico y ferrohornblenda normalmente asociada a microclina intersticial. Las episienitas del complejo plut6nico de La Pedriza se alinean según fracturas N85E, y son transformación hidrotermal del plutón de granito porfídico con cordierita, de la zona intermedia de dicho complejo, Estas rocas llegan a presentar una mineralogía de composición peralcalina, con albita, microclina residual, egirina, riebeckita y escaso cuarzo como fases principales. Las episienitas de Villacastín tienen una paragénesis de grado medio metamórfico y baja presión (se estima una temperatura aproximadamente 450°-500°C y una presión menor de 0,6 kb), mientras que las de La Pedriza serían de menor temperatura, en condiciones de grado bajo metamórfico (temperaturas de aproximadamente 250°-450°C y una presión menor de 0,5 kb), Estos tipos de episienitas anfibólico-piroxénicas no son conocidos en otras áreas del Hercínico ibéricoThis paper describes two particular cases of granites which are dequartzified and alkalinified along their fractures planes, The outcrops are small and lenticular, shaped « S m width) with scatce longitudinal continuity « 200 m length) and centimetrical transits with the not transformated granite. In the pluton of Villacastín this piesienite types appeat in fractures NSOE defining outocrops with a complex compositional banding whose more transformated varieties ate composed by albitized plagioclase, ferroaugite and ferrohornblende, usually associated with interstitial microcline. The episienites of the plutonic complex of La Pedriza ate aligned in N8SE fractures, and they ate an hydrothermal transformation of the porphiritic cordierite granites of the intermediate zone of this complexo These rocks reach a peralkaline mineralogy with albite, residual microcline, aegirine, riebeckite and lirnited quanz as main phases. The episienites of the Villacastín have a middle metatnorphic grade patagenesis of low pressure (an estimated temperatures of 4S0o-S00.oC and pressures les than 0.6 kb), whereaS the ones ofla Pedriza ate of lower temperature, i. e. low metamorphic grade (temperatures of 2S0o-4S0°C and a pressure less than 0.5 kb). These pyroxenic-amphibolitic episienites have not been described in others ateas of the Iberian Hercynian BeltDepto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasFALSEpu

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 event.Los 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 previoDepto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de Ciencia e Innovación (MICINN)Universidad Complutense de Madridpu

Microbial weathering of Fe-rich phyllosilicates and formation of pyrite in the dolomite precipitating environment of a Miocene lacustrine system

Sedimentary sequences composed of selenite gypsum, mudstone, and dolomite microbialites cropping out in the eastern part of the Madrid Basin were deposited in a mudflat – saline-lake system during the Miocene. In some dolomite beds enriched in detrital grains, dolomite crystals occur intergrown with framboidal pyrite clusters that in many cases are arranged along the associated phyllosilicate cleavages, which accounts for the mineral disaggregation, and eventually for the mineral replacement. Highresolution FE-SEM investigations across the interface between the pyrite and the phyllosilicate show that a single phyllosilicate is replaced by iron sulphides in different microsites, with retention of some aspects of the parent structure. Pyrite formed in the sediment as texturally complex framboids, coexisting with amorphous to poorly crystalline iron-rich sulphide phases, and include carbonaceous filaments with biogenic isotopic signatures (δ 13 C = -22.95 ‰). Microprobe analytical data indicate the weathered phyllosilicates to be significantly depleted in Fe relative to Si and Al. The selective release of Fe has not involved the mineral transformation to secondary phyllosilicates, which suggests a preferential microbial colonization of the Fe-bearing minerals rather than a thermodynamically driven degradation of those minerals. Depleted δ 34 S values in pyrite further suggest that sulphate microbial reduction to sulphide was active within the sediment during the alteration of the silicates. This paper gives new insight into the microbial weathering of phyllosilicates and the subsequent formation of pyrite through a sequence of intermediate products enriched in iron. These processes were coeval with the microbially-mediated precipitation of dolomite, which further reinforces the role of the microbes in the formation of the sulphides

Mineralogical characterization of peridotite xenoliths from the Los Tormos volcano (Calatrava volcanic field)

Los xenolitos de peridotitas del volcán Los Tormos (campo volcánico de Calatrava) proporcionan información valiosa sobre el manto litosférico del centro de España. Se estima que provienen de profundidades de 35-50 km (10-15.2 kbar), equilibrados a temperaturas de 961-1185 ºC, en el campo de estabilidad de peridotitas con espinela. Los modelos de fusión obtenidos a partir de la composición química de los minerales primarios de la peridotita (ortopiroxeno, clinopiroxeno y espinela) muestran un bajo grado de fusión parcial (hasta un 10%), aunque una harzburgita indica un mayor grado de fusión (cerca del 15%). El xenolito de wehrlita se interpreta como un tipo metasomatizado debido a la posible interacción con fundidos alcalinos subsaturados en sílice que eemplazarían el ortopiroxeno primario de la lherzolita. La similitud entre las presiones estimadas para el equilibrio mineral de la peridotita y la cristalización de megacristales/fenocristales de clinopiroxeno en la nefelinita sugiere la formación de cámaras magmáticas mantélicas.Peridotite xenoliths from the Los Tormos volcano Calatrava volcanic field) provide significant information about the lithospheric mantle of Central Iberia. Studied samples are classified as spinel-bearing lherzolite, harzburgite and wehrlite. Chemical composition of olivine and two pyroxenes shows similar Mg# ranges, providing evidence of mineral equilibrium. Geothermobarometic estimates give depths in the spinel peridotite stability field (35-50 km, 10-15.2 kbar), and temperatures of 961-1185 °C. Major element mineral melting models in the lherzolite xenoliths record a low partial melting degree (up to 10%) although one harzburgite indicates a higher degree of melting (about 15%). The wehrlite sample could result from metasomatism by interaction with alkaline silica-undersaturated melts that replaced primary lherzolite orthopyroxene. Similar estimated pressures for peridotite mineral equilibration and for clinopyroxene megacryst/phenocryst crystallization in the host nephelinite suggest the formation of mantle magma chambers, where alkaline magmas entrained megacrysts (cumulates) and peridotite wall-rocks.Ministerio de Ciencia e Innovación (MICINN)Universidad Complutense de Madrid/Banco de SantanderDepto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Geological, geophysical and geochemical structure of a fault zone developed in granitic rocks: Implications for fault zone modeling in 3-D

The structure of a fault zone developed in granitic rocks can be established on the basis of the spatial variability of geological, geophysical and geochemical parameters. In the North Fault of the Mina Ratones area (SW Iberian Massif, Spain), fault rocks along two studied traverses (SR-2 and SR-3 boreholes) exhibit systematic changes in mineralogy, geochemistry, fabrics and microstructures that are related to brittle deformation and alteration of granite to form cataclasite and subsequent gouge. The spatial distribution and intensity of these changes suggest a North Fault morphology that is consistent with the fault-core/damage-zone model proposed by Chester et al. (1993) to describe a fault zone architecture. North Fault damage zone thickness can be defined by the development of mechanically related mesoscopic faults and joints, that produce a Fracture Index (FI)>10. High FI values are spatially correlated with relative low seismic velocity zones (VP<5 km/s and VS<2.5 km/s in the well-logs), more probably related to a high concentration of fractures and geochemical alteration produced by meteoric water-granite interaction along fault surfaces. This correlation is the base of a geostatistical model proposed in the final part of this study to image the fault zone architecture of a granitic massif

Depleted lherzolite xenoliths from the leucititic Morrón de Villamayor volcano (Calatrava volcanic field, Spain)

This work reports petrographical, mineralogical and geochemical data of a suite of mantle xenoliths from the Morrón de Villamayor volcanic centre (MVM), which belongs to the Calatrava Volcanic Field (CVF), a part of the Circum-Mediterranean Anorogenic Cenozoic Igneous Province. The MVM volcano is the only centre belonging to the first stage of the CVF magmatism, an ultrapotassic stage that provides a sampling of the subcontinental lithospheric mantle beneath central Spain. The MVM peridotites record a variable host melt interaction (spongy texture and reaction zones), more intense toward xenolith-host leucitite contacts that it was formed during transport within the volcanic melt. The studied mantle xenoliths are spinel lherzolites equilibrated at lower temperatures (618–942 °C) and similar pressure ranges (8.8–13.6 kbar) than other studied CVF mantle xenoliths, and record a lithospheric mantle before the main volcanic Calatrava event. The MVM lherzolites could be classified in two groups by their chondrite-normalized rare earth element (REE) clinopyroxene patterns: group-1 lherzolites show flat pattern, and group-2 lherzolites display a LREE-depleted (N-MORB like) pattern. Group-1 lherzolites represent a more residual lithospheric mantle (up to 9 partial melting %) than group-2 lherzolites (~ 5%), the latter with similar partial melting degrees than other mantle xenoliths from the CVF. Moreover, group-1 lherzolites have undergone a post partial melting refertilization by different metasomatic agents: silicate and alkaline silicate melts different in composition to host leucitite. A primary clinopyroxene concentrate from these MVM lherzolites provides high Sr (87Sr/86Sr = 0.706104) and intermediate Nd (143Nd/144Nd = 0.512830) initial radiogenic ratios, not previously recorded in this volcanic field. This sampling proves the presence of an enriched isotope mantle pole in xenolith suites of the Calatrava area, trending in composition to an intermediate DMM-EMII component

Metasomatism in the lithospheric mantle beneath the Calatrava volcanic field: constraints from peridotite xenoliths of the Cerro Gordo volcano

The Calatrava volcanic field (CVF) comprises about 250 volcanic centers in an area of ~5000 km2. Detailed studies on mantle xenolith suites have been focused on the Cerro Pelado (Villaseca et al., 2010, and referencestherein) and the El Aprisco (González-Jiménez et al., 2014) volcanoes, and on other CVF edifices (e.g., Andía et al., 2018; García Serrano et al., 2019). The Cerro Gordo (CG) volcano is a composite edifice made by a scoria cone with short lava flows, which was partially destroyed during an explosive stage generating a maar with pyroclastic surge deposits. Some lenticular cm-sized mantle fragments appear dispersedly within these pyroclastic layers. All the sampled peridotite xenoliths are spinel lherzolites, most of them having amphibole in accessory modal amounts. No previous reference on mantle xenoliths was known from this volcano

Peridotite and pyroxenite xenoliths from the El Palo volcano (Calatrava Volcanic Field, Spain)

El centro volcánico de El Palo contiene varios tipos de xenolitos ultramáficos (peridotitas y piroxenitas) que registran diversos procesos ocurridos en el manto litosférico. Estos enclaves están equilibrados a temperaturas altas (1072– 1127ºC) y presiones muy variadas (7.3–13.8 kbar), dependiendo del tipo de enclave muestreado. El conjunto de xenolitos equilibrados a mayor profundidad son peridotíticos. Consisten en dos grupos de lherzolitas y una werhlita, que, en este orden, muestran gradualmente más enriquecimiento en Fe-Ti y patrones más fraccionados de REE normalizados al condrito. Los enclaves clinopiroxeníticos representarían acumulados fraccionados de magmas basálticos alcalinos a distintos niveles del manto y equilibrados en el límite con la corteza. La websterita, con mayor contenido en Al2O3 y menor en Cr2O3, está equilibrada a mayor profundidad que las clinopiroxenitas y su origen podría estar relacionado con magmas más subalcalinos.The El Palo volcano contains several groups of ultramafic (peridotite and pyroxenite) xenoliths that record enrichment processes within the lithospheric mantle. These mantle xenoliths are equilibrated at high temperatures (1072–1127ºC), though the pressures are very varied (7.3-13.8 kbar) depending on the xenolith type sampled. Xenoliths equilibrated at greater depth are mostly peridotites. They comprise two lherzolite groups and one werhlite sample, gradually more enriched in Fe-Ti and correlated with more fractionated chondrite-normalized REE patterns. Clinoyroxenites represent cumulates fractionated from alkaline basaltic magmas at shallower mantle depths and equilibrated near the crust-mantle boundary. The websterite is equilibrated at a greater depth than clinopyroxenites and its origin may be related to subalkaline melts.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEMinisterio de Ciencia e Innovación (MICINN)Universidad Complutense de Madridpu

"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