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

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

Caracterización de los minerales del dique gabroideo de Messejana - Plasencia en su sector nor-oriental

In the gabbroic Messejana–Plasencia dyke we have distinguished three types of pyroxenes: enstatite, pigeonite and augite. Enstatite and augite have been the first minerals that have crystallized at high T (1100ºC – 1150ºC). Pigeonites (@1000ºC) are later and finally appear symplectic two-pyroxene intergrowths around the orthopyroxene. Plagioclase have a continuous zonation with cores of An75 towards rims of An18. Occasionally, there is a micrographic intergrowth of orthoclase–quartz or albite-quartz interstitially to the plagioclase and pyroxene framework. Three types of amphibole have been distinguished: ferroedenite, magnesiohornblende and ferroactinolite. The estimated temperature range of amphibole crystallization is low suggesting that most amphiboles are subsolidus. Finally, biotite are FeO and TiO2 rich, its crystallization being late when residual liquids were enriched in those elements

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

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