The Hospitais Massif is an elliptical-shaped plutonic body, with a WNW-ESE long axis, located in the western Ossa-Morena Zone domains in Portugal. This massif, composed essentially of tonalites, is part of
an igneous association, ranging from gabbros to granites, intrusive in a gneissic-migmatitic complex, as part of the Évora High-grade Metamorphic Terrains. The tonalites present magmatic fabrics associated
with a moderately developed foliation, recording syntectonic crystallization and cooling, co-planar to steep foliations observed within the surrounding sheared gneisses and migmatites. From field and
petrographic studies, it is possible to suggest that the deformation of the tonalites and associated basic rocks was synchronous with Variscan progressive shearing of the gneisses and migmatites envelope.El Macizo de Hospitais corresponde a un plutón elíptico de orientación ONO-ESE localizado en los domínios occidentales de la Zona de Ossa Morena, en Portugal. Este macizo, constituido por una asociación ígnea
que incluye desde gabros hasta granitos y en la que son dominantes las tonalitas, está intruida en un complejo de gneises y migmatitas perteneciente a los Terrenos de Alto-grado Metamórfico de Évora. Las
tonalitas muestran fabricas magmáticas asociadas a una foliación moderadamente desarrollada durante la cristalización y enfriamiento, paralelas a la foliación subvertical observada en los gneises y migmatitas del encajante. Las evidencias de campo y los estudios petrográficos permiten sugerir que la deformación de las tonalitas y rocas básicas asociadas que afloran cerca de Montemor-o-Novo, fue contemporánea con el
progresivo cizallamiento Varisco observado en los gneises y migmatitas del encajante

Moita, P.

Pereira, M. F.

Santos, J. F.

Repositório Institucional da Universidade de Aveiro

51Tonalites from the Hospitais Massif (Ossa-Morena Zone, SWIberian Massif, Portugal)I: Geological setting and petrographyTonalitas del Macizo de Hospitais (Zona de Ossa Morena, SO Macizo Ibérico, Portugal).I: Encuadre geológico y petrografíaP. Moita (1), M.F. Pereira (1) y J.F. Santos (2)(1) Grupo de Tectónica- Centro de Geofísica de Évora, Dept. Geociências, Universidade de Évora, Apt 94, 7002 –554 Évora, Portugal(2) Departamento Geociências, Universidade de Aveiro, 3810-193 Aveiro, PortugalABSTRACTThe Hospitais Massif is an elliptical-shaped plutonic body, with a WNW-ESE long axis, located in thewestern Ossa-Morena Zone domains in Portugal. This massif, composed essentially of tonalites, is part ofan igneous association, ranging from gabbros to granites, intrusive in a gneissic-migmatitic complex, aspart of the Évora High-grade Metamorphic Terrains. The tonalites present magmatic fabrics associatedwith a moderately developed foliation, recording syntectonic crystallization and cooling, co-planar tosteep foliations observed within the surrounding sheared gneisses and migmatites.  From field andpetrographic studies, it is possible to suggest that the deformation of the tonalites and associated basicrocks was synchronous with Variscan progressive shearing of the gneisses and migmatites envelope.Key words: tonalites, gabbros, dioritic enclaves, Variscan orogeny, Ossa-Morena ZoneRESUMENEl Macizo de Hospitais corresponde a un plutón elíptico de orientación ONO-ESE localizado en los domíniosoccidentales de la Zona de Ossa Morena, en Portugal. Este macizo, constituido por una asociación ígneaque incluye desde gabros hasta granitos y en la que son dominantes las tonalitas, está intruida en uncomplejo de gneises y migmatitas perteneciente a los Terrenos de Alto-grado Metamórfico de Évora. Lastonalitas muestran fabricas magmáticas asociadas a una foliación moderadamente desarrollada durante lacristalización y enfriamiento, paralelas a la foliación subvertical observada en los gneises y migmatitas delencajante. Las evidencias de campo y los estudios petrográficos permiten sugerir que la deformación delas tonalitas y rocas básicas asociadas que afloran cerca de Montemor-o-Novo, fue contemporánea con elprogresivo cizallamiento Varisco observado en los gneises y migmatitas del encajante.Geogaceta, 37 (2005), 51-54ISSN: 0213683XIntroductionGeologically, south-central Portugal ispart of SW Iberian Massif where is locatedthe Ossa-Morena Zone (OMZ), one of themajor geotectonic divisions of the westernEuropean Variscan Chain (Julivert et al.,1972). One important characteristic of theOMZ is the presence of distinguishablestructural, paleogeographic and magmatic-metamorphic domains, as recognized byChacón et al. (1983), Apalategui et al. (1990)and Oliveira et al., (1991). In the western Ossa-Morena Zone exists a large area coveringapproximately 75 km2 (the Évora Massif; e.g.Carvalhosa, 1983; Quesada and Munhá, 1990)where the magmatism and high-grademetamorphism related with the Variscanorogeny is well exposed. Recently, with thepurpose of clarifying the complex internalstructure of the Évora Massif, Pereira et al.(2003) defined the Évora High-GradeMetamorphic Terrains (EHMT) as a majortectonic unit composed by a complexassemblage of migmatites and gneisses,intruded by syn-orogenic basic to acidplutonic rocks and late-orogenicporphyritic granites. The same authorsinterpret these high-grade metamorphic rocksas the result of Variscan HT-LP metamorphismdeveloped over Neoproterozoic (Serie Negra)and Lower Palaeozoic rocks.The Hospitais Massif (Carvalhosa andZbyszweski, 1994) occupies the northwestend of the EHMT, close to the town ofMontemor-o-Novo, and it corresponds to atonalitic body with an elliptical shape (15kmlong x 6km width) oriented sub-parallel to theregional Variscan foliation (WNW-ESEstriking deformation fabrics) in the region. Thetonalitic pluton intrudes mainly gneisses andmigmatites, except in its northern boundary,where it contacts a prominent amphiboliticlayer. To the west, Tertiary sediments coverboth tonalites and country rocks.52Gabbroic rocks occur within theEHMT, as several WNW-ESE elongatedbodies (Fig.1), intrusive in the gneissic-migmatitic complex. Their lengths usuallyfall between several hundred meters andfew kilometres.Field relations and PetrographyThis work is focused on the followingrock types: (1) tonalite, constituting themain lithology of the Hospitais Massif(HM); (2) leucotonalitic layers, locallyintercalated in the former lithology; (3)microgranular dioritic enclaves hosted bythe tonalite; (4) gabbros, forming smallintrusions in the EHMT, but outside theHospitais Massif.Main tonaliteThe dominant tonalite in the HospitaisMassif is monotonous, with smallpetrographic variation. Texturally, it ishypidiomorphic granular and shows mediumto coarse grain size. A WNW-ESE foliation,defined by the alignment of mafic minerals(biotite and amphibole), is often observed(Fig.2). Concerning the mineralogy, thedominant phases in the tonalite are plagioclase(40-50%) and quartz (20-30%), whereas theferromagnesian silicates do not exceed 20 to30% of rock volume. The relative proportionsof amphibole and biotite are quite variablefrom sample to sample: either one or the othermay be the most important mafic phase. Themost common accessory minerals are apatite,oxides, zircon and allanite.Plagioclase has composition ofandesine and forms subhedral grains(length: 2-4 mm) that commonly displayeither continuous or discontinuous complexzoning (Fig. 3A). Sometimes, these grainscontain inclusions of sub-millimetriccorroded plagioclase crystals. Anotheressential felsic mineral is quartz, whosegrains are always anhedral with contoursadapting to the forms of plagioclase, biotiteand amphibole. Wavy extinction and/orsubgranulation reveal that quartz wassubmitted to recovery processes.Two types of amphibole - hornblendeand cummingtonite, typically in closeassociation (Fig. 3B)- may be found in thetonalite. Hornblende is systematicallypresent and is one of the most importantmafic phases, whilst cummingtonite occursin a highly variable proportion betweensamples, from almost absent to 20-30% of totalamphibole. Typically, amphibole grains are 2-4 mm long and have subhedral forms. Whenthe two amphiboles are present, they associateto each other, with the hornblende rimming thecummingtonite. Occasionally, the biggestcrystals of amphibole develop a poikilitictexture, including small euhedral andsubhedral crystals of plagioclase, oxidesand apatite. The other major mafic mineral– biotite - forms subhedral crystals, whosemaximum length sometimes achieves 6mm, and commonly envelops opaques andapatite.Considering the accessory phases, theirmost significant features are the following:opaque minerals are commonly spatiallyrelated to the main mafic minerals, showingeuhedral and subhedral shapes; apatiteforms small (tenths of mm) euhedralprismatic crystals; zircon was almostalways observed as euhedral grains withhundredths or tenths of mm; allanite wasfound only sporadically, as anhedral tosubhedral grains hosted by amphibole orbiotite.Although primary minerals are usuallywell preserved in the collected samples,plagioclase may contain sericite, whereasbiotite is occasionally replaced by chloriteplus epidote.Leucotonalitic LayersLocally, it is possible to observe somecentimetric layers impoverished in maficphases (Leucotonalitic Layers – LTL, Fig.2B) that are commonly parallel or slightlyoblique to the foliation, although they alsocan be seen, rarely, cutting that structure.Within these discordant layers, it is possibleto identify an internal fabric defined by thealignment of undeformed grains of maficminerals and feldspars.At the two existing quarries in theHospitais Massif, there is a verticalmagmatic layering characterized by thealternation of (1) bands of particularlyquartz-rich and mafic-poor tonalite, and (2)levels with modal composition similar tothat of the main tonalite.Mafic enclavesDark-coloured enclaves, displayingwell-rounded outlines, ovoid forms and sharplimits, occur sparingly within the maintonalite. They show variable dimensions, from2 to 40 cm long and from 0.5 to 6 cm wide, andrepresent, when present, a small percentage ofthe outcrop area: 1-2% and sometimes less.Mineralogically, all the studied enclaves showdominance of amphibole. Two petrographicgroups based on the type of the amphibole,relative volume of ferromagnesian mineralsand granularity were defined: hornblendicenclaves (HbE) and cummingtonitic-hornblendic enclaves (Cm-HbE).The HbE enclaves are medium-grained(2mm) and have a hypidiomorphic granulartexture. Modally, they may be classified asFig. 1.- a) Geological sketch map of the Évora Massif located at the westernmost domains of theOssa-Morena Zone (OMZ). b) Simplified geological map of the Évora Massif with the subdivision intwo major tectonic units: the Évora high-grade Metamorphic Terrains (EHMT) and the Montemor-o-Novo Shear Zone (MNSZ), with location of tonalites from the Hospitais massif; After Pereira et al.,2003 (élocation of samples that were studied for geochemistry analysis by Moita et al., this volume)Fig. 1.- a) Esquema geológico con la localización del Macizo de Évora en los domínios occidentalesde la Zona de Ossa-Morena (OMZ). b) Mapa geológico simplificado del Macizo de Évora con la subdivi-sión en los Terrenos Metamórficos de Alto-grado de Évora (EHMT) y la Zona de Cizalla de Montemor-o-Novo (MNSZ), con la localización de las tonalitas del Macizo de Hospitais; Modificado de Pereira et al.,2003 (localización de las muestras analizadas en el estudio geoquímico por Moita et al., en este volumen).53diorites: hornblende is the most importantmineral (40-50%), followed by plagioclase(30-50%) and quartz (5%). Cummingtoniteis characteristically absent, but, sometimes,hornblende shows mimetic features(specially “ghosts” of a previous finetwinning) after the ferromagnesianamphibole. Hornblende occurs as euhedral/subhedral crystals (~ 2mm) occasionallycontaining plagioclase, apatite and opaqueinclusions. When in contact withplagioclase, amphibole grains tend to adaptto the feldspar contours. Crystals ofplagioclase (andesine-labradorite) typicallyexhibit subhedral forms and displaycomplex continuous zoning. The oxideminerals have subhedral habit and, in somesamples, reach 4-5% of the mode. In theseenclaves, biotite occurs in small amounts,but it may show a poikilitic development.Quartz is anhedral with lamellarsubgranulation.The Cm-HbE enclaves aredistinguished from the previous typemainly by (1) the significant amounts ofboth amphiboles (hornblende andcummingtonite), (2) the seriate texture withdominance of relatively small grains(<2mm) and (3) the systematic presence, asan accessory phase, of biotite. Theseenclaves have modal compositions typicalof diorites or microdiorites, sinceplagioclase (50-60%) and amphibole (30-35%) are largely dominant. Quartz (~5%)and biotite (1-2%) are other significantphases, whilst opaques and apatite onlyoccur in very small amounts. Plagioclase(labradorite) grains are anhedral and theirdimensions vary from 0.5 to 1-2mm (rarely4mm). The biggest crystals haveconcentric zoning - sometimes ofcomplex nature - and diffuse twinning, incontrast with the sub-millimetric grains,which show well-defined twinning andminor zoning. In several cases, welldeveloped plagioclase preserve cores thatseem to have been reworked. Amphibolesare usually present in aggregates of small(usually, less than 250ìm in length) andirregularly shaped grains. In those sets ofcrystals, both hornblende andcummingtonite are present, in a proportionof approximately 60:40, and theferromagnesian amphibole is confined to acentral position. Biotite is a subordinatephase occurring as small and elongatedgrains; it is occasionally replaced bychlorite and epidote. The crystals ofapatite, characteristically euhedral andelongated (axial ratios 3:1 - 5:1 and someacicular), are usually associated with thefelsic minerals. Quartz is anhedral withlamellar subgranulation and opaques aremanly associated with the felsic minerals.Fig. 2.- Field relations within Hospitais Massif: A) Elliptic flattened enclaves parallel with themain tonalitic foliation, B) Development of LTL, cutting the tonalitc foliation, C) Developmentof a LTH around the microgranular mafic enclaveFig. 2.- Relaciones de campo en el Macizo de Hospitais: A) Enclaves elípticos y aplastadosparalelos con la foliación principal de la tonalita, B) Desarrollo de LTL cortando la foliación dela tonalita, C) Desarrollo de LTH en el borde de un enclave diorítico contenido en las tonalitas.54Leucotonalitic halosSurrounding the mafic enclaves it issometimes possible to observe 1-3cm thickleucotonalitic halos (LTH) (Fig.2C). Thegrain size of the halo is similar to that of thehost tonalite. The felsic minerals(plagioclase and quartz) stronglypredominate, whereas the mafic phases(hornblende ± cummingtonite ± biotite)represent only 5% of the modalcomposition. Zircon occurs as an importantaccessory phase. Plagioclase showscomplex and discontinuous zoning, as wellas some deformation features like wedgetwinning. The quartz is more abundant inthe halos than in the surrounding tonaliteand shows lamellar subgranulation.Mafic rocksGabbros occur as small isolated outcropswith no direct contact with the HM. They arepredominantly medium grained rocks withhypidiomorphic granular textures, but, insome cases, poikilitic textures developed.Brown hornblende (30-40%), clinopyroxene(20-30%) and plagioclase (30-40%) are theessential minerals, whilst opaques andapatite are the accessory phases. Theamphibole commonly envelops the otherprimary minerals, especially plagioclase, andit may constitute oikocrysts (reaching 5mm).Pyroxene is present as isolated corrodednuclei (0.25-0.5mm) inside amphibole or aseuhedral crystals (0.25-1mm) whenassociated with plagioclase. The grains ofcalco-sodic feldspar (labradorite-bytownite)are usually subhedral and variable in size:crystals inside hornblende are small, from0.25mm to 0.5mm with round edges, whileoutside they may be up to 2mm long. Apatiteconstitutes anhedral grains, althoughsometimes elongated, reaching 0.5mm. Theopaque grains have usually round contours andsmall dimensions, and are mainly included inamphibole. Comparatively with the otherslithologies, the mafic rocks exhibit more oftenthe development of secondary minerals, namelygrowth of sericite, inside plagioclase, andactinolite, at hornblende and pyroxeneexpenses.ConclusionsThe Hospitais Massif is a Variscan syn-tectonic plutonic body that comprisesmedium-grained tonalites withleucotonalitic layers and microgranularenclaves. These enclaves have typicaligneous compositions and are sometimessurrounded by hololeucocratic halos.Outside this massif, but in the same sectorof the Évora High-Grade MetamorphicTerrains, several minor gabbroic syn-tectonic intrusions can be found. Thisspatial and chronological associationsuggests that the gabbros, the maficenclaves and the different tonalitic faciesmay represent the primitive andintermediate terms of a cogenetic igneoussuite, as will be discussed in the companionpaper (Moita et al., this volume), formedduring the Variscan orogeny.The tonalites present magmatic fabricsassociated with a moderately developedfoliation, recording syntectoniccrystallization and cooling, co-planar tosteep foliations observed within thesurrounding sheared gneisses andmigmatites. Locally, it is possible toobserve LTL commonly parallel or slightlyoblique to the foliation with internal fabricdefined by the alignment of undeformedgrains of mafic minerals.The enclaves display well-roundedoutlines, ellipsoidal forms (with maximumaxes sub-parallel to the foliation oftonalites) and sharp limits. These structuralcharacteristics suggest that they wereformed when the crystallization of themagma was not complete, but then theywere affected by strain in a progressivedeformation process. In fact, the fabricdisplayed by tonalites seems to be ofmagmatic origin and controlled by regionaldeformation since some microstructuresdeveloped on feldspars, biotite and quartzare possible to observe.In some places, there are discordantLTL with an internal foliation oblique to thefoliation of the main tonalite, implying that,at some point during the crystallization ofthe tonalite, tensional stress-originatedfractures immediately replenished with theremaining magma, after which shearingcontinued to deform both lithologies.Field relationships reveal thatcrystallization and cooling of the maintonalites and gabbros are synchronous withprogressive shearing of the gneisses andmigmatites envelope, with both intrusionand country rock showing steep fabrics sub-parallel to regional Variscan structures.AcknowledgmentsSupported by the project MODELIB35630/99-Termocronologia e ModelaçãoTectonometamórfica na Cadeia VariscaIbérica, SAPIENS Proj99 FCT, the Grupode Tectónica – Centro de Geofísica deÉvora budget for 2003-2004. This work is acontribution to IGCP project 497 (IUGS-UNESCO).ReferencesApalategui, O., Eguiluz, L. y Quesada, C.(1990). En: Pre-Mesozoic Geology ofIberia, (R.D.Dallmeyer y E. Martinez-Garcia Eds.), Springer-Verlag, 399-409.Carvalhosa, A. (1983). Comunicaçao doServicio Geolgico Portugal, 69 (2), 201-208.Carvalhosa, A. y Zbyszweski, G. (1994).Carta Geológica 1:50000, 35-D(Montemor-o-Novo), I.G.M.Chacón, J., Oliveira, V., Ribeiro, A. yOliveira, J.T. (1983). En: Libro JubilarJ.M.Rios, IGME, 490-504.Julivert, M., Fontboté J.M., Ribeiro, A. yConde, L.E.N. (1972). Mapa Tectónicode la Peninsula Ibérica y Baleares,1:1000 000, IGME.Oliveira, J.T., Oliveira, V. y Piçarra, J.M.(1991). Cuaderno LaboratorioXeolóxico de Laxe, 16, 221-250.Pereira, M.F., Silva, J.B. y Chichorro, M.,(2003). Geogaceta, 33, 71-74.Quesada, C. & Munhá, J.M. (1990). En:Pre-Mesozoic Geology of Iberia(R.D.Dallmeyer y E. Martinez-Garcia,Eds.). Springer-Verlag, 314-320.Fig. 3.- Photomicrographs showing some textural aspects of the main tonalite: A) Complexzoning of plagioclase, B) Common association of cummingtonite (Cmg) and hornblende (Hbl).Fig. 3.- Microfotografías con aspectos texturales de la tonalita: A) Zonado complejo de plagio-clasa, B) Asociación común entre cummingtonita (Cmg) y hornblenda (Hbl).

Tonalitas del Macizo de Hospitais (Zona de Ossa Morena, SO Macizo Ibérico, Portugal): I: Encuadre geológico y petrografía

The Hospitais Massif is an elliptical-shaped plutonic body, with a WNW-ESE long axis, located in the
western Ossa-Morena Zone domains in Portugal. This massif, composed essentially of tonalites, is part of
an igneous association, ranging from gabbros to granites, intrusive in a gneissic-migmatitic complex, as
part of the Évora High-grade Metamorphic Terrains. The tonalites present magmatic fabrics associated
with a moderately developed foliation, recording syntectonic crystallization and cooling, co-planar to
steep foliations observed within the surrounding sheared gneisses and migmatites. From field and
petrographic studies, it is possible to suggest that the deformation of the tonalites and associated basic
rocks was synchronous with Variscan progressive shearing of the gneisses and migmatites envelopeEl Macizo de Hospitais corresponde a un plutón elíptico de orientación ONO-ESE localizado en los domínios
occidentales de la Zona de Ossa Morena, en Portugal. Este macizo, constituido por una asociación ígnea
que incluye desde gabros hasta granitos y en la que son dominantes las tonalitas, está intruida en un
complejo de gneises y migmatitas perteneciente a los Terrenos de Alto-grado Metamórfico de Évora. Las
tonalitas muestran fabricas magmáticas asociadas a una foliación moderadamente desarrollada durante la
cristalización y enfriamiento, paralelas a la foliación subvertical observada en los gneises y migmatitas del
encajante. Las evidencias de campo y los estudios petrográficos permiten sugerir que la deformación de
las tonalitas y rocas básicas asociadas que afloran cerca de Montemor-o-Novo, fue contemporánea con el
progresivo cizallamiento Varisco observado en los gneises y migmatitas del encajant

Moita, Patricia

Pereira, Manuel Francisco

Arias Montano: Institutional Repository of the University of Huelva

Tonalitas del Macizo de Hospitais (Zona de Ossa Morena, SO Macizo Ibérico, Portugal).
I: Encuadre geológico y petrografía

http://rabida.uhu.es/dspace/bitstream/10272/8900/2/Tonalites.pdf

Tonalitas del Macizo de Hospitais (Zona de Ossa Morena, SO Macizo Ibérico, Portugal): I: Encuadre geológico y petrografía

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