Volcanism of the Garrotxa area is part of the Neogene volcanism of NE Spain (NE Spain volcanic province, NESVP). This volcanism is a manifestation of the magmatic activity consequence of the rift-type extensional tectonics that affect the eastern margin of Iberia since late Oligocene and that involved the opening of the Valencia trough. Volcanic activity in the NESVP started weakly in the Cadaqués zone 15 Ma ago, (C. Lewis, personal communication), followed by the Empordà (9 to 12 Ma in Alt Empordà, 6 Ma in Baix Empordà), La Selva field (5 to 7 Ma) and the Tordera fault system (2 Ma). Finally, activity took place at the Garrotxa volcanic field (GVF) from 300,000 to 10,000 years ago, with an eruptive episode every 15,000 to 20,000 years approximately (C. Lewis personal communication)

Aulinas Juncà, Meritxell

Fernández Turiel, José Luis

Gasperini, D.

Gimeno Torrente, Domingo

Gisbert Pinto, Guillem

Hernández de la Cruz, A.

English

Diposit Digital de la Universitat de Barcelona

 101 macla nº 15.  septiembre 2011 revista de la sociedad española de mineralogía  Origin of Green Clinopyroxene Cores of Lavas from the Garrotxa Volcanic Field (Spain) / GUILLEM GISBERT PINTO (1,*), DOMINGO GIMENO TORRENTE (1), MERITXELL AULINAS JUNCÀ (1), JOSE LUIS FERNANDEZ-TURIEL (2), DANIELA GASPERINI (1), AGUSTÍN HERNÁNDEZ DE LA CRUZ (1) (1) Departament de Geoquímica, Petrologia i Prospecció Geològica. Universitat de Barcelona. C/ Martí Franquès s/n. 08028 Barcelona (2) Institut de Ciències de la Terra Jaume Almera. CSIC. C/ Solé i Sabarís s/n. 08028 Barcelona INTRODUCTION.  Volcanism of the Garrotxa area is part ofthe Neogene volcanism of NE Spain (NESpain volcanic province, NESVP). Thisvolcanism is a manifestation of themagmatic activity consequence of therift-type extensional tectonics that affectthe eastern margin of Iberia since lateOligocene  and that involved the openingof the Valencia trough. Volcanic activityin the NESVP started weakly in theCadaqués zone 15 Ma ago, (C. Lewis,personal communication), followed bythe Empordà (9 to 12 Ma in Alt Empordà, 6 Ma in Baix Empordà), LaSelva field (5 to 7 Ma) and the Torderafault system (2 Ma). Finally, activity tookplace at the Garrotxa volcanic field (GVF)from 300,000 to 10,000 years ago, withan eruptive episode every 15,000 to20,000 years approximately (C. Lewis personal communication).  Evolution of the NESVP was highlyinfluenced by the evolution of theNeogene extensional fracturation whichgenerated a complex system of horstand graben structures and allowed theupwelling of magmas generated by decompression in the upper mantle inthe three main volcanic fields (Empordà,Selva and Garrotxa). Geophysical datashow that the area containing theNESVP is characterized by a crustal thinning and high thermal gradient, witha thinned lithosphere of 60-70 km, and a maximum crustal thickness of about28 km. The NESVP is mainly characterized by alkaline basaltic rocks — basalts and basanites — both potassic and sodic inaffinity, with the only known exception ofVilacolum and Arenys d'Empordàtrachytes (Gimeno, 1995; Diaz et al.,1996, Gisbert Pinto, 2008). Mineralogical composition of rocks is simple. In most cases small olivine,clinopyroxene and plagioclase crystalsare found inside a glassy matrix, usuallyrich in iron oxides. At some selectedlocalities the alkaline basaltic rocks ofthese regions contain xenoliths ofseveral types: mantle xenoliths, crustal magmatic xenoliths, with widespread accidental sedimentary xenolithsincorporated during the opening of thevents. The mantellic sources of magmas have been interpreted as the result of atwo-fold mantle metasomatism process(Bianchini et al., 2007).  PETROGRAPHY AND MINERALCHEMISTRY.  Petrography.  Rocks are aphanitic, light to dark grey,with some small (mm) phenocrysts andxenoliths (up to cm). Microscopic texture is porphyritic, hipohyaline toholocrystalline, with phenocrysts,microcrysts and xenocrysts ofclinopyroxene, olivine and plagioclase.Matrix between microcrysts is made ofopaque minerals and feldspars, andlocally leucite.   Olivine is mainly present as phenocrysts,but also as microphenocrysts, frequentlywith skeletal or elongated growing.Evidences of resorption can be found(probably corroded xenocrysts).  Plagioclase is found as clear and freshidio- to hipidiomorphic microlites with albite twinning; phenocrysts are scarce. Xenocrysts of rounded allotriomorphic morphologies, with internal sieve textureand a thin skeletal overgrown rim can befound.   Clinopyroxene shows the widest range of composition, morphologies and originsof all minerals. It is mostly present as phenocrysts and microphenocrysts,either free or in glomerules andagglomerates, but also as xenocrysts and in xenoliths. The bigger ones tend to have a core of rounded shape and sharp boundary mantled by clinopyroxene in equilibrium with the melt, which gives a final idiomorphic shape to the crystals, in some cases with some degree of skeletal growing. There are several typologies of core; the main ones are: slightly-pleochroic green cores (Fig. 1), brown cores and pinkish brown cores.Cores can be either intact or present some degree of corrosion (in sieve texture). Small crystals usually do not show core. Microphenocrysts are mainly idiomorphic of equidimensional or short prismatic habit. Clinopyroxene in equilibrium with the melt (which forms the mantle over cores, phenocrysts without core, and microphenocrysts) is zoned and its color changes from brown at the centre to pinkish brown at the edges. Zonation is more dominant at the edges, being in some cases oscillatory.  fig 1. Example of pyroxene with green core. The crystal is 2 mm wide.  Mineral chemistry  Olivine phenocrysts show a composition of Fo88-72, being chemicallyhomogeneous in most of their volume, with the Fo content only decreasing in a thin band by the edge.   Plagioclase microlites are mainly labradorite, with minor bytownite, ( An71-54)(Or1-3) Average composition (An65) is the same for all rock compositions.  Opaque minerals are Fe-Ti oxides of the magnetite-ulvospinel series, containing also Al and Mg. FeO ranges from 47 to palabras clave: Campo volcánico de la Garrotxa, Xenocristales, Piroxenos. key words: Garrotxa Volcanic Field, Xenocrysts, Pyroxenes.  resumen SEM 2011 * corresponding author: ggisbertp@ghotmail.com  102 macla nº 15.  septiembre 2011 revista de la sociedad española de mineralogía   64 wt% and TiO2 from 7 to 23 wt%. Al2O3 ranges from 2 to 16 wt%, and MgO from 2 to 8 wt%.   Most of the analysed pyroxenes have diopside composition whereas few plotin the hedenbergite and augite fields(Fig. 2). Chemical analyses reveal that brown cores have the same compositionas centres of non-cored phenocrysts. Pinkish-brown cores show higher Al2O3contents, as well as more FeOt and less MgO and CaO, resembling thecomposition of rims. Green cores showhigher contents of FeOt, Na2O and MnO, and are depleted in MgO, TiO2 and Al2O3. Despite the higher content in Na2O, this is not enough to claimdiopside/hedenbergite/augite to besodic. Cores are in most caseschemically homogeneous. Mantlecompositions are the same as those of the non-cored phenocrysts: from core to rim SiO2 and MgO decrease, while TiO2and FeOt increase. The transition fromcore to overgrowing implies in mostcases a sharp increase (more or lessmarked depending on the type of core)of the MgO content (reverse zoning), and from there to the edge of the crystal asteady decrease. TiO2 shows an oppositebehaviour.  WHOLE-ROCK GEOCHEMISTRY.  Rocks from the GVF are basalts, basanites and traquibasalts incomposition. Regular chemical trendscan be observed in major elementversus SiO2 or MgO binary diagramsshowing that the magmatic eventsoccurring in the GVF underwent similarmagmatic evolutionary processes. These trends can be explained in terms offractional crystallisation of olivine,pyroxene and opaque minerals of themagnetite-ulvospinel series.   DISCUSSION AND CONCLUSIONS.  Reversely zoned clinopyroxenes found in GVZ have also been observed in otheralkali basalts related to the opening ofthe Miocene European Rift (e.g. Eiffel,Duda and Schmincke, 1995) and in other alkali provinces (e.g CanaryIslands, Aulinas et al., 2010). The different origins proposed for suchreverse zonation include a geneticrelationship between cores and mantle-rims, considered cognate phases.According to this model, the inverse zonation may be caused by an increaseof fO2 conditions (i.e. oxidation) or by sinking of the crystal into a convectingcrustal magma chamber in whichcrystals settle in hotter, Mg-rich, lower portions (Borely et al., 1971). Another explanation of the origin of such inversely zoned phenocrysts is the interpretation of green cores asaccidental xenoliths related to wall rockdebris entrained by the rising magma(Barton & Bergen, 1981). Finally, thegreen clinopyroxene cores could be interpreted as cognate xenocrysts produced during crystallisation of anevolved magma that subsequentlyinteracted with a more primitive hotmagma (Brooks and Printzlau, 1978).   In our case, the rounded shape of thegreen cores, the sharp optical andchemical boundary between green cores and mantles, the high Fe/Mg ratios ofcores, and the coexistence of normallyand reversely zoned clinopyroxene in a single sample cannot be explained by achange in the melt composition.Consequently, these petrographicfeatures require a xenocrystic origin of the green cores.  ACKNWOLEDGEMENTS.  This work was funded by Research Group PEGEFA (2005SGR-795, AGAUR, DURSI de la Generalitat de Catalunya) and Spanish research project CGL2007-63727/BTE (MEC).  REFERENCES.   Aulinas, M., Gimeno, D., Fernandez-Turiel, J. L., Font, L., Perez-Torrado, F.J., Rodriguez-Gonzalez, A.,  Nowell, G.M. (2010): Small-scale mantle heterogeneity on the source of the Gran Canaria (Canary Islands) Pliocene-Quaternary magmas. Lithos, 119, 377-392. Barton, M. & Bergen, M.J. (1981): Green clinopyroxenes and associated phases in a potassium-rich lava from the Leucite Hills, Wyoming. Contrib. Mineral. Petr., 77, 101-114. Bianchini, G., Beccaluva, L., Bonadiman, C., Nowell, G., Pearson, G., Siena, F., Wilson, M. (2007): Evidence of diverse depletion and metasomatic events in harzburgite–lherzolite mantle xenoliths from the Iberian plate (Olot, NE Spain): Implications for lithosphere accretionary processes. Lithos 94, 25–45. Borely, G.D., Suddaby, P., Scott, P. (1971):Some xenoliths from the alkali rocks of Tenerife, Canary Islands. Contrib. Mineral. Petr., 31, 102-114. Brooks, C.K. & Printzlau, I. (1978): Magma mixing in mafic alkaline volcanic rocks, the evidence from relict phenocrysts phases and other inclusions. J. Volcanol. Geoth. Res., 4, 315-331. Diaz, N., Gimeno, D., Losantos, M., Segura, C., (1996): Las traquitas de Arenys d’Empordà(Alt Empordà, NE de la Península Ibérica). Geogaceta, 20, 572-575. Duda, A. & Schmincke, H-U. (1985): Polybaric differentiation of alkali basalt magmas, evidence from green-core clinopyroxenes (Eifel, FRG). Contrib. Mineral. Petr., 91, 340–353. Gimeno, D., (1995): Las rocas de textura traquítica de Vilacolum (Girona). Una corrección del Catálogo de la colección de rocas grandes-bloques, del Museo del Parque de Barcelona basada en la revisión de la paragénesis mineral y la geoquímica. Treballs del Museu de Geologia de Barcelona 4, 211-223. Gisbert Pinto, G. (2008): Estudi petrològic i geoquímic del vulcanisme recent de la Garrotxa. Master’s Final Project, Universitat de Barcelona. 33 pp. Morimoto, N. (chairman), (1989):Nomenclature of Pyroxenes. Can. Mineral.,27, 143-156.     Green coreCenterNear rimRimEnWoFsPigeoniteAugiteDiopside HedenbergiteEnstatite Ferrosilite fig 2. Projection of pyroxenes from one sample in the En-Fs-Wo classification diagram (Morimoto et al., 1989). 

Origin of green clinopyroxene cores of lavas from the garrotxa volcanic field (spain)

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Origin of green clinopyroxene cores of lavas from the garrotxa volcanic field (spain)

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