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Origin of Green Clinopyroxene Cores of Lavas from the Garrotxa Volcanic

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Origin of Green Clinopyroxene Cores of Lavas from the Garrotxa Volcanic macla nº 15 . septiembre 2011 revista de la sociedad española de mineralogía 101 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. rich in iron oxides. At some selected equilibrium with the melt, which gives a localities the alkaline basaltic rocks of final idiomorphic shape to the crystals, Volcanism of the Garrotxa area is part of these regions contain xenoliths of in some cases with some degree of the Neogene volcanism of NE Spain (NE several types: mantle xenoliths, crustal skeletal growing. There are several Spain volcanic province, NESVP). This magmatic xenoliths, with widespread typologies of core; the main ones are: volcanism is a manifestation of the accidental sedimentary xenoliths slightly-pleochroic green cores (Fig. 1), magmatic activity consequence of the incorporated during the opening of the brown cores and pinkish brown cores. rift-type extensional tectonics that affect vents. The mantellic sources of magmas Cores can be either intact or present the eastern margin of Iberia since late have been interpreted as the result of a some degree of corrosion (in sieve Oligocene and that involved the opening two-fold mantle metasomatism process texture). Small crystals usually do not of the Valencia trough. Volcanic activity (Bianchini et al., 2007). show core. Microphenocrysts are mainly in the NESVP started weakly in the idiomorphic of equidimensional or short Cadaqués zone 15 Ma ago, (C. Lewis, PETROGRAPHY AND MINERAL prismatic habit. Clinopyroxene in personal communication), followed by CHEMISTRY. equilibrium with the melt (which forms the Empordà (9 to 12 Ma in Alt the mantle over cores, phenocrysts Empordà, 6 Ma in Baix Empordà), La Petrography. without core, and microphenocrysts) is Selva field (5 to 7 Ma) and the Tordera zoned and its color changes from brown fault system (2 Ma). Finally, activity took Rocks are aphanitic, light to dark grey, at the centre to pinkish brown at the place at the Garrotxa volcanic field (GVF) with some small (mm) phenocrysts and edges. Zonation is more dominant at the from 300,000 to 10,000 years ago, with xenoliths (up to cm). Microscopic texture edges, being in some cases oscillatory. an eruptive episode every 15,000 to is porphyritic, hipohyaline to 20,000 years approximately (C. Lewis holocrystalline, with phenocrysts, personal communication). microcrysts and xenocrysts of clinopyroxene, olivine and plagioclase. Evolution of the NESVP was highly Matrix between microcrysts is made of influenced by the evolution of the opaque minerals and feldspars, and Neogene extensional fracturation which locally leucite. generated a complex system of horst and graben structures and allowed the Olivine is mainly present as phenocrysts, upwelling of magmas generated by but also as microphenocrysts, frequently decompression in the upper mantle in with skeletal or elongated growing. the three main volcanic fields (Empordà, Evidences of resorption can be found fig 1. Example of pyroxene with green core. The Selva and Garrotxa). Geophysical data (probably corroded xenocrysts). crystal is 2 mm wide. show that the area containing the Plagioclase is found as clear and fresh NESVP is characterized by a crustal idio- to hipidiomorphic microlites with Mineral chemistry thinning and high thermal gradient, with albite twinning; phenocrysts are scarce. a thinned lithosphere of 60-70 km, and Xenocrysts of rounded allotriomorphic Olivine phenocrysts show a composition a maximum crustal thickness of about morphologies, with internal sieve texture of Fo88-72, being chemically 28 km. and a thin skeletal overgrown rim can be homogeneous in most of their volume, The NESVP is mainly characterized by found. with the Fo content only decreasing in a alkaline basaltic rocks — basalts and thin band by the edge. basanites — both potassic and sodic in Clinopyroxene shows the widest range of affinity, with the only known exception of composition, morphologies and origins Plagioclase microlites are mainly Vilacolum and Arenys d'Empordà of all minerals. It is mostly present as labradorite, with minor bytownite, ( An71- trachytes (Gimeno, 1995; Diaz et al., phenocrysts and microphenocrysts, 54)(Or1-3) Average composition (An65) is 1996, Gisbert Pinto, 2008). either free or in glomerules and the same for all rock compositions. Mineralogical composition of rocks is agglomerates, but also as xenocrysts simple. In most cases small olivine, and in xenoliths. The bigger ones tend to Opaque minerals are Fe-Ti oxides of the clinopyroxene and plagioclase crystals have a core of rounded shape and sharp magnetite-ulvospinel series, containing are found inside a glassy matrix, usually boundary mantled by clinopyroxene in also Al and Mg. FeO ranges from 47 to palabras clave: Campo volcánico de la Garrotxa, Xenocristales, key words: Garrotxa Volcanic Field, Xenocrysts, Pyroxenes. Piroxenos. resumen SEM 2011 * corresponding author: [email protected] macla nº 15 . septiembre 2011 revista de la sociedad española de mineralogía 102 Wo ACKNWOLEDGEMENTS. Green core Diopside Hedenbergite This work was funded by Research Center Group PEGEFA (2005SGR-795, AGAUR, Near rim DURSI de la Generalitat de Catalunya) and Spanish research project CGL2007- Augite Rim 63727/BTE (MEC). REFERENCES. Aulinas, M., Gimeno, D., Fernandez-Turiel, J. Pigeonite L., Font, L., Perez-Torrado, F.J., Rodriguez- Gonzalez, A., Nowell, G.M. (2010): Small- scale mantle heterogeneity on the source Enstatite Ferrosilite of the Gran Canaria (Canary Islands) En Fs Pliocene-Quaternary magmas. Lithos, 119, 377-392. Barton, M. & Bergen, M.J. (1981): Green fig 2. Projection of pyroxenes from one sample in the En-Fs-Wo classification diagram (Morimoto et al., clinopyroxenes and associated phases in a 1989). potassium-rich lava from the Leucite Hills, Wyoming. Contrib. Mineral. Petr., 77, 101- 64 wt% and TiO2 from 7 to 23 wt%. magnetite-ulvospinel series. 114. Al2O3 ranges from 2 to 16 wt%, and MgO Bianchini, G., Beccaluva, L., Bonadiman, C., from 2 to 8 wt%. DISCUSSION AND CONCLUSIONS. Nowell, G., Pearson, G., Siena, F., Wilson, M. (2007): Evidence of diverse depletion Most of the analysed pyroxenes have Reversely zoned clinopyroxenes found in and metasomatic events in harzburgite– diopside composition whereas few plot GVZ have also been observed in other lherzolite mantle xenoliths from the Iberian in the hedenbergite and augite fields alkali basalts related to the opening of plate (Olot, NE Spain): Implications for (Fig. 2). Chemical analyses reveal that the Miocene European Rift (e.g. Eiffel, lithosphere accretionary processes. Lithos 94, 25–45. brown cores have the same composition Duda and Schmincke, 1995) and in Borely, G.D., Suddaby, P., Scott, P. (1971): as centres of non-cored phenocrysts. other alkali provinces (e.g Canary Some xenoliths from the alkali rocks of Pinkish-brown cores show higher Al2O3 Islands, Aulinas et al., 2010). The Tenerife, Canary Islands. Contrib. Mineral. contents, as well as more FeOt and less different origins proposed for such Petr., 31, 102-114. MgO and CaO, resembling the reverse zonation include a genetic Brooks, C.K. & Printzlau, I. (1978): Magma composition of rims. Green cores show relationship between cores and mantle- mixing in mafic alkaline volcanic rocks, the higher contents of FeOt, Na2O and MnO, rims, considered cognate phases. evidence from relict phenocrysts phases and other inclusions. J. Volcanol. Geoth. and are depleted in MgO, TiO2 and Al2O3. According to this model, the inverse Res., 4, 315-331. Despite the higher content in Na2O, this zonation may be caused by an increase Diaz, N., Gimeno, D., Losantos, M., Segura, C., is not enough to claim of fO2 conditions (i.e. oxidation) or by (1996): Las traquitas de Arenys d’Empordà diopside/hedenbergite/augite to be sinking of the crystal into a convecting (Alt Empordà, NE de la Península Ibérica). sodic. Cores are in most cases crustal magma chamber in which Geogaceta, 20, 572-575. chemically homogeneous. Mantle crystals settle in hotter, Mg-rich, lower Duda, A. & Schmincke, H-U. (1985): Polybaric compositions are the same as those of portions (Borely et al., 1971). Another differentiation of alkali basalt magmas, the non-cored phenocrysts: from core to explanation of the origin of such evidence from green-core clinopyroxenes (Eifel, FRG). Contrib. Mineral. Petr., 91, rim SiO2 and MgO decrease, while TiO2 inversely zoned phenocrysts is the 340–353. and FeOt increase. The transition from interpretation of green cores as Gimeno, D., (1995): Las rocas de textura core to overgrowing implies in most accidental xenoliths related to wall rock traquítica de Vilacolum (Girona). Una cases a sharp increase (more or less debris entrained by the rising magma corrección del Catálogo de la colección de marked depending on the type of core) (Barton & Bergen, 1981). Finally, the rocas grandes-bloques, del Museo del of the MgO content (reverse zoning), and green clinopyroxene cores could be Parque de Barcelona basada en la revisión from there to the edge of the crystal a interpreted as cognate xenocrysts de la paragénesis mineral y la geoquímica. steady
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