Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEsubmitte

Chicharro Álvarez, Eva

Pérez Soba, Cecilia

Villaseca González, Carlos

Docta Complutense

Book of Abstractsposters40PT.016NA-LI-F-METASOMATISM RELATED TO A HIGHLY FRACTIONATED PERALUMINOUS I-TYPE GRANITE (LA PEDRIZA, SPAIN, IBERIAN VARISCAN BELT)Pérez-Soba C1, Villaseca C1, Chicharro E2 - 1Universidad Complutense Madrid - Petrologia y Geoquimica, 2Universidad Complutense Madrid - Cristalografïa y MineralogiaNa-rich metasomatism is usually related to carbonatite or alkaline complexes. In this work we study an oustanding outcrop of albitites on the basis of: (i) their genetic connection to a peraluminous I-type leucogranite, and (ii) the very unusual paragenesis of Li-Na-rich amphiboles occurring within it. These rocks were previously interpreted as tectonic-derived episyenites; nonetheless, new field, petrographical and geochemical studies make questionable this model.Albitites outcrop as subvertical lense-shaped bodies of variable sizes, from 1 to 80 m in length, and width up to 6 m. La Pedriza albitites spread over an area of about 0.25 km2, within a 1 km width band from the contact with the La Pedriza I-type leucogranite. The parent rocks of the albitites are porphyritic cordierite-bearing granite and biotite microgranite intrusions, whose main textural characteristics are preserved (isovolumetric metasomatism). Albitites are white coloured with dark-blue irregular mafic nodules. Contacts with host granitoids are sharp and irregular at cm-scale, and no zonation within the body is observed. Single outcrops differ from each other by mineralogical and geochemical features, defining a metasomatic family. Some aplite dikes crossing these bodies show similar metasomatic transformations.The main mineral association of albitites consists of relict magmatic mineralogy enclosed by newly formed albite (70-85% modal), some K-feldspar, and interstitial nodules (10-20%) of quartz-pyroxene (aegirine-augite) and/or Li-Na-amphiboles. Accessories are titanite, magnetite, andradite, epidote, Mg-Li-rich mica (tainiolite), zircon, ilmenite, rutile, fluorite and apatite. The interstitial character of the main metasomatic asemblage suggests an infiltrational metasomatism.La Pedriza albitites are classified as alkaline and metaluminous (close to peralkaline values). Compared to parental-rocks, albitites show enrichments in Na2O, FeO, MgO, MnO, Li, F, Be, Zn, Sc and Sn, while SiO2, K2O, Rb, Cs and W are depleted. Their Na-Li-F-Be-rich chemistry is typical of metasomatic fluids derived from igneous intrusions. The lack of significant REE, Y, Nb, Ta, Th and U enrichment contrasts with Na-metasomatism associated with alkaline complexes. The gradual increase of Na2O is negatively correlated with K2O and Rb, and positively correlated with FeO, CaO, Y and REE. Identical Sr-Nd isotopic signatures between albitites and host granites suggest that magmatic fluids might be dominant in the metasomatic transformation.Estimations based on oxygen thermometry yields a range of 520 to 635°C for albitite formation. Secondary aqueous fluid inclusion data indicate a minimum temperature of 397 ± 10°C for albitite formation, thus defining a mid-to-high-T hydrothermal metasomatism. The La Pedriza albitites are not related with fracturing and are not episyenites s.s., owing to the clear coprecipitation of quartz and alkaline mafics. We interpret this metasomatism as Na-Li-F-rich solutions linked with the fractionation of the nearby La Pedriza I-type granite. This massif is a cryptozoned pluton, reaching in this area the highest concentrations in Na, Rb, Nb, Ta, Y, HREE, F, Li, Sn, Mn and Cs, as a result of extensive fractional crystallization. This is an unusual Na-Li-F-rich contact metasomatism related to the fractionation of I-type peraluminous granitic melts.

English

Na-Li-F-Metasomatism related to a highly fractionated peraluminous I-type granite (La Pedriza, Spain, Iberian Variscan Belt)

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Na-Li-F-Metasomatism related to a highly fractionated peraluminous I-type granite (La Pedriza, Spain, Iberian Variscan Belt)

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