Li-Na-metasomatism related to I-type granite magmatism: A case study of the highly fractionated La Pedriza pluton (Iberian Variscan belt)

Abstract

This study describes rare albitites associated with highly fractionated I-type granitic melts, showing a mineral paragenesis that resembles those of high-temperature metasomatized rocks (fenites) associated with alkaline and carbonatitic intrusions. The studied albitites occur as lenticular bodies on the meter scale, hosted in peraluminous granites, and are scattered in a narrow sector next to the peraluminous and highly fractionated I-type La Pedriza leucogranite (LPL) pluton in the Spanish Central System. Albitites show sharp contacts and preserve textural features of the host granites (i.e., isovolumetric metasomatism). These rocks consist of a variably albitized magmatic feldspar framework (70–85 modal %) and interstitial domains where singular Na-Li-Fe3+-rich amphiboles and pyroxenes crystallized with albite or quartz. Titanite, andradite-grossular garnet, Mg-Li-rich mica, magnetite, fluorite, and apatite are metasomatic accessory minerals. Mass balance indicates enrichments of Na2O, FeO, MgO, MnO, Li, F, Be, and some HFSE (high-field-strength elements, such as Zn, Sc, and Sn) and LILE (large-ion lithophile elements) depletion. U-Pb data from metasomatic titanite yield 307 ± 10 Ma, which is coeval with the LPL and within the range of the regional granite emplacement (about 309–298 Ma). The LPL also shows strong compositional affinities (F, Li, Na, Sn, and Zn enrichment) with albitites. Similar Sr-Nd-δD isotopic signatures between albitites and host granites suggest that magmatic fluids might be dominant during metasomatism. However, the heterogeneity of δ18O, which shifts toward lower values in the more distant outcrops, suggests contribution of meteoric in origin low-salinity waters during prograde metasomatism. P-T-fO2 conditions are estimated at 0.2–0.3 GPa, 645–700 °C, and relatively oxidized conditions. We propose that Na-HFSE-flux-rich residual melts of the LPL expelled two immiscible hot liquids: an alkaline low-density liquid and a high-density hydrosilicate liquid (HSL). The low-density fluid dissolved magmatic quartz and provided albitization of the magmatic feldspar framework by a diffusive mechanism. Quartz dissolution created microchannels for infiltration of the relatively low viscosity HSL, forming tubular channels. This fluid was enriched in network modifiers (Li, Na, F, Be) and cations (Ca, Al) released after albitization, and it precipitated in these microchannels to form the new metasomatic paragenesis (quartz, albite, uncommon Li-rich mafic silicates, and accessory minerals).