Publication: Petrogenetic relationships between Variscan granitoids and Li-(F-P)-rich aplite-pegmatites in the Central Iberian Zone: Geological and geochemical constraints and implications for other regions from the European Variscides
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Publication Date
2018-04
Authors
Roda Robles, E.
Pesquera, Alfonso
Gil-Crespo, Pedro P.
Vieira, Romeu
Lima, A.
Garate-Olave, Idoia
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Elsevier
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Abstract
The Central Iberian Zone (CIZ) is characterised by a large volume of Variscan granitic intrusions, which can be grouped into five types: (1) two-mica peraluminous leucogranites (S1); (2) P-rich highly peraluminous granites (S2); (3) P-poor moderately peraluminous granites (S3); (4) moderately to low peraluminous granites (S4); and (5) I-type low peraluminous granites (I). Though not as abundant as granites, aplite-pegmatite rocks are nonetheless widespread in this region, occurring either as fields of aplite-pegmatite dykes or as leucogranitic cupolas. They are commonly enriched in Li-(F-P) minerals such as spodumene, petalite, micas, and phosphates of the amblygonite-montebrasite and triphylite-lithiophilite series. Many of the Li-rich bodies show an aplitic texture, frequently with the development of layered units. Coarse crystals are also common, but are mostly smaller than 12 cm long. They usually do not show internal fractionation, zoning or a quartz-core, and often have high values in Li2O (>1.3 wt%), and high Na, F and P contents. Evidence in support of a petrogenetic link among peraluminous granites and aplite-pegmatite bodies is provided by field relationships and geochemical affinities. The Li-(F-P)-mineralisation is closely related to the S1 series in the northern CIZ realm, whereas in the southern portion of the CIZ the parental granites correspond to the S2 series. The granites of the S3 and S4 series, and the I-type granite suite are not related to the Li mineralisation. The S1 and S2 granites are interpreted to derive mainly from the partial melting of highly peraluminous, Ca-poor and P-rich Neoproterozoic metasediments during the Variscan orogeny. The melts are presumed to evolve favoured by a high content in fluxing components, such as P, F, B, Li, and H2O, which contributed to the lowering of viscosity, solidus temperature and polymerisation degree. This is in parallel to the increasing of the diffusion rates and mobility of the highly fractionated melts. The residual melts, enriched in incompatible elements such as Li and F, as well as B, Sn and other rare elements, tend to accumulate at the top of the granitic cupolas. Therefore, Li-rich dykes as observed in many aplite-pegmatite fields of the CIZ are arguably the result of the opening of the system, whereas Li-rich granitic cupolas form when the system remains closed. Lithium-rich rocks, comparable to those of the CIZ, are found in other parts of the European Variscan Belt also related to P-rich, Ca-poor, highly peraluminous S-type granites originated during the Variscan Orogeny. Accordingly, we postulate the existence of an extense Li-metallogenetic province including mainly the CIZ in Spain and Portugal, the Massif Central in France, the Bohemian Massif in the Czech Republic and Germany, the western Carpathians in the Slovak Republic, and the Cornwall region in the south west of England.