Los leucogranitos equigranulares del Domo del Tormes (Zona Centro Ibérica): discriminación geoquímica mediante Biplot Canónico y significado petrogenético

Equigranular leucogranites from the Tormes Dome (Central Iberian Zone): geochemical discrimination using Canonical Biplot and petrogenetic meaning

Miguel López-Plaza milplaz@usal.es (Departamento de Geología, Universidad de Salamanca, Plaza de los Caídos s/n, 37008, Salamanca España)
Francisco Javier López-Moro
fjlopez@usal.es (same address as M. López-Plaza)
Santiago Vicente-Tavera
svt@usal.es (Departamento de Estadística, Universidad de Salamanca, Espejo 2, 37007 Salamanca, España)
José Luis Vicente-Villardónvillardon@usal.es (same address as S. Vicente-Tavera)

e-Terra
volume 5 - nº 4 - 2008

e-terra.geopor.pt

Palabras clave: Domo anatéctico del Tormes, leucogranitos equigranulares, discriminación geoquímica, Biplot Canónico, vectores de diferenciación.

Key-words: Variscan anatectic Tormes Dome, Equigranular leucogranites, Geochemical discrimination, Canonical Biplot, Differentiation factors.

Resumen: Alrededor del  65 % de las rocas plutónicas del domo varisco y anatéctico del Tormes (Zona Centro-Ibérica) son  leucogranitos equigranulares, que incluyen tipos graníticos de grano grueso, medio y fino. La afinidad cortical (granitos de tipo S) se manifiesta, entre otras, por las siguientes características geoquímicas: 1) elevado índice A/CNK; 2) modelos de normalización de multicomponentes con pendiente negativa, indicando un enriquecimiento en elementos incompatibles y móviles, como Rb y K, así como anomalías negativas en Nb, Ti, Sr, Ba y Eu; 3) altos valores de fraccionación de tierras raras, con un rango de (La/Lu)n entre 28 y 75; y 4) altos valores de δ18O (10,3-12,9 ‰). A pesar de las diferencias en tamaño de grano, los granitos de dos micas equigranulares del Domo del Tormes muestran un alto grado de solapamiento composicional, tanto geoquímico como en las proporciones mineralógicas. A fin de discriminar los distintos grupos establecidos se ha aplicado la metodología estadística multivariante denominada Biplot Canónico referida a elementos mayores y trazas. La aplicación se llevó a cabo primeramente para siete grupos graníticos, incluyendo los granitos biotíticos y los turmaliníferos; después, se restringió a los granitos de dos micas.  Mediante el primer procedimiento se obtuvo un 95,9 % de absorción total para los tres primeros ejes. El plano 1-3 nos permite discriminar los grupos y proyectarlos en un factor general e hipotético de diferenciación (y/o fusión parcial) que puede incluir información sobre el nivel cortical de emplazamiento, la mineralogía y las características geoquímicas mismas. Analizando la información suministrada por el plano 1-2 se deduce un factor de enriquecimiento en fluidos mediante los elementos Cs y Ta, apareciendo los granitos con turmalina en la parte más enriquecida de dicho factor. El segundo procedimiento ha permitido, por un lado, confirmar los dos procesos anteriores, y por otro lado, una discriminación perfecta de los diferentes grupos de granitos de dos micas. La variabilidad en SiO2, Rb, Cs y Ta, como elementos incompatibles, unida a la del CaO, Sr y Ba, como elementos compatibles, no puede ser explicada satisfactoriamente sólo por tasas variables de fusión parcial, sino que se requieren otros procesos adicionales, como cristalización fraccionada y enriquecimiento en fluidos.

Abstract: About 65 % of plutonic rocks from the Variscan Tormes Dome (Central Iberian Zone) consist of equigranular leucogranites, including coarse-grained, medium-grained and fine-grained granite types. A crustal affinity (S-type granite melts) can be inferred, among others, from the following geochemical features: 1) a high A/CNK index; 2) negatively-slopped normalized spidergrams, showing an enrichment of incompatible and mobile elements, such as Rb and K, as well as negative anomalies in Nb, Ti, Sr, Ba and Eu; 3) high REE fractionation values, (La/Lu)n ranging from 28 to 75, and 4) high δ18O (10.3-12.9 ‰). Despite textural differences, mainly in relation to grain size variation, a compositional overlapping is worth noting, making any geochemical approach difficult to discriminate the two-mica granite groups. Variable partial melting degrees after glandular gneisses is unlikely to be the only reason to account for the linear trend on the ACF diagram, since such a trend is characteristic of leucogranite belts from NW Iberian Massif. To allow a better discrimination of the two-mica granite groups, a statistical methodology, the Canonical Biplot, was applied to a matrix consisting of 32 columns (major and trace-elements) and 42 rows (granite samples from the Tormes Dome). These latter, in turn, were firstly divided into seven granite groups (first analysis), including biotite granites and tourmaline granites, and secondly, into four two-mica granite groups (coarse-grained granites from central areas, coarse-grained granites from south western areas, medium-grained granites and fine-grained granites). Several biplot representations were obtained for the matrix adding confidence circles to the granite group markers in such a way that their circles were projected onto the direction representing a hypothetical magmatic differentiation vector. If no overlap is found we can conclude that there is a certain difference between granite groups, allowing us a geochemical discrimination. The biplot representation for the first analysis results in a 95.9 % inertia absorption for the first three axes. The main plane 1-3 allows a discrimination to be made by considering the projection onto a general differentiation vector that can include information on crustal emplacement level, mineralogy and the geochemical features themselves. In particular, the more shallowly emplaced medium-grained granites display a higher degree of differentiation than the coarse-grained granites from the deeper central area, both granite groups having similar chondrite-normalized patterns. The main plane 1-2 provides information that allows the role played by fluids to be discriminated, affording a fluid-enrichment factor in relation to the content in Cs and Ta, both elements showing  similar behaviour. Tourmaline-bearing granites plot on the highest fluid-enriched side, followed by the peripheral coarsed-grained granites from the SW area, whereas the fine-grained granites appear on the “depleted side”, this latter suggesting a possible fluid-exsolution process linked to the late-orogenic decompressional evolution of the Tormes Dome. The biplot representation for the second analysis permits us to confirm these magmatic processes, leading to a perfect discrimination between two-mica granite groups using the residual plane 2-3 (23.7 % of the total inertia). The comparison of the statistical results with the chemical variation diagrams allow us to infer that the incompatible-element enrichment cannot be accounted for by variable partial melting degrees. Additional processes such as pelitic xenolith assimilation, restite unmixing and fluid enrichment are possible to have been involved to explain the enrichment in mobile elements.

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