Peridotites and mafic igneous rocks at the foot of the Galicia Margin: an oceanic or continental lithosphere? A discussion

Jacques Kornprobst; Gilles Chazot

doi:10.21701/bolgeomin.127.2-3.002

Autores/as

Jacques Kornprobst Laboratoire Magmas et Volcans, Université Blaise Pascal et CNRS
Gilles Chazot Laboratoire de Géosciences marines, Université de Bretagne Occidentale et CNRS

DOI:

https://doi.org/10.21701/bolgeomin.127.2-3.002

Palabras clave:

Margen de Galicia, transición Océano-Continente, rifting, manto litosférico, contaminación mantélica

Resumen

En el fondo marino al pie del Margen de Galicia (latitudes correspondientes a España y Portugal) aflora un complejo ultramáfico/máfico, compuesto por varios tipos de peridotitas y piroxenitas, así como dioritas anfíbólicas, gabros, doleritas y basaltos. Por razones cronológicas y estructurales (gabros emplazados dentro de peridotitas antes de la fractura continental) esta unidad no se puede asignar a la corteza oceánica del Atlántico. La recopilación de todos los datos petrológicos y geoquímicos disponibles sugiere que las peridotitas se derivan del manto litosférico subcontinental, profundamente transformado durante el “rifting” cretácico. Así, en las harzburgitas litosféricas se emplazaron tempranamente diques de websterita extraídos del reservorio mantélico MORB empobrecido (DMM); ulteriores procesos de deformación (boudinage) y dispersión tectónica de estos diques en las peridotitas, durante las etapas de deformación iniciales del rifting, dieron lugar a la formación de lherzolitas fértiles pero isotópicamente empobrecidas. Las websteritas estériles pero isotópicamente enriquecidas podrían representar residuos de fusión en las peridotitas, tras una significativa fusión parcial y extracción del fundido, en relación con la erosión térmica de la litosfera. Estos últimos fundidos son probablemente la fuente de la cristalización metasomática de anfíbol marrón en algunas peridotitas, así como del emplazamiento de diques de diorita anfibólica. Los fundidos extraídos directamente de la astenosfera se emplazaron como gabros en el interior del manto subcontinental. La mezcla de estos fundidos DMM con los fundidos enriquecidos extraídos de la litosfera dio lugar a las composiciones isotópicas intermedias -entre DMM y el reservorio OIB o basaltos de isla oceánica- observadas en doleritas y basaltos, ninguno de los cuales se caracteriza por una típica firma N-MORB. Un manto litosférico enriquecido, presente antes de la dislocación del margen de Galicia, concuerda bien con los datos del dique Messejana (Portugal) y, en general, con los de todas los toleitas continentales de la Provincia Magmática del Atlántico Central (CAMP).

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