Carboniferous-Permian mafic magmatism in the Variscan belt of Spain and France: implications for mantle sources
G. Perini, J. M. Cebria, J. Lopez-Ruiz, M. Doblas, 2004. "Carboniferous-Permian mafic magmatism in the Variscan belt of Spain and France: implications for mantle sources", Permo-Carboniferous Magmatism and Rifting in Europe, M. Wilson, E.-R. Neumann, G. R. Davies, M. J. Timmerman, M. Heeremans, B. T. Larsen
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Carboniferous-Permian magmatism in the Spanish Central System, Iberian Ranges, Cantabrian Chain, Pyrenees (Maladeta plutonic complex) and the French Massif Central includes a range of mafic calc-alkaline and shoshonitic rock types, as well as amphibole-bearing lamprophyres (spessartites) and minor alkaline lamprophyres (camptonites). Subalkaline basalts with intermediate characteristics between enriched mid-ocean ridge basalts (E-MORB) and the mafic calc-alkaline rocks also occur in the Pyrenees (Panticosa, Cinco Villas and La Rhune). The incompatible trace-element characteristics of the least differentiated subalkaline rocks and lamprophyres indicate that variably enriched mantle sources were involved in their genesis. High large ion lithophile element/high-field-strength element (LILE/HFSE), light rare earth element (LREE) HFSE and low Ce/Pb ratios in the calc-alkaline and shoshonitic rocks require either assimilation of crustal rocks plus fractional crystallization (AFC) of the parental mafic magmas or melting of a previously subduction-modified mantle source. In the Cantabrian Chain and the Massif Central, melting of a subduction-modified mantle source seems more likely. In the Central System, Iberian Ranges and Maladeta area the lack of any evidence for a contemporaneous subduction system suggests that AFC processes were more likely to be responsible for the crustal signature of the magmas. The alkaline camptonites from the Central System were generated from an enriched mantle source, which had lower LREE/HFSE and LILE/HFSE ratios than the source of the older calc-alkaline magmas from the same area. The incompatible trace-element patterns and ratios (e.g., Y/Nb, Zr/Nb) of the subalkaline basalts from Panticosa, Cinco Villas and La Rhune suggest that they were generated from similar parent magmas, formed by mixing of partial melts of an asthenospheric source and a crustal component.
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Widespread extension occurred within the Variscan orogen and its northern foreland during Late Carboniferous to Early Permian times. This was associated with magmatism and with a fundamental change, at the Westphalian-Stephanian boundary, in the regional stress field, coincident with the termination of orogenic activity and onset of dextral translation between North Africa and Europe. Rifting propagated across basement terranes with different ages and thermal histories. Most of the rift basins developed on relatively thin lithosphere; however, the highly magmatic Oslo Graben initiated within the edge of a craton. Early Stephanian regional uplift is contemporaneous with the onset of magmatism, inviting speculation that it might have been induced by a thermal anomaly within the upper mantle. The contributions to this volume suggest that the geodynamic setting in which magmatism occurred was complex, involving wrench tectonics, slab detachment, and delamination or thermal erosion of the base of the lithosphere.