Petrology and Geochemistry of Highly Differentiated Tholeiitic Magmas: Granophyres in the Messejana–Plasencia Great Dyke (Central Iberia)

Abstract

The Messejana–Plasencia great dyke (MPGD) is a Late Triassic tholeiitic gabbro intrusion related to the Central Atlantic Magmatic Province. Its large outcrop extent (~530 km), combined with its prolongation below the Duero basin (additional 100 km), makes it one of the world’s largest dykes known. We have studied felsic granophyric bodies appearing in its northernmost segment at different scales, from mm-sized (interstitial micrographic pockets) to felsic dykes of up to 10 m thick and 1.5 km long, intruding within the gabbros. Significant differences exist in the mineral and whole-rock composition of gabbros and granophyres, including the Sr–Nd isotopic ratios. The chemical variation in the gabbros is coherent with fractionation of olivine, clinopyroxene and plagioclase at depth. However, the presence of a compositional gap between gabbros and granophyres (absence of intermediate compositions) and the formation of these late-stage intergranular felsic melts within the gabbro mesostasis suggest that they could be derived by liquid immiscibility. The Sr–Nd isotopic heterogeneity in the MPGD gabbros and the presence of zircons with Variscan ages (~286 Ma), inherited from granulitic rocks, indicate that the mafic magmas experienced some degree of lower crust assimilation during fractionation close to the Moho depth. On the contrary, the scarce xenocrystic Variscan zircon crystals found in a granophyric dyke within the MPGD gabbro display similar textures and ages (~299 Ma) to those of the country rock granites and point to contamination at a different crustal level.