Migrating Cretaceous–Eocene Magmatismin the Serra do Mar Alkaline Province,SE Brazil: Melts from the Deflected Trindade Mantle Plume?

The Serra do Mar province extends for ∼500 km as plutonic complexes, some with volcanic remnants, and dyke swarms along coastal SE Brazil. The igneous rocks are emplaced into metamorphics of the Brasiliano (Pan-African) Ribeira mobile belt, which abuts the southern margin of the Archaean São Francisco craton. Syenites and phonolites dominate most complexes but mafic–ultramafic dykes (MgO <16.4%) are also widespread, together with sparse mafic lavas. The mafic rocks are alkali basalts, basanites and melanephelinites, with rare leucitites and minettes (s.l.). Major and trace elements of all but a few of the strongly potassic rock types are indistinguishable from ocean-island basaltic (OIB, s.l.) magmatism. Isotopically (Sr–Nd), the Serra do Mar suite overlaps the field for OIB (εNd up to +2.7) and aseismic ridges, especially Walvis. In general, the most potassic compositions have the lowest εNd values (−1.4 to −3.3). We consider the most plausible source for Serra do Mar mafic magmas to be predominantly sub-lithospheric convecting mantle, with sporadic input from lithospheric metasomites. Additionally, the two most Mg-rich dykes have high 87Sr/86Sri (>0.707), despite εNd in the same range as nearby mafic …