Petrology of metabasaltic dykes in the Diamantina region, Minas Gerais, Brazil.

A sequence of mafic dykes is found in the Diamantina region (Minas Gerais, Brazil) at the eastern border of the southern São Francisco Craton. The dykes have been distinguished into four groups on the basis of petrography and geochemical characteristics. Groups 1 to 3 cut the rock sequence older than Meso-Proterozoic, but do not cut the Neo-Proterozoic rocks, thus constraining their emplacement age. Group 1 is constituted by fine grained, sometimes foliated metabasites, which only exceptionally preserve relics of the primary mineral assemblage, and are located only in the lowest stratigraphic units of the Espinhaço Supergroup, suggesting an older emplacement age with respect to the other groups. Group 2 and 3 metabasites are better preserved than those of Group 1. Group 4 represents a single igneous body virtually non metamorphic. Its age is not constrained, but it is similar to Mesozoic dykes occurring further south in this region. In all groups, composition is basaltic with tholeiitic affinity. Metamorphic element mobility substantially affected only the LILE, whereas igneous variation trends are preserved for all the other elements. The various groups differ for their incompatible trace element composition and ratios. These ratios are more similar to OIB (Ocean Island Basalt) values rather than to any other magma type. Geochemical evidence rules out any important influence of crustal contamination, fractional crystallisation, or variable degrees of melting of a common source material as an explanation for the inter-group variability. It is inferred that the geochemistry of the different groups reflects complementary characteristics and differences of their mantle sources. These latter are attributed either to a metasomatic enrichment of a variably depleted premetasomatic mantle by a component with OIB characteristics, or, alternatively (our preferred interpretation), to the melting of the metasomatised mantle at different depths. The metasomatised sources underwent extensive melting, producing tholeiitic melts retaining OIB-like geochemical characteristics. By analogy with OIB, the metasomatic component may be plume-related. Dyke emplacement may be controlled by passive crustal rifting induced by plume-related mantle diapirs. In such a scenario, the Group 1 samples could be related to the initial rifting phase and plume impingement in the lithosphere, whereas Groups 2 and 3 could represent advanced stages of crustal thinning and melting of the plume head source. The youngest Group 4 dykes presumably represent a limited and local occurrence of the Mesozoic mafic magmatism which affects the area of the Serra do Espinhaçho.