Petrological and Sr-Nd evidence bearing on Early Proterozoic magmatic events of the subcontinental mantle: Sao Francisco craton (Uaua, NE-Brazil)

Abstract

Early Proterozoic (2.0–2.4 Ga), unmetamorphosed mafic dykes intrude Archean (3.1–2.7 Ga) terrains of the northern Sao Francisco craton (Uaua, NE-Brazil). The dykes are composed of evolved [atomic Mg/(Mg+Fe2)<0.6] two-pyroxene quartz tholeiites, the compositional variations of which are compatible with gabbro fractionation from different parental melts. The incompatible trace-element patterns indicate that the parental melts derived from partial melting of chemically heterogeneous garnet peridotite sources. Geochemistry and Sr-Nd isotopes imply some contamination of the Uaua dyke magma by continental crustal components during emplacement. Sr-isotopes suggest that the “uncontaminated” dykes may be related to time-integrated depleted mantle materials, while Nd-isotopes suggest mantle sources similar to the bulk Earth composition. The “uncontaminated” dykes have a positive Nb-anomaly and incompatible element contents which do not support appreciable enrichment of large ion lithophile and light rare earth elements in the source(s) throughmantle metasomatism and/or crustal components related to dehydration of a subducting slab, and an anorogenic ensialic emplacement is inferred. The Uaua dykes are chemically and isotopically distinct from the Early–Middle Proterozoic (1.9–1.7 Ga) unmetamorphosed dykes from W-Uruguay (Rio de La Plata craton) which are believed to have originated from different garnet peridotite sources. This suggests that Uaua–Uruguay compositional differences may reflect different Archean mantle differentiation and evolution of the cratonization processes. If this possibility proves to be correct, then magma genesis from various South American cratons would be by independent cratonic processes and their imprints on the subcratonic mantle.