Paleoproterozoic potassic–ultrapotassic magmatism: Morro do Afonso Syenite Pluton, Bahia, Brazil

Abstract

The Morro do Afonso Syenitic Pluton (MASP) is a small, bimodal suite of lamprophyric and syenitic rocks, located in the Serrinha Nucleus, São Francisco Craton, northeastern Brazil. The unit consists predominantly of alkali feldspar syenite with numerous mafic xenoliths and dykes that are predominantly lamprophyric. The syenite shows geochemical and isotopic features typical of the Roman potassic magmas, i.e. low TiO2 (up to1.2%), low K2O/Al2O3 (0.33–0.49), strong enrichment in LILE, and high LILE/HFSE ratios. Fractionation, mainly of clinopyroxene and amphibole, explains the transition from lamprophyre to syenite. Magma mixing is required to explain the most evolved compositions. MASP syenites have Paleoproterozoic crystallization ages (2.11 Ga). The Nd isotope composition of mafic and felsic phases are similar (εNdmafic = −2.36 to −2.61 and εNdfelsic = −2.47) and they have Archaean model ages (TDM = 2.56–2.58 Ga) that match the age of an inherited zircon grain (ca. 2.6 Ga). MASP is similar to many other post-orogenic syenitic plutons distributed over a wide area of São Francisco Craton (SFC), which are closely comparable in age and also typically associated with ultrapotassic lamprophyre. The present results suggest that a major proportion of the potassic–ultrapotassic syenitic rocks of the SFC were emplaced in a narrow time interval from 2111 to 2080 Ma, limiting the time of metamorphism peak of the Transamazonian Orogeny to at least 2.1 Ga. These plutons show some evidence of having interactions with older felsic crustal rocks and are interpreted to be derived from partial melting of lithospheric mantle that was enriched by subduction.