Lithospheric to asthenospheric transition in Low-Ti flood basalts from southern Parana, Brazil

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    Abstract

    Two geochemically distinct Low-Ti magma types (Gramado and Esmeralda) are distinguished within the flood basalt sequences of southern Parana on the basis of certain element abundances and ratios. Esmeralda magmas have lower Ce/Sm and (Sr-87/Sr-86)(i), and higher Ti/Zr and epsilon(Nd), than Gramado magmas. Detailed stratigraphical work indicates that there was a temporal progression from Gramado- to Esmeralda-type magmas. This compositional shift cannot simply be explained in terms of a declining extent of crustal contamination of an asthenosphere-derived melt with time, and instead it seems that the two magma types evolved from distinct parental magmas. Although compositional variations of Gramado magmas are dominated by crustal assimilation and fractional crystallisation, geochemical features of the inferred parental magmas [low Ti/Y of < 300 and low Nb/La of <0.8, negative epsilon(Nd)-values similar to -3 and high (Sr-87/Sr-86)(i) 0.708-0.710] are indicative of a significant contribution from the continental mantle lithosphere. Gramado magmas also show a marked regional variation which might imply that the mantle source was compositionally heterogeneous over a length scale of a few hundred km. It is suggested that variations within the Esmeralda magma type are controlled by mixing between an asthenospheric melt with epsilon(Nd) > +4 and a Gramado-type magma, at relatively shallow crustal levels, followed by fractional crystallisation. The asthenospheric component has relatively depleted incompatible element characteristics comparable to MORE. Thus, Low Ti lava sequences of southern Parana record a shift from a predominantly lithospheric to an asthenospheric signature with time. Parana hood basalts appear to have been generated in response to lithospheric extension associated with the opening of the South Atlantic over a region of anomalously hot mantle, attributed to the presence of the Tristan mantle plume. However, the distinctive geochemical signature of the modern Tristan plume, seen on the islands of Tristan da Cunha, Gough and Inaccessible, is not evident in the compositions of Low-Ti magmas in southern Parana.

    Original languageEnglish
    Pages (from-to)124
    Number of pages24
    JournalChemical Geology
    Volume127
    Issue number1-3
    Publication statusPublished - 10 Jan 1996

    Keywords

    • PB-ISOTOPE EVIDENCE
    • CRUSTAL CONTAMINATION
    • MANTLE HETEROGENEITY
    • PLATEAU BRAZIL
    • TRACE-ELEMENT
    • SR-ISOTOPE
    • ND-ISOTOPE
    • GEOCHEMISTRY
    • ATLANTIC
    • LAVAS

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