Abstract Protogranular spinel-peridotite mantle xenoliths
and their host sodic alkaline lavas of Cretaceous to
Paleogene age occur at the same latitude ≈26°S in central
eastern Paraguay and Andes. Na- alkaline lavas from both
regions display similar geochemical features, differing
mainly by higher Rb content of the Paraguayan samples.
Sr, Nd, and Pb isotope ratios are also similar with
predominant trends from depleted to enriched mantle
components. The mantle xenoliths are divided into two
main suites, i.e. relatively low in potassium and incompatible
elements, and high in potassium and incompatible
elements. The suite high in potassium occurs only in
Paraguay. Compositions of both suites range from lherzolite
to dunite indicating variable “melt extraction”. Clinopyroxenes
from the xenoliths display variable trace element
enrichment/depletion patterns compared with the pattern of
average primitive mantle. Enrichment in LREE and Sr
coupled with depletion of Nb, Ti and Zr in xenoliths from
both areas are attributed to asthenospheric metasomatic
fluids affecting the lithospheric mantle. Metasomatism is
apparent in the sieve textures and glassy drops in
clinopyroxenes, by glassy patches with associated primary
carbonates in Paraguayan xenoliths. Trace element geochemistry
and thermobarometric data indicate lack of
interaction between xenoliths and host lavas, due to their
rapid ascent. Sr and Nd isotope signatures of the Andean
and Paraguayan xenoliths and host volcanic rocks plot
mainly into the field of depleted mantle and show some
compositional overlap. The Andean samples indicate a
generally slightly more depleted mantle lithosphere. Pb
isotope signatures in xenoliths and host volcanic rocks
indicate the existence of a radiogenic Pb source (high U/Pb
component in the source) in both areas. In spite of the
distinct tectonic settings, generally compressive in the
Central Andes (but extensional in a back-arc environment),
and extensional in Eastern Paraguay (rifting environment in
an intercratonic area), lavas and host xenoliths from both
regions are similar in terms of geochemical and isotopic
characteristics
