Composition and Sm-Nd isotopic data of the lower crust beneath San Luis Potosí, central Mexico: Evidence from a granulite-facies xenolith suite

Abstract

Quaternary volcanics from San Luis Potosí (SLP) in central Mexico sampled a xenolith suite comprising uppermantle nodules (spinell lherzolites, hornblende pyroxenites, spinel pyroxenites) and lower-crustal xenoliths. The latter consist of granulite-facies, mafic to intermediate metaigneous rocks and high-grade metasediments. Precursors of the metaigneous suite range from gabbroic to tonalitic compositions. Whole-rock REE patterns permit classification of metaigneous rocks into cumulates and fractionated melts.Mafic xenoliths consist of plagioclase ± clinopyroxene ± orthopyroxene ± garnet ± amphibole ± apatite ± rutile ± scapolite. Garnet has reacted extensively with clinopyroxene, forming corona textures of secondary plagioclase, orthopyroxene and amphibole. Intermediate xenoliths are garnet-free and quartz-bearing. All samples display similar Ca-Al zoning patterns documented in orthopyroxene, clinopyroxene and plagioclase which are, along with the presence of coronas around garnet, interpreted as representing partial re-equilibration stages from 940 ± 60°C and 7–11 kbar down to lower temperatures and pressures. There is no indication of a secondary heating event in these xenoliths.Sm-Nd isotopic compositions of the spinel lherzolites indicate a heterogeneous composition of the mantle segment beneath SLP. Similar Sr- and Nd-isotopic signatures reveal that the hornblende pyroxenites are high-pressure cumulates linked to the host basanites. Four mainly intermediate metaigneous granulites from a single vent, which are free of secondary amphibole, and two spinel pyroxenites, regarded as the most primitive member of the metaigneous suite, define a regression line in a Sm-Nd isochron diagram, yielding an age value of 1248 ± 69 Ma and an initial ϵNd-value of +2.3 Metaigneous xenoliths containing secondary amphibole plot significantly above the regression line, which is probably due to the introduction of externally derived Nd during amphibole formation under retrograde p-T conditions. Discussing the geological significance of the calculated age value, there are some arguments for interpreting this age as representing the intrusion age of the magmatic precursors. This gives evidence for formation of lower crust of basic to intermediate compositions during the mid-Proterozoic, now situated beneath SLP.