COMPOSITION AND SM-ND ISOTOPIC DATA OF THE LOWER CRUST BENEATH SAN-LUIS-POTOSI, CENTRAL MEXICO - EVIDENCE FROM A GRANULITE-FACIES XENOLITH SUITE

Quaternary volcanics from San Luis Potosi (SLP) in central Mexico samp led a xenolith suite comprising upper-mantle nodules (spinel lherzolit es, hornblende pyroxenites, spinel pyroxenites) and lower-crustal xeno liths. The latter consist of granulite-facies, mafic to intermediate m etaigneous rocks and high-grade metasediments. Precursors of the metai gneous suite range from gabbroic to tonalitic compositions. Whole-rock REE patterns permit classification of metaigneous rocks into cumulate s and fractionated melts. Mafic xenoliths consist of plagioclase +/- c linopyroxene +/- orthopyroxene +/- garnet +/- amphibole +/- apatite +/ - rutile +/- scapolite. Garnet has reacted extensively with clinopyrox ene, forming corona textures of secondary plagioclase, orthopyroxene a nd amphibole. Intermediate xenoliths are garnet-free and quartz-bearin g. All samples display similar Ca-Al zoning patterns documented in ort hopyroxene, clinopyroxene and plagioclase which are, along with the pr esence of coronas around garnet, interpreted as representing partial r e-equilibration stages from 940 +/- 60 degrees C and 7-11 kbar down to lower temperatures and pressures. There is no indication of a seconda ry heating event in these xenoliths. Sm-Nd isotopic compositions of th e spinel lherzolites indicate a heterogeneous composition of the mantl e segment beneath SLP. Similar Sr- and Nd-isotopic signatures reveal t hat the hornblende pyroxenites are high-pressure cumulates linked to t he host basanites. Four mainly intermediate metaigneous granulites fro m 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 epsilon(Nd)-value of 2.3. Metaigneous xenoliths containing secondary amphibole plot signifi cantly above the regression line, which is probably due to the introdu ction of externally derived Nd during amphibole formation under retrog rade p-T conditions. Discussing the geological significance of the cal culated age value, there are some arguments for interpreting this age as representing the intrusion age of the magmatic precursors. This giv es evidence for formation of lower crust of basic to intermediate comp ositions during the mid-Proterozoic, now situated beneath SLP.