Mineralogy and Petrology (2000) 70: 25±35 Clinopyroxenes from metasomatized spinel-peridotite mantle xenoliths from Nemby (Paraguay): crystal chemistry and petrological implications F. Princivalle 1 , M. Tirone 2 , and P. Comin-Chiaramonti 3 1 Dipartimento di Scienze della Terra (DST), University of Trieste, Trieste, Italy 2 Department of Geosciences, University of Arizona, Gould-Simpson Building, Tucson, AZ, USA 3 Dipartimento di Ingegneria Chimica, dell'Ambiente e delle Materie Prime (DICAMP), University of Trieste, Trieste, Italy With 4 Figures Received February 1, 1999; revised version accepted November 26, 1999 Summary The Nemby mantle xenoliths are represented by spinel peridotites variably affected by ``metasomatic processes'' in Proterozoic times (1.2±0.5 Ga). Clinopyroxene composi- tional variations mainly re¯ect variably degrees of basalt extraction and mantle ``metasomatic processes''. Clinopyroxene crystal chemistry allows to estimate the equilibration pressure by V(Cell) and V(M1) relationship. The investigated clinopyr- oxenes yielded a pressure range of 12±18 kb. Therefore, the Nemby xenoliths are representative of about 20 km thickness of subcontinental lithospheric mantle of the South America platform at Tertiary times (ca. 46 Ma). Zusammenfassung Klinopyroxene aus metasomatischen Spinell-Peridotit-Mantel Xenolithen aus Nemby (Paraguay): Kristallchemie und petrologische Bedeutung Die Mantelxenolithe aus Nemby sind Spinellperidodite, die in verschiedenem Ausmaû im Proterozoikum (1.2 bis 0.5Ga) durch ``metasomatische Prozesse'' beein¯usst wurden. Variationen in der Zusammensetzung der Klinopyroxene lassen variable Ausmaûe von Basaltextraktion und Mantelmetasomatose erkennen. Die Kristallchemie der Pyroxene erm oglicht es, den Gleichgewichtsdruck durch V(Cell) und V(M1) Beziehungen zu bestimmen. Die untersuchten Klinopyroxene zeigten Drucke im