Alkaline lamprophyres from southern Tuscany are early Cretaceous, ultrabasic, primary mantle melts that have not undergone significant magmatic differentiation. New mineralogical, geochemical and Sr, Pb and Nd isotopic evidence shows contrasting geochemistries with “crustal-like” and mantle geochemical features. High Mg#'s, however, coupled with high compatible elements, rule out any notable mantle melt dilution by bulk crustal contamination. Variations of Zr/Hf and Ta/Nb indicate a source containing residual titanates while their REE geochemistry suggests low degrees of partial melting and possible metasomatism of their source by carbonatitic melts. Arrays in isotope ratio diagrams are consistent with mixing between two distinct mantle end members. One is FOZO-like in character, and supports the involvement of asthenosphere, while the second has 87Sr/86Sr initial values that are much higher than FOZO (> 0.70641). High 87Sr/86Sr signatures are present in the other Italian alkaline lamprophyres as well as other potassic–ultrapotassic and carbonatitic rocks of Italy. Our preferred model involves melting of a two component, metasomatised mantle, which can be tied into plume-related magmatism.
Lamprophyres of Italy: Early Cretaceous Alkaline lamprophyres of Southern Tuscany, Italy
STOPPA, Francesco;SCHIAZZA, MARIANGELA;
2013-01-01
Abstract
Alkaline lamprophyres from southern Tuscany are early Cretaceous, ultrabasic, primary mantle melts that have not undergone significant magmatic differentiation. New mineralogical, geochemical and Sr, Pb and Nd isotopic evidence shows contrasting geochemistries with “crustal-like” and mantle geochemical features. High Mg#'s, however, coupled with high compatible elements, rule out any notable mantle melt dilution by bulk crustal contamination. Variations of Zr/Hf and Ta/Nb indicate a source containing residual titanates while their REE geochemistry suggests low degrees of partial melting and possible metasomatism of their source by carbonatitic melts. Arrays in isotope ratio diagrams are consistent with mixing between two distinct mantle end members. One is FOZO-like in character, and supports the involvement of asthenosphere, while the second has 87Sr/86Sr initial values that are much higher than FOZO (> 0.70641). High 87Sr/86Sr signatures are present in the other Italian alkaline lamprophyres as well as other potassic–ultrapotassic and carbonatitic rocks of Italy. Our preferred model involves melting of a two component, metasomatised mantle, which can be tied into plume-related magmatism.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
