Geochemical and isotopic (Nd-Sr-Hf-Pb) evidence for a lithospheric mantle source in the formation of the alkaline Monteregian Province (Quebec)

We present new major element and isotopic (Nd-Sr-Hf-Pb) data and modelling from alkaline rocks of the Monteregian Igneous Province of southern Quebec (Canada) that constrain the mantle source and the magmatic origin of these rocks. The whole-rock chemical composition of the intrusions is consistent with fractional crystallization of an assemblage of olivine [+ or -] clinopyroxene (± plagioclase) derived from ocean island basalts (OIB)-like magmas, and variations in the Sr and Nd isotope compositions suggest as much as 20% crustal contamination. The bulk of the Nd-Sr-Hf and Pb isotopic data form a tight cluster between a depleted mantle end-member (HIMU, high-U/Pb mantle) and an enriched mantle (EMI) end-member and are thought to reflect a sub-continental lithospheric mantle that was metasomatized by a convecting asthenospheric plume. Variations in these isotopic compositions along the west-east axis of the Monteregian Province (from the Oka carbonatite to the Mount Shefford intrusion) may reflect various degrees of mixing between HIMU and EMI enriched mantle reservoirs. Anomalously low [sup.207]Pb/[sup.204]Pb and [sup.208]Pb/[sup.204]Pb isotopic ratios from some of the intrusions likely indicate incorporation of an Archean component within the lithospheric mantle. We propose a model in which Monteregian magmatism formed from melting of a predominantly Proterozoic metasomatized lithospheric mantle in response to lithospheric extension during the opening of the North Atlantic Ocean at ca. 124 Ma. L'origine de la Province Ignee des Monteregiennes, Sud Quebec (Canada), est fortement debattue, principalement pour savoir lequel du modele du point chaud ou du modele du rift continental prevaut. Dans cette etude, nous presentons de nouvelles donnees geochimiques et isotopiques (Nd-Sr-Hf-Pb) des roches de la Province Ignee des Monteregiennes afin de contraindre la source mantellique et l'origine de ce magmatisme. La composition chimique des intrusions montre la presence de cristallisation fractionnee d'un assemblage olivine ± clinopyroxene (± plagioclase) derive de magmas de type OIB. Les variations de compositions isotopiques en Sr et Nd suggerent un maximum de 20% de contamination bien que les donnees isotopiques du Sr, Nd, Hf et Pb montrent un regroupement de points entre le pole mantellique type HIMU (appauvri) et le pole type EMI (enrichi). Ces compositions isotopiques semblent ainsi refleter un manteau lithospherique sub-continental metasomatise par la remontee d'un manteau asthenospherique convectif. Une evolution Ouest-Est de la composition isotopique (Nd, Sr, Hf, Pb) est observee, des carbonatites d'Oka vers l'intrusion de Shefford. Cette tendance parait refleter la variation du taux de melange entre les reservoirs HIMU et EMI; la signature isotopique HIMU est dominante a l'Ouest tandis que la signature isotopique EMI est dominante vers l'Est. De faibles rapports isotopiques en [sup.207]Pb/[sup.204]Pb et [sup.208]Pb/[sup.204]Pb sont observes dans certains echantillons et indiquent l'incorporation d'un materiel Archeen dans le manteau lithospherique Proterozoique. Ces resultats indiquent qu'un manteau lithospherique metasomatise est a l'origine du magmatisme des Monteregiennes, possiblement en reponse a une extension lithospherique durant l'ouverture de l'Ocean Atlantique Nord vers 124 Ma.
Introduction Magmas of continental igneous provinces range from large volume tholeiitic flood basalts such as the Deccan Province and Columbia River basalts (e.g., Hawkesworth and Gallagher 1993, and references therein) [...]