Interaction among upper crustal, lower crustal, and mantle materials in the Port Mouton pluton, Meguma Lithotectonic Zone, southwest Nova Scotia

The Port Mouton pluton is unique among the Late Devonian peraluminous grani toid bodies in the Meguma Lithotectonic Zone of southwestern Nova Scotia in its lithological heterogeneity, extensive physical and chemical interactio n with the country rocks, clear evidence for mingling and mixing with mafic magmas, and highly abundant pegmatites. New U-Pb age determinations on mon azite establish an intrusion age of 373 +/- 1 Ma, similar to the ages of ot her Meguma Lithotectonic Zone granitoid plutons and mafic intrusions. Field relations, petrology, and geochemistry define three stages of intrusion of the Port Mouton pluton: (i) early stage, discontinuously exposed around th e outer margin of the pluton, dominated by coarse-grained tonalite-granodio rite, and with Rb/Sr < 0.55, Eu/Eu* > 0.40, and Gd-N/Lu-N < 2; (ii) middle stage, occupying the interior of the pluton, dominated by medium-grained gr anodiorite-monzogranite, and with Rb/Sr > 0.55, Eu/Eu* < 0.40, and Gd-N/Lu- N > 2; and (iii) late stage, consisting of abundant minor sheets throughout the pluton, dominated by fine-grained tonalite, granodiorite, and leucogra nite that are similar to rocks of the early and middle stages. The Port Mou ton pluton shows a wider range of Sr-87/Sr-86(i) (0.7036-0.7154), and a wid er range and generally higher epsilon Nd-i (-3.72 to +2.12), than other gra nitoid rocks in the Meguma Lithotectonic Zone, potentially reflecting a com plex, partially equilibrated, interaction among mantle, lower crust, and up per crust. Field, petrological, and chemical evidence for the involvement o f mantle-derived magmas and melting of upper crust permit modelling of the Port Mouton pluton granitoid compositions by three simultaneous mixing equa tions. These mixing model results suggest that the early stage granitoid ro cks can form from simple three-component mixing relationships when the bulk distribution coefficients between residuum and melt for Sr and Nd range fr om 1.05 to 1.18, or two-component mixing combined with fractionation of mat erial like the known felsic lower crust. The middle stage granitoid rocks o nly yield solutions involving two-component mixing and fractionation of mat erial unlike the known felsic lower crust. We conclude that the Late Devoni an mafic magmas played a major role in the formation of granitoid magmas in the Meguma Lithotectonic Zone by supplying heat and material to cause part ial fusion of the Avalon lower crust.