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 granitoid bodies in the Meguma Lithotectonic Zone of southwestern Nova Scotia in its lithological heterogeneity, extensive physical and chemical interaction with the country rocks, clear evidence for mingling and mixing with mafic magmas, and highly abundant pegmatites. New U–Pb age determinations on monazite establish an intrusion age of 373 ± 1 Ma, similar to the ages of other 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 the outer margin of the pluton, dominated by coarse-grained tonalite-granodiorite, and with Rb/Sr < 0.55, Eu/Eu* > 0.40, and GdN/LuN < 2; (ii) middle stage, occupying the interior of the pluton, dominated by medium-grained granodiorite-monzogranite, and with Rb/Sr > 0.55, Eu/Eu* < 0.40, and GdN/LuN > 2; and (iii) late stage, consisting of abundant minor sheets throughout the pluton, dominated by fine-grained tonalite, granodiorite, and leucogranite that are similar to rocks of the early and middle stages. The Port Mouton pluton shows a wider range of 87Sr/86Sri (0.7036-0.7154), and a wider range and generally higher εNdi (–3.72 to +2.12), than other granitoid rocks in the Meguma Lithotectonic Zone, potentially reflecting a complex, partially equilibrated, interaction among mantle, lower crust, and upper crust. Field, petrological, and chemical evidence for the involvement of mantle-derived magmas and melting of upper crust permit modelling of the Port Mouton pluton granitoid compositions by three simultaneous mixing equations. These mixing model results suggest that the early stage granitoid rocks can form from simple three-component mixing relationships when the bulk distribution coefficients between residuum and melt for Sr and Nd range from 1.05 to 1.18, or two-component mixing combined with fractionation of material like the known felsic lower crust. The middle stage granitoid rocks only yield solutions involving two-component mixing and fractionation of material unlike the known felsic lower crust. We conclude that the Late Devonian mafic magmas played a major role in the formation of granitoid magmas in the Meguma Lithotectonic Zone by supplying heat and material to cause partial fusion of the Avalon lower crust.

Petrochemistry, tectonic history, and Sr–Nd systematics of the Liscomb Complex, Meguma Lithotectonic Zone, Nova Scotia

The Liscomb Complex (area ca. 240 km2), located in the Meguma Lithotectonic Zone of the Canadian Appalachians, consists of three main lithological components: high-grade gneisses, mafic plutons, and peraluminous granitoid bodies. Field relations and 40Ar/39Ar dating (369–377 Ma) embracing all three lithological groups show that the complex is post-Acadian. The gneisses occur as a domal uplift and represent a mixed volcano-sedimentary package that is structurally, metamorphically, and chemically distinct from the surrounding low-grade metawackes and metapelites of the Meguma Group. The mafic intrusions (quartz gabbro to quartz diorite) have major and trace element compositions (e.g., Ti–Zr–Y, Nb–Zr–Y, Th/Yb – Ta/Yb, rare earth elements) typical of within-plate or volcanic arc materials. The peraluminous granitoid rocks range from two-mica granodiorites to leucomonzogranites, and are mineralogically and chemically very similar to granitic rocks elsewhere in the Meguma Zone. Neodymium and strontium isotopic analyses show that (i) the gneisses have a wide range of εNd and initial Sr isotopic ratios, with Nd model ages that are generally younger than those of the Meguma Group; (ii) the mafic intrusive rocks represent magmas derived from slightly depleted mantle sources (εNd +3.3 to +1.4), in part modified by crustal contamination (εNd +0.5 to −5.0); and (iii) the granitoid rocks isotopically overlap both the South Mountain Batholith and the intermediate gneisses of the Liscomb Complex. The combined field, petrological, and chemical evidence suggests that underplating by mafic magmas, followed by thermal doming of the gneisses, diapirism through the Meguma Group, anatexis, and multiple intrusion of both mafic and felsic magmas best explain the observed relationships in the Liscomb Complex. This mechanical model may also apply to granite generation throughout the Meguma Zone.