Petrogenesis of the peralkaline Flowers River Igneous Suite and its significance to the development of the southern Nain Batholith

The Flowers River Igneous Suite of north-central Labrador comprises several discrete peralkaline granite ring intrusions and their coeval volcanic succession. The Flowers River Granite was emplaced into Mesoproterozoic-age anorthosite–mangerite–charnockite–granite (AMCG) -affinity rocks at the southernmost extent of the Nain Plutonic Suite coastal lineament batholith. New U–Pb zircon geochronology is presented to clarify the timing and relationships among the igneous associations exposed in the region. Fayalite-bearing AMCG granitoids in the region record ages of 1290 ± 3 Ma, whereas the Flowers River Granite yields an age of 1281 ± 3 Ma. Volcanism occurred in three discrete events, two of which coincided with emplacement of the AMCG and Flowers River suites, respectively. Shared geochemical affinities suggest that each generation of volcanic rocks was derived from its coeval intrusive suite. The third volcanic event occurred at 1271 ± 3 Ma, and its products bear a broad geochemical resemblance to the second phase of volcanism. The surrounding AMCG-affinity ferrodiorites and fayalite-bearing granitoids display moderately enriched major- and trace-element signatures relative to equivalent lithologies found elsewhere in the Nain Plutonic Suite. Trace-element compositions also support a relationship between the Flowers River Granite and its AMCG-affinity host rocks, most likely via delayed partial melting of residual parental material in the lower crust. Enrichment manifested only in the southernmost part of the Nain Plutonic Suite as a result of its relative proximity to multiple Palaeoproterozoic tectonic boundaries. Repeated exposure to subduction-derived metasomatic fluids created a persistent region of enrichment in the underlying lithospheric mantle that was tapped during later melt generation, producing multiple successive moderately to strongly enriched magmatic episodes.

Granulite-facies regional and contact metamorphism of the Tasiuyak paragneiss, northern Labrador: textural evolution and interpretation

The Tasiuyak paragneiss at the western margin of the Nain Plutonic Suite has been subjected to two granulite-facies metamorphic events: (i) regional metamorphism during the Paleoproterozoic Torngat orogeny, and (ii) contact metamorphism due to emplacement of the Mesoproterozoic Nain Plutonic Suite. Regional metamorphism led to partial melting of pelitic rocks and the development of a locally well-preserved sequence of prograde and retrograde textures. These textures are partly controlled by bulk composition and formed in the pressure–temperature (P–T) field of the continuous reaction: biotite + sillimanite + plagioclase + quartz = garnet + K-feldspar + melt, along a hairpin P–T path with peak conditions of ~8–10 kbar (0.8–1.0 GPa) and up to 870 °C in the NaKFMASH (Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O) system. These textures controlled the development of the contact metamorphic assemblages. Contact metamorphism of the pelitic rocks between the Tessiarsuyungoakh intrusion and the Makhavinekh Lake pluton led to growth of orthopyroxene-cordierite symplectite after garnet–biotite, and cordierite–spinel symplectite after garnet–sillimanite. These phase associations attest to reactions in specific microtextural settings, some of which produced a second generation of partial melt. Maximum temperatures were above ~750 °C and pressures were lower than those of the regional metamorphism. The aureole around the Makhavinekh Lake pluton is ~4 km wide and shows a progressive development of the contact metamorphic assemblages toward the pluton. In contrast, the contact metamorphic overprint is incipient around the Tessiarsuyungoakh intrusion, which developed a ~20 m wide contact aureole and is most prominent in screens of paragneiss within that intrusion.

Emplacement of the Nain anorthosite: diapiric versus conduit ascent

Estimation of settling velocities of large orthopyroxene megacrysts, found within anorthosite intrusions, are calculated and compared with ascent rates achieved by diapirism and conduit propagation. Calculations suggest that diapirism is far too slow to be an appropriate ascent mechanism for anorthositic crystal mush and favour conduit emplacement. The intrusions of the Nain Plutonic Suite (NPS) are located along the Abloviak shear zone, which marks the boundary between the Nain and Churchill provinces, and within the zone of juxtaposition of the Saglek and Hopedale blocks of the Nain Province. These crustal weaknesses have probably controlled the emplacement and distribution of the intrusions. Contact relations between intrusions of anorthosite and their gneissic host rock provide evidence for two emplacement styles within the NPS, the first typified by strongly deformed and recrystallized rocks, and the second by an outer border zone of mafic rocks. It is proposed that these differences in intrusive style are due to differences in ductility contrast between the magma and its surrounding host rocks, such that those intrusions emplaced into the thermally softened shear zone have deformed margins, whereas those intruded into the cooler Archaean crust have undeformed margins.

Country-rock contamination of marginal mafic granulites bordering the Nain Plutonic Suite: implications for mobilization of Sr during high-grade contact metamorphism

The marginal mafic granulites that locally border the Nain Plutonic Suite (NPS) have a range of initial Nd-isotope ratios that overlap with that of the NPS anorthosites and associated Nain dykes. The similarity in Nd-isotope data suggests that gneissic Archaean country rocks have contaminated all the anorthosites, marginal mafic granulites, and dykes. Sr-isotope data for the mafic granulites and dykes support a country rock contamination scenario but preclude wholesale assimilation of rocks such as the host Archaean tonalite gneisses as the sole contaminant. Initial epsilonSr values of +10 to +403 and +0.9 to +242 for the mafic granulites and dykes, respectively, are significantly higher than values for NPS country rocks examined thus far. The elevated initial εSr values are therefore interpreted to result from the introduction of radiogenic Sr into the granulites and dykes via Sr-rich fluids, generated by the breakdown of Rb-rich mineral phases such as biotite in the country rocks during NPS.