Country-rock contamination of marginal mafic granulites bordering the NainPlutonic 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 th e NPS anorthosites and associated Nain dykes. The similarity in Nd-isotope data suggests that gneissic Archaean country rocks have contaminated all th e anorthosites, marginal mafic granulites, and dykes. Sr-isotope data for t he mafic granulites and dykes support a country rock contamination scenario but preclude wholesale assimilation of rocks such as the host Archaean ton alite gneisses as the sole contaminant. Initial epsilon(Sr) values of +10 t o +403 and +0.9 to +242 for the mafic granulites and dykes, respectively, a re significantly higher than values for NPS country rocks examined thus far . The elevated initial es, 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 bioti te in the country rocks during NPS.