Response of the Rb–Sr system in biotite during contact metamorphism in the aureole of the Makhavinekh Lake Pluton, Labrador

Abstract. High-temperature contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, Labrador, led to progressive consumption of 1850 Ma garnet formed during upper-amphibolite facies regional metamorphism that produced migmatitic paragneiss (Tasiyuak Gneiss). Biotite Rb–Sr isotope measurements were carried out in situ by laser ablation ICP-MS/MS allowing biotite in a variety of textural settings to be characterized. This natural laboratory provides important information about the nature of Rb–Sr closure temperature (Tc) as a function of textural setting in high-grade metamorphic rocks. Intact biotite inclusions armoured in garnet preserved in the outer aureole (>4 km from the contact) display a range of Rb–Sr isochron ages between ∼1850 and ∼1322 Ma consistent with a zone of partial retention of Sr in biotite. Isotopic resetting in the outer aureole was controlled by microfractures in garnet that provided short-circuit diffusion pathways for redistribution of radiogenic Sr into plagioclase-bearing contact metamorphic assemblages; biotite inclusions isolated from microfractures retain 1850 Ma Rb–Sr isochron ages. Biotite grains falling along a ∼1322 Ma isochron attest to efficient intra- and intercrystalline Sr diffusion at T≥500 °C on timescales of ≥5 Myr. Samples in the central part of the contact aureole (3.7 to 1.1 km from the contact) contain partly resorbed biotite surrounded by contact metamorphic Opx + Crd coronal assemblages in addition to armoured inclusions in relict garnet. These display similar Rb–Sr behaviour to outer aureole samples with the exception that ∼1322 Ma biotite domains display higher Rb/Sr due to more extreme loss of Sr. In the inner aureole, where garnet was completely consumed by contact metamorphic assemblages, a new generation of biotite neoblasts grew textural equilibrium with Opx + Crd. This biotite preserves Rb–Sr ages ≤1322 Ma with initial 87Sr/86Sr best interpreted as a mixture of radiogenic Sr accumulated in regional biotite and whole-rock Sr liberated from low-Rb/Sr regional metamorphic garnet, apatite, and plagioclase. This study reveals how the exact textural setting of biotite in high-grade metamorphic rocks influences the preservation of Rb–Sr ages and demonstrates that there is no universal closure temperature for biotite Rb–Sr. It also reveals that in situ Rb–Sr dating of granulite-facies rocks might provide robust chronometric data if grains isolated from intergranular diffusion are systematically evaluated to reveal zones of partial retention.