Response of 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 MS/MS ICP-MS allowing biotite in a variety of textural settings to be characterized. This natural laboratory provides important constraints on 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 (>4km from the contact) display a range of Rb-Sr isochron ages between ~1850 Ma 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 retained 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 has been completely consumed by contact metamorphic assemblages, a new generation of biotite neoblasts occurs in 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 or UHT rocks might provide robust chronometric data if grains isolated from intergranular diffusion are systematically evaluated to reveal zones of partial retention.