Dating and tracing highly fractionated granitic systems: resolving isotopic disturbance and metal source in the 2.7 Ga Tanco pegmatite

Abstract

Rare-metal pegmatites are primary repositories of Li, Cs, Ta, and Sn, yet their genesis remains debated, largely due to difficulties in constraining the timing, source, and evolution of highly evolved granitic systems. These challenges are particularly pronounced in Archean terranes, where radiation damage and later thermal events promote isotopic disturbance. The Tanco pegmatite in Canada, a classic Li–Cs–Ta (LCT) system, provides an ideal case to address these issues. Previous U–Pb and Rb–Sr datasets have been used to indicate pegmatite formation at ∼2.64 Ga from S-type melts, followed by prolonged subsolidus fluid activity lasting for tens of millions of years. New in situ U–Pb analyses of cassiterite and columbite–tantalite (n = 28) define a lead-loss array with an upper-intercept age of 2.70 ± 0.03 Ga, representing primary crystallization of the Tanco pegmatite. This age overlaps with that of the nearby Birse Lake pluton, suggesting that pegmatite emplacement was contemporaneous with regional magmatism and potentially genetically related to the pluton. Muscovite from the Tanco pegmatite is exceptionally enriched in Rb (>10⁴ ppm) with high ⁸⁷Rb/⁸⁶Sr ratios (10⁴–10⁶), resulting in negligible common Sr and enabling single-mineral Rb–Sr dating. The wide spread in muscovite apparent Rb–Sr ages (1.9–2.8 Ga, n = 374) necessitates redistribution of radiogenic ⁸⁷Sr long after pegmatite crystallization. Because little radiogenic ⁸⁷Sr would have accumulated during pegmatite cooling, subsolidus hydrothermal activity at that stage cannot explain the observed age dispersion. Instead, the scattered ages likely record diffusion-driven radiogenic ⁸⁷Sr redistribution during a later thermal overprint, consistent with the cassiterite/columbite-tantalite U–Pb lower-intercept age of 0.3 ± 0.1 Ga. Importantly, anomalously high (⁸⁷Sr/⁸⁶Sr)_initial values do not indicate an S-type source, but instead reflect radiogenic Sr redistribution. Cassiterite δ¹⁸O values of 4.7 ± 0.8 ‰ (2SD; n = 28) correspond to equilibrium zircon δ¹⁸O values of ∼7.5 ± 0.8 ‰ at 600 ± 50 °C, indicating derivation from a hybrid melt containing <30% S-type component, rather than a purely S-type source. The extreme enrichment of incompatible elements in the Tanco pegmatite reflects advanced fractional crystallization, which progressively concentrates rare metals in the residual melt. Our results reconcile inconsistencies in existing datasets, suggest a genetic link between the Tanco pegmatite and the Birse Lake pluton, and demonstrate that isotope systems in highly evolved granitic rocks are readily modified by diffusion and thermal/hydrothermal overprinting, requiring integrated multi-isotopic approaches to constrain their genesis.