Geochronology and geochemistry of rare-element pegmatites from the Superior Province of Canada

Smith, Stephen Raymond (2001). Geochronology and geochemistry of rare-element pegmatites from the Superior Province of Canada. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000e7db

Abstract

The crystallisation age and thermochronology of late- to post-orogenic, rare-lithophile element (Li, Cs, Ta) pegmatites that are chemically associated with peraluminous granites have been determined from U-Pb TIMS and LA-MC-ICP-MS analyses of columbite-tantalite, and UV -laser ⁴⁰Ar/³⁹Ar analyses of mica.

In the northwest Superior Province, U-Pb ages reveal that the Pakeagama Lake pegmatite crystallised at 2673 ±8Ma. Rare-element pegmatites in the western Superior Province yield crystallisation ages of 2649 ±4Ma and 2644 ±8Ma for the Separation Rapids group, and 2665 ±lOMa for the Mavis Lake group, showing that they are temporally associated with adjacent peraluminous granites. Replacement zones of columbite-tantalite grains from the pegmatites show disturbance to the U-Pb system, probably caused by interaction with residual fluids and sub solidus metasomatic processes that lasted for -30Ma after crystallisation.

Mica from rare-element pegmatites is characterised by distinct compositional zoning with muscovite cores and F-, Li- and Rb-rich zinnwalditellepidolite rims. The zinnwalditellepidolite rims that replace the original muscovite were most likely precipitated from a residual melt or fluid phase during the late-magmatic to hydrothermal period of pegmatite formation.

Within these micas, ⁴⁰Ar/³⁹Ar apparent ages young towards the grain boundary and also towards the core - rim boundary, which suggests that the compositional zones form separate argon diffusion domains. However, similar ⁴⁰Ar/³⁹Ar apparent ages are found within F-rich zinnwaldite/lepidolite and coexisting muscovite indicating that diffusion rates may be comparable.

Mica from rare-element pegmatites of the Superior Province yield intra-grain ⁴⁰Ar/³⁹Ar apparent age variations of up to 800Ma within single crystals. Theoretical slow cooling models do not match the measured ⁴⁰Ar/³⁹Ar profiles and cannot account for the large apparent age variations. Thermal history models that produce the best fit to apparent age profiles include two re-heating events; a minimum 350°C re-heating at -2450Ma and circa 300°C re-heating at 1850Ma, both for a period of at least 10Ma. These apparent ages are concordant with two major Proterozoic events affecting the Superior Province; intrusion of the Matachewan - Hearst dyke swarms and the Trans Hudson Orogeny, respectively.

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