Gray and 4 co-authors reported in 2010 that felsic volcanic rocks from the ca. 335 Ma Cumberland Hill Formation of southern New Brunswick consist of peralkaline rhyolite and trachyte. These rocks are interpreted to have undergone extensive fractional crystallization from an alkali basaltic magma, although their different geochemical signatures indicate that the rhyolite was not derived from the trachyte via continuous fractional crystallization.
Geologic Map of New Brunswick (To enlarge, click (here)
The rhyolite is highly enriched in incompatible trace elements including uranium (up to ca. 20 ppm). Cumberland Hill peralkaline rhyolite suggests that a potentially economic source of uranium exists in the basin, based on the elevated uranium concentrations and high potential for remobilization into surrounding sedimentary units.
The authors further concluded that felsic volcanic rocks from the Cumberland Hill Formation consist of peralkaline rhyolite and trachyte. Both contain elevated concentrations of incompatible trace elements, with the rhyolite more enriched and fractionated than the trachyte. Although both rock types appear to be genetically related, differences in their geochemical signatures imply the rhyolite was not derived from the trachyte by continuous fractional crystallization.
Examination of the Th/Ta ratio also indicated crustal contamination cannot account for the observed compositional contrast. A preferred model is that the trachyte and rhyolite originated from the same or highly similar parent magma, and represent separate magma pulses with the latter being younger, having undergone more extensive fractionation prior to emplacement.
The absence of a negative Nb anomaly suggests the parent magma was not derived from subcontinental lithospheric mantle, but more plausibly originated from an asthenospheric mantle source. Lastly, Cumberland Hill rhyolite contains elevated U concentrations, with U hosted within the glassy groundmass. Since U can be leached and re-deposited, the rhyolite could be a source of sediment-hosted U mineralization within the basin.
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