A REVISED PETROGENIC HISTORY FOR THE LATE PALEOZOIC FELSIC VOLCANIC ROCKS OF THE HARVEY FORMATION, SOUTHERN NEW BRUNSWICK, CANADA

The Late Devonian Harvey Formation (Fm) in southern New Brunswick, Canada contains felsic volcanic and volcaniclastic rocks with elevated uranium concentrations that have received attention as possible uranium resources. Due to the petrographic similarity of the Harvey Fm to other global U-rich, topaz-bearing rhyolites (e.g., the Piskahegan Fm, also in southern New Brunswick), a common petrogenetic history has been suggested. However, this is based only on limited geochemical, lithologic, and petrographic analysis of the Harvey Fm. The current model for the origin of felsic rocks of the Harvey and Piskahegan Fms is partial melting of crustal rocks at ~600-800°C, based on mineralogical studies. The emplacement of basaltic magma during the initial phases of lithospheric extension is the hypothesized heat source for crustal melting. Supporting evidence for this hypothesis includes glassy melt inclusions preserved within quartz phenocrysts that have a peraluminous nature.. However, our preliminary analyses of melt inclusions from the Harvey Formation indicate considerably lower fluorine (<0.02%F) and water content than previously documented implying that the Harvey Fm was emplaced at higher temperatures than the compared Piskahegan Fm. Furthermore, there are differences in the REE patterns of the Harvey and Piskahegan Fms, whereby the Harvey Fm has a relatively flat chondrite-normalized pattern and the Piskahegan Fm REE pattern is fractionated with enrichment in LREE and depletion in HREE. Collectively, these results challenge the previously accepted hypothesis for petrogenesis of the Harvey Fm, and support more complex – and possibly different – formation mechanisms for U-rich volcanic and volcaniclastic rhyolites in southern New Brunswick. Clearly, a more detailed understanding is needed on the petrogenesis of the Harvey Fm, considering its current economic value and the projected future global demand for uranium ore.