Petrogenesis Petrogenesis of Neoproterozoic Allanite-bearing Granitic Pegmatites, Perch Falls, New Brunswick: insights from mineral-chemical, whole-rock geochemistry, and U-Pb geochronology

Abstract

A small swarm of eight granitic pegmatite dykes cuts the late Neoproterozoic Perch Falls Granodiorite in the Perch Falls area of southern New Brunswick. Gamma-ray spectrometry reveals notable variability in K, eU, and eTh (6.5–75 ppm), suggesting localized enrichment within the dyke system. These dykes display whole-rock geochemical signatures typical of NYF-type pegmatites, including minor LREE enrichment. The dykes contain elevated SiO₂ concentrations ranging from 70-75.55 wt%, accompanied by variable Fe₂O₃(T) and CaO concentrations. Trace-element concentrations include La (3–20 ppm), Ce (5.8–38.8 ppm), Nd (2.2–18.5 ppm), and Nb (up to 158 ppm). Petrographic analyses reveal that these pegmatites were overprinted by two hydrothermal fluids following crystallization. An early iron-rich hydrothermal fluid overprinted the pegmatites. Copper and sulfur occur together as chalcopyrite localized within iron-rich vein networks. A later calcium-rich hydrothermal fluid overprinted the iron-rich veins and altered associated micas to chlorite. This alteration transformed magmatic epidote into titanite and produced antiperthitic textures within plagioclase. U–Pb dating of zircon and magmatic titanite was conducted to constrain the timing of pegmatite formation. The zircon grains were highly fractured and affected by fluid-mediated alteration, resulting in secondary U–Th-rich microdomains that rendered the zircon unsuitable for precise geochronology. Magmatic titanite from sample PF-25-003 yields a U–Pb date of 533.4 ± 2.9 Ma, which overlaps within analytical uncertainty with published crystallization ages for the Prince of Wales Granite (e.g., 540 ± 3.5 Ma). These results indicate that the pegmatitic dykes are genetically related to the Prince of Wales Granite and represent a more evolved magmatic phase.