Browsing by Author "McFarlane, Christopher R. M."

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    In situ multiphase LA-ICP-MS U-Pb Geochronology of terrestrial impact structures
    (University of New Brunswick, 2020-08) McGregor, Maree; Spray, John G.; McFarlane, Christopher R. M.
    In an attempt to improve the chronologic record of impact events on Earth, U-Pb geochronology has been conducted on shocked and thermally metamorphosed accessory phases (zircon, titanite and apatite) from several terrestrial impact structures using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). All dated phases occur as inherited grains derived from the underlying target lithologies and now occur within impact melt-bearing breccias and clast-laden melt rocks. This study provides the first application of the apatite U-Pb geochronometer from a terrestrial impact structure, and emphasizes the complexity of dating inherited grains within impact melt-bearing lithologies. Unlike newly-grown (igneous) grains within impact melt sheets, the results presented here highlight the challenges of obtaining precise and accurate impact ages from variably reset grains within complex lithologies. This approach requires an understanding of the relationship between isotopic resetting, extreme pressure-temperature (P-T) conditions and variable temperature-time (T-t) histories realized during impact events, impact-induced deformation microstructures, solid-state recrystallization, and pre-impact radiation damage within inherited grains. The results from this study have contributed to the chronological record of terrestrial impact events and the current understanding of U-Pb isotope systematics within U-bearing accessory phases during hypervelocity impact events. The use of a multiphase, in situ LA-ICP-MS U-Pb geochronological approach has permitted an inter-phase assessment on the comparative reliability of zircon, apatite and titanite as impact chronometers, while also providing insights into the U-Pb isotope systematics of these phases under extreme P-T-t conditions. The results reveal that isotopic resetting in apatite is thermally induced, inferred to be the result of apatite’s lower closure temperature and rapid Pb diffusivities. Consequently, apatite is determined to be more susceptible to isotopic resetting during short-lived temperature excursions compared to zircon and titanite. As such, this study demonstrates that, in the absence of coherent impact melt sheets and where structures remain tectonically undeformed, apatite is the most reliable U-Pb geochronometer for accurately dating terrestrial impact structures. Under the same P-T-t conditions, zircon and titanite are found to be less useful impact chronometers and may yield geologically unmeaningful ages. Similar to apatite, isotopic resetting in titanite is primarily thermally induced. However, due to numerous factors including its higher closure temperatures and slower Pb diffusivities, titanite is prone to incomplete isotopic resetting, and is considered a particularly complex U-Pb impact chronometer. Unlike apatite and titanite, isotopic resetting in zircon is deformation enhanced. In addition to recrystallization-driven Pb loss, the results presented here demonstrate that, for the first time, radiation damage within pre-impact zircons facilitates isotopic resetting. However, metamict zircons are found to be susceptible to recent Pb loss and common Pb contamination, with lower intercept ages typically yielding anomalously young impact ages that are consistantly unreliable. The application of multiphase in situ LA-ICP-MS U-Pb geochronology has provided the first higher precision age constraints for four terrestrial impact structures in Canada: Nicholson Lake (387 ± 5 Ma), Lac La Moinerie (453 ± 5 Ma), Steen River (141 ± 4 Ma) and Brent (452.8 ± 2.7 Ma). Excluding Nicholson Lake, all structures yield ages that overlap with biological extinction events, with Steen River forming at, or close to, the Jurassic-Cretaceous boundary, and both Lac La Moinerie and Brent forming at, or close to, the Sandian-Katian boundary in the Upper Ordovician.
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    Petrogenetic analysis of arc-related Devonian magmatic rocks in relation to the formation of magmatic hydrothermal Cu±Mo±Au deposits in the New Brunswick segment of the Northern Appalachians
    (University of New Brunswick, 2024-10) Yousefi, Fazilat; Lentz, David R.; McFarlane, Christopher R. M.
    Devonian adakitic porphyritic intrusive rocks in New Brunswick (NB) are associated with porphyry Cu-Mo-Au mineralization and are similar to Cu-Mo-Au porphyries in Québec and Maine, implying a shared genesis. These granitoids are similar petrologically and feature oxidized I-type arc-like compositions with SiO2 ≥66.5 wt.% and Al2O3 >15.5 wt.%, among other distinctive geochemical characteristics. Trace element compositions characterize the link between adakite formation, slab failure, and porphyry Cu-Mo-Au system potential. These rocks are enriched in Cs, Rb, and Ba, but depleted in Nb, Ta, P, and Ti, reflecting subduction-related magmatism. The slab breakoff process involves the breaking of part of the subducted oceanic plate, leading to asthenosphere upwelling and selective partial melting of the slab, the suprasubduction zone lithospheric mantle, the upwelling mantle, and the basal continental crust; transpressional to transtensional tectonics can facilitate magma ascent and emplacement in the post-subduction setting. Zircon, because of its refractory nature and prevalence in igneous rocks, is useful for assessment of magmatic evolution, specifically its oxygen and U-Th-Pb isotopic compositions. This information can be used to predict magmatic fertility in terms of Cu ± Mo ± Au porphyry mineralization. LA-ICP-MS analyses of zircons from the adakitic intrusions investigated reveal diverse compositions, reflecting varied magmatic conditions. The Zr/Hf and Ce/Ce* values provide insights into porphyry fertility, aiding mineralization potential assessment. This study also employs petrography, µXRF-EDS mapping, and LA-ICP-MS to analyze titanite compositions in these oxidized I-type granitoids. Titanite is enriched in high field strength elements (Ta, Zr, Hf, Th, U), rare earth elements, and Sr and can serve as a petrogenetic and metallogenic indicator. Zr-in-titanite thermometry reveals varied crystallization temperatures (784° to 1098°C) among the intrusions investigated. Magnetite phenocrysts in these intrusions have euhedral to subhedral morphology and lack oscillatory zoning (according to SEM-BSE imaging). The diverse trace element compositions offer insight into the magmatic to magmatic-hydrothermal systems. EPMA analysis reveals variable concentrations of SiO2, Al2O3, MgO, Mn, Fe, Ti, Cr, and V in magnetite, with distinguishing features for various deposit systems. When comparing Ti+V with Al+Mn concentrations in magnetite crystals, all samples exhibit similar crystallization temperatures (>500ºC).