Advances in samarium-neodymium geochronology: applications to early earth garnet, hydrothermal carbonate, and high temperature metamorphic systems
Sullivan, Nora Cleary
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This study utilizes recent advances in the analysis of neodymium (Nd) and samarium (Sm) by thermal ionization mass spectrometry (TIMS) to constrain timescales of heating and fluid flow within the lithosphere. Garnet grows during metamorphism and can be linked to its pressure and temperature of growth, while carbonate mineralization ages can be linked to hydrothermal fluid flow. The ability to date these common minerals makes it possible to unravel the timing and duration of tectonometamorphic processes. Garnet from the Eoarchean Nuvvuagittuq Supracrustal Belt (NSB) in Québec, Canada yields an age of 2574.66 ± 0.72 Ma. This is the most precise Neoarchean age reported for this terrane and was achieved through a modified partial dissolution procedure designed to remove the effects of much older (up to 3.8 Ga) inherited mineral growth. An accurate age for the most recent metamorphic garnet in the NSB is critical, as the (controversial) Hadean Nd model age calculations for this terrane depend on the time at which the 147Sm/144Nd systematics were last altered. Carbonate mineralization ages are explored for a diverse group of thirty samples using a novel sequential acid extraction procedure. Through this procedure it is possible to constrain carbonate mineralization in a variety of geologic settings (metamorphic fluid flow, hydrothermal and ore-forming systems) to ± 20 Myr. Zoned and bulk garnet geochronology at ten sites within a unusually localized area (~5 km2) of high temperature granulite-facies metamorphism in Bristol, New Hampshire reveals multiple brief (<1 Myr) periods of garnet growth focused between 393 and 386 Ma, at peak temperatures of 730-850°C. Comparison with garnet growth ages and temperatures , in central Massachusetts (364 Ma at 950°C) and Connecticut (341 Ma at 1000°C) reveal a regional trend of pulsed high temperature garnet growth spanning ~60 Myr from north to south, the same time span bracketed by associated igneous rocks dated via zircon geochronology. Ultrahigh metamorphic temperatures were achieved during the Acadian Orogeny in New England in localized, short pulses, likely related to igneous heating and/or associated fluid flow above and beyond the regional heating due to tectonic overthickening.