P-t-time paths for mid to lower crustal metamorphism in orogenic belts: linking metamorphism with plutonism and toward a better understanding of garnet sm-nd and lu-hf geochronology

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Garnet is an exceedingly useful mineral for interpretation and identification of magmatic and metamorphic processes because it records pressure, temperature, and time (P-T-t) as it grows during metamorphism. Temperatures >650°C, required for partial melting of metasedimentary rocks, can chemically and isotopically reset garnet grains by diffusion, obscuring the growth history and biasing interpretations. However, in rocks that reach temperatures >850°C, large garnet grains can preserve a protracted growth history. Garnet major and trace element zoning is often used to identify preserved growth zoning or diffusional re-equilibration. Trace element zoning of Lu and Sm are particularly useful for interpreting the meaning of garnet ages because the two chronometers used to date garnet are based on the decay of these two elements: Lu to Hf and Sm to Nd. However, these two chronometers seldom yield the same result, often differing by 10’s of millions of years or more, casting uncertainty on the meaning of ages. This dissertation utilizes three suites of rocks in order to better understand discrepancies between the two widely used isotopic systems in garnet and the use of garnet ages for tectonic interpretations. This work uses garnet-bearing metamorphic samples from a variety of tectonic settings and terranes, including the metasedimentary rocks exposed within the southern Coast Mountains batholith, British Columbia; Precambrian metasedimentary rocks in northern New Mexico; and lower-crustal granulites exposed in the exhumed Zealandia magmatic arc, Fiordland, New Zealand. The results of this study highlight the usefulness of garnet geochronometry when chemical zoning is known, permitting connection of garnet ages to P-T conditions and geological processes. The low to mid-amphibolite facies conditions recorded in northern New Mexico and the Coast Mountains of British Columbia facilitate the preservation of prograde zoning in garnet. Garnet in these two settings grew in response to crustal thickening via thrust faulting and plutonism. However, the granulite facies conditions recorded in New Zealand are in sharp contrast. Only the largest garnet grains record prograde growth conditions, while the smallest garnet re-equilibrated at lower temperatures.

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Geology, Geochemistry