Timing and Rates of Precambrian Crustal Genesis and Deformation in Northern South America
Gehrels, George E.
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PublisherThe University of Arizona.
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AbstractThe Amazon Craton is one of the largest Precambrian landmasses on Earth, yet its crustal growth history and evolution are relatively poorly constrained. This dissertation addresses the timing and modes of continental growth and deformation of a sizable portion of the central and western Guyana Shield, mostly using a combination of texturally-resolved U-Pb, Lu-Hf and δ¹⁸O analysis of zircon. These results provide a wealth of new information that have major implications for the role of Amazonia in Precambrian supercontinent models, its significance in the global record of crustal generation and the possible tectonic processes that were involved in its construction. It is argued that, contrary to the previously accepted Proterozoic accretionary model for the growth of northwestern Amazonia, the new data suggest that sizable portions of the shield formed during an episodic extraction event in the Neorchean. This hypothesis challenges the current tectonic model for the evolution of Amazonia, and provides evidence for the continued importance that mechanisms inducing episodic crustal generation had after subduction-driven plate tectonics had been established on Earth. During the last decade, the study of crustal growth processes by means of U-Pb-Hf-O analysis of zircon has considerably improved our understanding of the mechanisms that drive planetary differentiation. However, zircon has a significant limitation, and is that basic and ultrabasic rocks have very low fertility to form this mineral owing to their low SiO₂ composition. Therefore, the global zircon U-Pb-Hf repository is biased towards the felsic portion of the crust, leaving the mafic archive mostly unconstrained. Undersaturated rocks can form baddeleyite, a phase with high affinity for Hf and U, low affinity for Yb, Lu and Pb, and slow diffusion rates with respect to all these chemical species. Consequently, baddeleyite is an extremely robust carrier of geochronological (by using the U-Pb system) and tracer (by using the Lu-Hf system) information for the origin of mafic rocks. This dissertation provides a method for the fast, accurate and precise analysis of U-Pb-Hf isotopes in baddeleyite, a tool that in the future may prove crucial for studying the timing and processes associated with the formation of mafic crust on Earth.
Degree ProgramGraduate College