From grain to terrane: Integrated geochemical and structural studies in granulites.

摘要

Granulites are diagnostic of tectonic regimes that thicken and heat the continental crust. Granulite petrogenesis commonly involves one or more episode of tectonic burial, deformation-related recrystallization, partial melting, fluid fluxing, and/or tectonic exhumation, over millions to tens of millions of years. This dissertation explores the possibility that granulites preserve tectonic and process information in the spatial patterns of structural and compositional variation, especially when considered over multiple length scales.;Chapter I treats the structural record of solid-state flow in granulites from the Arunta block of central Australia. Outcrop-scale strain analysis, combined with map-scale structural and geologic mapping, shows that strain in the deep crust of orogens can be partitioned without being strongly localized. This work also demonstrates that the currently observed fabrics in the Arunta granulites accurately reflect variations in strain during Paleoproterozoic orogenesis.;The remaining three chapters are a set of closely related studies in granulites of the Adirondack Mountains (New York). This suite of papers explores the fine-scale compositional signatures of tectonic-scale processes through in situ geochemical microanalysis of the mineral titanite (also called sphene), with an emphasis on the use of secondary ion mass spectrometry (SIMS). Chapter II is a short proof-of-concept study demonstrating that subtle, diffusion-related d18O zoning in the mineral titanite is resolvable by SIMS and can be modeled to extract quantitative cooling rates for the Mesoproterozoic Grenville orogen. Chapter III is a study of grain-scale mass transport as recorded by oxygen isotopes. It expands upon the ideas in Chapter II and utilizes a larger oxygen isotope data set to investigate a range of intragrain d18O zoning mechanisms in relation to outcrop- and terrane-scale geologic processes, including shear-zone formation, fluid infiltration, and tectonic exhumation through gravitational collapse. Finally, Chapter IV combines multiple fine-scale (intragrain), in situ geochemical datasets to explain the mechanisms of U-Pb age resetting in titanite and locate regions within the grains that preserve accurate event ages.