Using Large Layered Intrusions as Analogues for Understanding Subduction Zone Hydrothermal Systems
The genesis of layered intrusions has been the focus of countless studies. Layered intrusions have historically been viewed as natural laboratories to understand the evolution of a single large magma chamber. Many contain platinum- and palladium-rich reef-type deposits, making layered intrusions particularly important economically. Further, layered intrusions may be a useful analogue for understanding subduction zone hydrothermal systems.This dissertation investigates layered intrusion genesis, specifically in relation to the suggested hydrothermal model of layered intrusion formation, which suggests migrating fluids may have remobilized economically important elements, creating the deposits observed in these intrusions today. The work is divided into eight chapters that explore three layered intrusions: the Bushveld Complex, South Africa, the Stillwater Complex, Montana, and the Skaergaard Intrusion, Greenland. New samples collected from the Stillwater Complex were analyzed for major and trace element compositions and radiogenic and stable isotopes. Investigations into the Bushveld Complex and Skaergaard Intrusion were based on previously published data. The second chapter examines evidence for fluid circulation in the Bushveld Complex, South Africa, as responsible for some of the geochemical and isotopic signatures present in the complex. Previous isotopic studies of Bushveld are combined with numerical modeling of footwall dehydration to suggest that diapir-like structures injected fluids into the Main Zone of the intrusion. This chapter further details the similarities between diapiric structures in the Bushveld Complex and those that have been modeled in subduction zone hydrothermal systems. The third chapter expands upon the Bushveld model, specifically in relation to the formation of iron-rich ultramafic pegmatoids and dunite pipes, which the work presented here suggests to be fluid-related. In the fourth chapter, strontium, neodymium, and lead isotopes are analyzed for rocks from the Stillwater Complex, Montana, to compare with the isotopic mixing model results of the Bushveld Complex. Initial isotopic ratios are used to explore various proposed models of complex formation. Results suggest isotopic heterogeneity during complex formation, whether due to heterogeneous source regions or crustal/fluid contamination. In the fifth chapter, stable isotope analyses (oxygen, hydrogen, and lithium) are used to better understand the formation of the pegmatoidal bodies thought to be related to fluids at Stillwater. Some evidence of fluid circulation may be observed in hydrogen and lithium isotopes. Geothermometry using oxygen isotopes is suggestive of lower cooling temperatures in the pegmatoids, and may provide evidence of mineral-scale disequilibrium attributable to fluid circulation. The sixth and seventh chapters utilize the thermodynamic modeling program MELTS to explore problems of layered intrusion evolution. Understanding the evolution of the liquids that formed various layered intrusions, and identifying magmas parental to layered intrusions, can pose a challenge. Using MELTS, bulk rocks can be synthetically remelted, and the evolution of the complex can be examined through analysis of estimated trapped liquid contents. The sixth chapter applies this method to the Stillwater Complex, while the seventh chapter extends this work to the Skaergaard intrusion in East Greenland. These investigations allow for examination of the magmatic processes operating alongside hydromagmatic processes in layered intrusions.
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