Investigating the secular geochemical and geodynamic evolution of accretionary orogens with zircon petrochronology: A case study from West Antarctica

ABSTRACTInvestigating the secular geochemical and geodynamic evolution of accretionary orogens with zircon petrochronology: A case study from West AntarcticabyDemian Alan Nelson Subduction-related processes in accretionary orogens modulate Earth’s geochemistry. Uncovering the secular geodynamic evolution of accretionary orogens, therefore, is the key to Earth’s geochemical history. A record of the secular geodynamic evolution of accretionary orogens is preserved in the tempo, geochemistry, and distribution of subduction-related magmatism. Zircon is ubiquitous in subduction-related magmas and provides a valuable time integrated geochemical proxy for the secular geodynamic evolution of accretionary orogens via U-Pb geochronology, Hf isotopes, and trace-elements (i.e., zircon petrochronology). The paleo-Pacific margin of Gondwana, including South America, eastern Australia, and West Antarctica, provides an ideal opportunity to investigate the secular geodynamic evolution of accretionary orogens with zircon petrochronology because it contains the most long-lived and best-preserved record of accretionary orogenesis on Earth. This dissertation represents the first comprehensive zircon petrochronologic study of Phanerozoic subduction and accretionary orogenesis in West Antarctica and combines the results with existing data from adjacent regions in eastern Australia and South America to refine our understanding of the secular evolution of accretionary orogens. Chapter 1 investigates volcaniclastic sedimentary rocks in the central Transantarctic Mountains and documents episodic isotopically depleted magmatism along the Antarctic margin of Gondwana. Comparisons of these data from the central Transantarctic Mountains demonstrate a shared tectonic history between Antarctica, Zealandia, and Australia that contrasts with that of South America. Chapter 2 applies zircon petrochronology to the plutonic rocks in eastern Marie Byrd Land and Thurston Island. These new data combined with existing data from the Antarctic Peninsula, western Marie Byrd Land, and central Transantarctic Mountains provides an ~450 million year geochemical and geodynamic history for the Antarctic margin of Gondwana. This record is then compared with compilations from South America and Australia to determine similarities and differences in geochemical and geodynamic evolution along the Gondwana margin. Chapter 3 traces the source of Permian volcanic deposits in central Antarctica to a voluminous volcanic province in South America, the Choiyoi Province, which may have contributed to Permian climate change and environmental degradation