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Scholarly Works (2 results)

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UC Santa Barbara Electronic Theses and Dissertations (2018)

ABSTRACT

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

by

Demian 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.

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Cover page: Investigating the secular geochemical and geodynamic evolution of accretionary orogens with zircon petrochronology: A case study from West Antarctica
  • Article
  • Peer Reviewed
UC Santa Barbara Previously Published Works (2022)
The Early Jurassic Butcher Ridge Igneous Complex (BRIC) in the Transantarctic Mountains contains abundant and variably hydrated silicic glass which has the potential to preserve a rich paleoclimate record. Here we present Fourier Transform Infrared Spectroscopic data that indicates BRIC glasses contain up to ~8 wt.% molecular water (H2Om), and low (<0.8 wt.%) hydroxyl (OH) component, interpreted as evidence for secondary hydration by meteoric water. BRIC glasses contain the most depleted hydrogen isotopes yet measured in terrestrial rocks, down to δD = -325 ‰. In situ 40Ar/39Ar geochronology of hydrated glasses with ultra-depleted δD values yield ages from 105 Ma to 72 Ma with a peak at c. 91.4 Ma. Combined, these data suggest hydration of BRIC glasses by polar glacial ice and melt water during the Late Cretaceous, contradicting paleoclimate reconstructions of this period that suggest Antarctica was ice-free and part of a global hot greenhouse.
    Cover page: Ultra-depleted hydrogen isotopes in hydrated glass record Late Cretaceous glaciation in Antarctica
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