Date of Award

5-2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Suzanne L. Baldwin

Keywords

Active and Passive Margins, Hf and Nd Isotopes, Lithospheric Rupture, Lu-Hf Garnet Geochronology, Mineral Records of Tectonic Processes, U-Pb Zircon Geochronology

Subject Categories

Earth Sciences

Abstract

Tectonically active regions provide important natural laboratories to glean information that is applicable to developing a better understanding of the geologic record. One such area of the World is Papua New Guinea, much of which is situated in an active and transient plate boundary zone. The focus of this PhD research is to develop a better understanding of rocks in the active Woodlark Rift, situated in Papua New Guinea's southernmost reaches. In this region, rifting and lithospheric rupture is occurring within a former subduction complex where there is a history of continental subduction and (U)HP metamorphism. The lithostratigraphic units exposed in the Woodlark Rift provide an opportunity to better understand the records of plate boundary processes at many scales from micron-sized domains within individual minerals to regional geological relationships.

This thesis is composed of three chapters that are independent of one another but are all related to the overall goal of developing a better understanding of the record of plate boundary processes in the rocks currently exposed in the Woodlark Rift. The first chapter, published in its entirety in Earth and Planetary Science Letters (2011 v. 309, p. 56 - 66), is entitled `Lu-Hf garnet geochronology applied to plate boundary zones: Insights from the (U)HP terrane exhumed within the Woodlark Rift'. This chapter focuses on the use of the Lu-Hf isotopic system to date garnets in the Woodlark Rift. Major findings of this study are that some of the rocks in the Woodlark Rift preserve a Lu-Hf garnet isotopic record of initial metamorphism and continental subduction occurring in the Late Mesozoic, whereas others only preserve a record of tectonic processes related to lithospheric rupture during the initiation of rifting in the Late Cenozoic.

The second chapter is entitled `Geochemical and geochronological constraints on the origin of rocks in the active Woodlark Rift of Papua New Guinea: Recognizing the dispersed fragments of an active margin'. This chapter uses a panoply of geochronological (U-Pb zircon) and geochemical (Lu-Hf and Sm-Nd isotopes, trace/REEs, and major elements) tools to investigate the origin the major lithostratigraphic units in the Woodlark Rift. Important findings in this chapter include the establishment of a tectonic link between sialic metamorphic rocks in the Woodlark Rift and the remnants of a Late Cretaceous aged bi-modal volcanic province along Australia's northern Queensland coast. This link is important because it identifies another rifted fragment of the former Australian continental margin in Gondwana, and demonstrates the complexity of recognizing the dispersed fragments of active margins.

Another important finding of this chapter is that Quaternary aged high silica rhyolites erupted in the western Woodlark Rift have mantle isotopic and geochemical signatures, and are therefore not the extrusive equivalents of partially melted metamorphic rocks found in the lower plates of large metamorphic core complexes. This is important because it signifies that lithospheric rupture has already occurred, despite the fact that mid-ocean ridge basalts are not yet being erupted and there are still topographically prominent metamorphic core complexes in the region. This chapter is not yet published, but is being prepared for submission to Gondwana Research.

The third chapter is entitled `Zircon growth in rapidly evolving plate boundary zones: Evidence from the active Woodlark Rift of Papua New Guinea". The original purpose of this chapter was simply to use U-Pb dating of zircons from felsic and intermediate gneisses in the Woodlark Rift to understand the history of rocks from (U)HP terranes that don't preserve the (U)HP metamorphic paragenesis. It was soon realized that the types of U-Pb zircon analyses typically performed on a SIMS instrument were going to be insufficient to fully understand the geochemical and geochronological records within zircons from these rocks. Because of this, traditional SIMS analyses for zircons from these rocks are augmented by U-Pb age and elemental depth profiles that elucidate the isotopic and geochemical nature of the sharp boundaries between different aged domains in these polygenetic zircons. The results presented in this chapter demonstrate that zircon U-Pb ages record specific plate boundary events that can be related to the development of the Woodlark Rift, and that traditional assumptions regarding geochemical equilibrium might not hold true in all situations.

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