Date of Award

December 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth Sciences

Advisor(s)

Suzanne L. Baldwin

Keywords

Elastic thermobarometry, Metamorphic petrology, Pressure-Temperature, Trace element thermobarometry, Ultrahigh pressure metamorphism

Subject Categories

Physical Sciences and Mathematics

Abstract

Subduction zones are tectonically active regions that produce seismicity and volcanism during plate convergence and ultimately recycle crustal material into the mantle. Since these regions dictate many global scale tectonic and geochemical processes (i.e. orogenesis, volatile flux into the mantle, etc.), it is important to understand the depth-temperature conditions of mineral reactions during the subduction-exhumation metamorphic cycle. (Ultra)high-pressure ((U)HP) metamorphic rocks, such as blueschists and eclogites, are formed during metamorphism of subducted crust and sediments along relatively cold geothermal gradients. (U)HP metamorphic rocks may be incorporated into the subduction zone accretionary wedge and exhumed, thereby providing a direct records of subduction zone conditions and processes. This PhD research is focused on the determination of pressure (P) – temperature (T) conditions of exhumed (U)HP subduction zone metamorphic rocks in order to better understand the metamorphic conditions of geologic processes within subduction zones.

This dissertation contains three separate studies which combine multiple thermobarometric methods to better constrain the petrologic history of exhumed subduction zone metamorphic rocks. Elastic thermobarometry, trace element thermobarometers, and numerical modeling approaches were integrated to determine P–T(–t) histories of (U)HP metamorphic rocks. In the first chapter of this dissertation, multi-diffusion domain numerical modelling of white mica 40Ar/39Ar and thermobarometric data were used to forward model continuous P–T–t paths of blueschist block exhumation from the western Baja terrane of Mexico. Results from this chapter provide new insights into the application of white mica P–T–t numerical modeling to constrain exhumation histories of (U)HP metamorphic rocks. In the second chapter, strain-based quartz-in-garnet elastic thermobarometric data was combined with Ti concentration measurements and a Ti-in-quartz solubility model to estimate P–T conditions of inclusion entrapment in garnet from a quartzofeldspathic gneiss from the (U)HP terrane of eastern Papua New Guinea. The quartz-in-garnet and Ti-in-quartz (QuiG-TiQ) method gives P–T constraints from a single mineral and does not introduce temperature estimates external of the host-inclusion system. Results from this chapter give new insights into the use of elastic thermobarometry to determine conditions of metamorphic mineral growth and inclusion entrapment. In the third chapter, the first characterization of mineralogical evidence for UHP metamorphism in the Appalachian orogen is presented. Multiple thermobarometric methods, including Zr-in-rutile trace element and quartz-in-garnet elastic thermobarometry, were combined with petrologic observations to characterize the prograde metamorphic conditions of garnet growth during subduction.

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