Abstract
Rare occurrences of high-pressure and low temperature rocks preserved in metamorphic terranes older than the Neoproterozoic raise questions concerning deep subduction and the onset of plate tectonics early in Earth’s history. If such conditions were more widespread in the early Earth, then evidence might be found buried in the sedimentary record and enclosed in robust detrital minerals, i.e., minerals that survive weathering. Rutile is an exceptional candidate as it is formed over a wide range of pressures and temperatures during metamorphism and it is ubiquitous in siliciclastic rocks. In this study, detrital rutile has been investigated as a tool to probe the record of eroded metamorphic source rocks and those which may not be exposed at the present, and shed light on crustal and tectonic processes operating in the Precambrian. To address and explore the uses of detrital rutile as a fingerprint of these processes I have applied different methodologies, including mineral concentration techniques, electron microprobe analysis, electron scanning microscopy with electron dispersive spectrometry and electron backscattering detectors, and laser ablation inductively coupled plasma mass spectrometry. These were used to identify, determine and quantify the elemental and isotopic composition of multiple rutile grains.Rutile grains found in a Paleoproterozoic unit formed as a rift-passive margin basin (Minas sequence, São Francisco Craton, Brazil) highlight competing crustal processes; they show the suitability of detrital rutile as a powerful tool to assist in ore exploration, but also the variability of rutile textures and trace element content in rutile precipitated from hydrothermal fluids. A new discrimination diagram based on Zr/Hf and Nb/Ta values is proposed as a way forward to assist in provenance studies. Rutile grains suffered post-deposition alteration, which, together with the presence of detrital uraninite and gold, serves as a common feature found in worldwide Paleoproterozoic basins from the rifted Kenorland supercontinent.
Trace element variability (in the order of thousands of ppm) within single rutile grains can be contentious in provenance studies. Despite rutile being known as a chemically robust mineral, these new findings suggest that at relatively low metamorphic grade conditions (sub-greenschist facies), but in the presence of complex-chemistry fluids (e.g. B, F, Na), rutile suffers dissolution, and reprecipitation. This imparts disturbance of the U-Pb isotopic system, as well as of other trace elements.
Rutiles from NW Scotland were retrieved from the Neoproterozoic Torridonian group (formed as a foreland basin) and from the Cambrian Advreck Group (formed as a rift basin) and show a marked metamorphic affinity with no signs of post-deposition alteration. Because rutile is sensitive to tectonometamorphic processes, the U-Pb distribution ages from these different units serve as the basis to propose a new tool to elucidate sedimentary tectonic settings. Convergent-related and extensional-related basins show different rutile cumulative probability age curves. By recognising and characterising the extent of convergent and collisional basins in the geological record it may be possible to provide new insight about early Earth tectonics.
A large dataset of detrital U-Pb and trace element rutile data is combined with data presented in this study to show that detrital rutile can be used to track the evolution of metamorphic gradients across the Archaean and Proterozoic Eons. Zr-in rutile thermometry data show the existence of high and low geothermal gradients, serving as evidence of paired metamorphism since at least 1.7-2.1 Ga. By combining detrital rutile age data with the global zircon age spectra, it provides further insights into crustal growth and recycling. Combined age spectra emphasize that production of new continental crust is not episodic but rather is probably continuous, consistent with modern plate tectonic and magmatic processes.
Altogether, this thesis serves as a contribution towards the discussion of crustal and tectonic processes operating since the early Earth focusing on the mineral rutile. I have provided a new discrimination diagram using trace elements, used rutile grains and their mineral inclusions to track multiple tectonometamorphic events that led to the early Paleoproterozoic worldwide gold endowment event, used the U-Pb distribution ages as a new tool to discriminate different depositional tectonic settings and provided a new way to probe the tectonometamorphic record in order to infer that modern plate tectonics was already fully operating at least by 2.1 Ga.
| Date of Award | Apr 2019 |
|---|---|
| Original language | English |
| Supervisor | Craig Storey (Supervisor) & James Darling (Supervisor) |