Properties and evolution of cratonic lithosphere
Creators: McIntyre, Timothy
Publisher: name: University of Alberta Library
Year: 2022

<p>Resolving important fundamental questions about the petrogenesis of cratonic lithosphere is difficult in ancient cratons that have long and complex histories. Several properties of cratons are poorly constrained and our understanding of their petrogenesis largely remains ambiguous. In the Archean Earth, the diversity of tectonic environments that gave rise to the building blocks of cratonic lithosphere and the timing of its amalgamation into coherent cratons is uncertain due to our limited understanding of the tectonic regimes that operated during this Eon. Equally unclear is the petrogenetic melting environment that gave rise to the highly refractory composition of cratonic lithospheric mantle. Finally, our understanding of the thermal evolution and thickness of cratons is limited by poor constraints on the abundances of heat producing elements in cratonic mantle throughout its depth. In this thesis, three studies are presented that focus on contributing insight into the timing of the onset of plate tectonics and the petrogenetic environments that gave rise to the earliest cratonic crust, the petrogenetic melting environment of lithospheric mantle that underpins the Proterozoic portion of the North Atlantic Craton, and lastly, an improved estimate of the heat producing element abundances in the various lithologies that comprise cratonic lithosphere. </p>

<p>In the first study, Chapter 2, the geochronology and petrogenesis of Archean crustal components of the Nagssugtoqidian orogen (NO), West Greenland were evaluated. The Qorlortoq gneiss, a component of this heterogeneous crust, was found to have much older ages than the surrounding gneiss. This allowed for new insights into the evolution of the Archean lithosphere comprising the NO. Measurements of U-Pb and Lu-Hf in zircon from the Qorlortoq gneiss yielded an age of 3177 ± 12 Ma with a weighted mean εHf of 1.7 ± 0.5. The hiatus in crustal production between this and the younger Archean components of the NO (&lt;2.87 Ga) imply a local tectonic regime dominated by stagnant lid or only episodic subduction. The crust in the NO provides a complimentary Archean history to regional studies, allowing for a better understanding of the timing and processes that led to the formation of the North Atlantic Craton in the Archean. </p>

<p>The second study, Chapter 3, focuses on the contributions of hydrous vs anhydrous melting regimes to generating the refractory compositions of cratonic lithospheric mantle in the Paleoproterozoic. This was done through a detailed geochronological and geochemical evaluation of a Paleoproterozoic orogenic peridotite – the Ussuit peridotite, West Greenland. The Ussuit peridotite was found to have formed in a sub-arc environment and provides potential geochemical proxies for comparison with mantle xenoliths. Re-Os isotope geochronology of the Ussuit peridotites dates the melting event responsible for their highly depleted compositions to ~2 Ga, which overlaps the Paleoproterozoic production of local and global oceanic arc-lithosphere incorporated into cratons in the Paleoproterozoic. Combined with geochemical similarities with Paleoproterozoic cratonic mantle xenoliths, this suggests the Ussuit peridotite could have equivalents in the cratonic lithosphere of other regions and that highly depleted sub-arc lithospheric mantle may constitute a fundamental component of the Proterozoic keels beneath cratons. </p>

<p>Heat production is a fundamental, but poorly constrained, property of the lithospheric mantle that is required to model the thermal structure of the cratonic lithosphere. In the final study, Chapter 4, heat producing element (K, Th, and U) concentrations in cratonic mantle roots were measured in the spectrum of minerals that make up cratonic peridotites. These new data were produced via laser ablation ICP-MS and together with a literature compilation of robust data were used to place better constraints on heat production in the cratonic lithospheric mantle. The resulting magnitude of heat generation in depleted peridotitic lithosphere was found to be between 0.00004 and 0.006 µW/m3. This new estimate is significantly lower than previous published estimates, that are in common use and can range up to 0.084 µW/m3, 1 to 3 orders of magnitude higher than the new estimate. Depending on model assumptions, using this new value for mantle heat production produces modelled estimated thicknesses of cratonic lithosphere as much as 10 to 80 km thinner than estimates that rely on some previous measurements of heat producing elements.</p>