Petrochronology and mineral chemistry of mid-crustal shear zones: new tools for tectonics and mineral exploration
Student thesis: Doctoral Thesis
Dating ductile shear zones is daunting because we have to demonstrate either that the chronometer of choice grew during shear zone operation or that crystal-plastic deformation induced age resetting. By adopting a petrochronological approach in this project combining petrographic, geochemical, U-Pb isotopic, and quantitative microstructural data U-Pb isotopic dates are linked with certain shear zone processes. The study area is the South Range of the world-class Sudbury Impact Structure. Specifically, mylonitic shear zones at the Creighton Mine (South Range, Sudbury) operated during three distinct tectonothermal events at ca. 1.75 Ga, 1.65 Ga, and 1.45 Ga. The age dating of texturally and geochemically characterised titanite grains from a shear zone exposed at the 5400 level of the Creighton Mine, indicates operation of the shear during the Mazatzalian – Labradorian orogeny (1.7 – 1.6 Ga). Meso-scale sulphide structures of mechanical remobilization, within the main body of the examined shear zone, show that this event facilitated the local-scale transfer of sulphides to satellite positions. Three age populations of ca. 1.75 Ga, 1.65 Ga, and 1.45 Ga are also prevalent in shear zones from deeper levels of the Creighton Mine. These age populations yield new insights into the orogenic history of the South Range and the Southern Province, and provide further constraints on the comparison of accretionary provinces of the North American Mid-continent and the Southwest United States. Taking into consideration the fluid-mediated and crystal-plasticity textural features in the examined titanite populations it is suggested that these dates record events of syndeformational fluid percolation. Within the 1.75 Ga textural population of titanite grains survived inclusions of inherited titanite grains with shock-metamorphic features. Microstructural and micro to nano-scale crosscutting relationships suggest that the shock wave during the 1.85 Ga impact event induced in these grains the growth of 75°/<010> and 108°/<010> shock microtwins. The nucleation of twins induced a work hardening effect that allowed their survival during the later polyorogenic reworking of the basin (1.75 to 1.45 Ga). U-Pb age dating of these grains yield accurately the age of impact (i.e. 1851 ± 12 Ma). In comparison, titanite grains located within Archaean target rocks ofthe Vredefort structure show identical crystallographic features and partial age resetting. The differential response is attributed to the different distance of the samples from the base of the impact melt sheet that was the dominant heat source. The ore-controlling character of the examined shear zones in the Sudbury mining camp can provide critical information about the exploration potential of these structures in metallogenetic settings. Preliminary mineral-chemical analysis, from major to trace element level, of fabric-forming silicates show distinct trends in the abundance of pathfinder elements (e.g. transition metals). Further, work that will collate the different datasets using multivariable statistical methods will be pursued in order to untangle the vectoring potential of different elements.
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