The Quandary of the Sask Craton: Origin and Evolution of the Lithospheric Mantle beneath the Sask Craton

  • Author / Creator
    Czas, Janina
  • Mantle xenoliths from the Cretaceous (~106 to ~95 Ma) kimberlites at Fort à la Corne (FALC) present a unique opportunity to study the lithospheric mantle beneath the newly recognised Sask Craton. The Sask Craton, a small terrane with Archean (3.2 - 2.5 Ga) crustal ages, is enclosed in the Paleoproterozoic (1.9 - 1.8 Ga) Trans Hudson Orogen (THO). Only limited research has been conducted on this craton, yet it hosts major diamond deposits within the FALC Kimberlite Field. This thesis presents the first study of major and trace elements, as well as isotopic data from diamondiferous and barren mantle xenoliths (peridotitic and eclogitic) from two volcanic centres (Star and Orion South) in the Fort à la Corne Kimberlite Field. To constrain the origin and evolution of the lithospheric mantle beneath this craton, the age and composition of the lithosphere are established, and diamond forming processes are assessed. Further, this study provides the opportunity to constrain the influence of the Trans Hudson Orogeny on the mantle keel and its effect on the diamond population. Based on the geochemistry of peridotite xenoliths from FALC, the garnet-bearing lithospheric mantle is dominated by moderately depleted lherzolite. Signatures of carbonatitic and kimberlitic melt metasomatism can be identified in the majority of the xenolith suite. Pressure and temperature conditions (840 to 1250 °C and 2.7 to 5.5 GPa) of the lithospheric root are similar to other cratons, the calculated geotherm is cool and compares well with a 38 mW/m2 reference geotherm. No Archean ages were recorded in the Os isotope composition, with the main mode of Re depletion ages spanning from 2.4 to 1.7 Ga. This provides evidence that the majority of the lithospheric mantle was depleted and stabilised in the Palaeoproterozoic, significantly later than the Archean crust. The timing of the dominant lithosphere formation is linked to the rifting (~2.2 Ga - 2.0 Ga), as well as the subsequent collision (1.9 - 1.8 Ga) of the Superior and Hearne craton during the Wilson cycle of the Trans Hudson Orogen. FALC eclogites have major element and oxygen isotope compositions consistent with an origin from subducted, seawater-altered oceanic crust. Diamond-free eclogites commonly have signatures indicative of a gabbroic origin, while diamond-bearing xenoliths are likely derived from basaltic protoliths. Temperatures calculated for the FALC eclogites span a broad range (740 to 1390 °C), though diamondiferous samples are restricted to the higher temperatures (1180 – 1390 °C). Both modal (diamond formation) and cryptic metasomatism affected the FALC eclogite suite. Intense melt metasomatism, which occurred in temporal proximity to host kimberlite magmatism, resulted in strong chemical gradients and heterogeneities in major, trace and even oxygen isotope values within the diamondiferous eclogites. Similar to the peridotitic sample suite, both carbonatitic, and proto-kimberlitic metasomatism can be identified in the diamondiferous FALC eclogites. Eclogite formation is likely linked to the subduction of oceanic crust during the Trans Hudson Orogeny. All diamonds in this study are intergrown with mantle minerals, with eclogitic assemblages dominating the sample suite. Three diamond suites (monocrystalline, aggregate and polycrystalline diamonds) were identified based on their morphology and chemistry. Monocrystalline diamonds from FALC have nitrogen and carbon systematics indicative of a mantle-derived source fluid, a subduction-related origin is likely for the polycrystalline diamonds. In the diamond aggregates nitrogen and carbon systematics are decoupled; mixing of mantle-like and recycled nitrogen, or the presence of nitrogen-bearing phases during diamond crystallisation could account for the observed disconnect between nitrogen and carbon isotope variations. The decoupling of carbon and nitrogen systematics suggests that diamond crystallisation is not fluid limited and Rayleigh fractionation does not play a major role at FALC. It is more likely that diamond crystallised from a supersaturated CHO-rich fluid/melt due to isochemical cooling. Unusual diamond brecciation and annealing patterns observed in the diamond aggregates are possibly linked to the intense melt metasomatism that affected their host eclogites. Considering the absence of Archean lithospheric mantle beneath the Sask Craton it is likely that all diamonds from FALC are Palaeoproterozoic in age. Further, nitrogen aggregation and platelet peak degradation suggests that at least some of the FALC diamonds formed during the THO.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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