Doctor of Philosophy
Dr. Desmond Moser, Dr. Frederick Longstaffe, Dr. William Davis
Zircon from lower crustal xenoliths can preserve unique crustal formation records of unexplored levels of the lithosphere. The first such records for the western Churchill province (Nunavut, Canada) are presented here. Zircon SIMS U-Pb geochronology and oxygen isotope analysis were conducted on kimberlite-hosted granulite-facies xenoliths: four metabasites (#9864; #9865; #9866; #9870) from the Rankin Inlet region, and five metabasites (#10162; #10163; #10167; #10169; #10179) and three metatonalite/anorthosites (#10158; #10164; #10168) from the Repulse Bay region. For the Rankin Inlet suite, oscillatory planar zoned, igneous cores from metabolite xenolith #9865 record dates between ca. 2.9–2.6 Ga, whereas complex to radial-sector zoned metamorphic cores from metabasite xenolith #9864 yield dates between ca. 2.0–1.9 Ga. Oscillatory planar zoned igneous cores from the Repulse Bay suite yield dates of ca. 3.50 and ca. 3.0–2.6 Ga (#10158; #10164; #10168). Metamorphic rims from all 12 xenoliths yield dates between ca. 1.75–1.70 Ga, which likely represent regional thermal reworking of the lower crust initiated by magma underplating along the crust-mantle boundary. Irregular, vein-like micro-structures and evidence for solid-state recrystallization are commonly observed using cathodoluminescence within the igneous and metamorphic cores, and likely represent zones affected by intracrystalline Pb-loss and/or Pb-redistribution during high-grade metamorphism.
The oxygen isotope results for zircon from four metabasites from the Rankin Inlet suite have δ18Ozrc values between +5.7 and +8.6‰. Three metatonalite/anorthosites and five metabasites from the Repulse Bay suite have δ18Ozrc values ranging from +5.6 to +8.3‰. The high δ18O values of the oldest igneous cores from xenoliths #10158 and #10164 indicate protolith formation from evolved, supracrustal-derived (i.e., previous contact with surface fluids) tonalite/anorthosite magmas. By comparison, the near-mantle δ18O values of igneous cores from metabasite xenolith #9865 and metatonalite/anorthosite xenolith #10168 suggest protolith formation from juvenile magmas. The mantle-like δ18O values of ca. 2.0–1.9 Ga metamorphic cores from xenolith #9864 indicate localized reworking of the lower crust caused by regional-scale plate convergence. The wide range of δ18O values for all ca. 1.75–1.70 Ga metamorphic rims indicates regional transient heating and reworking of juvenile and evolved, supracrustal-derived lower crust induced by ca. 1.75 Ga magma underplating.
Petts, Duane C., "Deep Crustal Evolution of the Western Churchill Province, Nunavut, Canada: Isotopic (U-Pb, O), Trace Element and Micro-Structural Analysis of Zircon from Lower Crustal Xenoliths" (2012). Electronic Thesis and Dissertation Repository. 591.