Geology and U-Pb geochronology of the rocks of the Eokuk Uplift, Northwestern Slave Province, Nunavut, Canada

Emon, Kellie A. (1998) Geology and U-Pb geochronology of the rocks of the Eokuk Uplift, Northwestern Slave Province, Nunavut, Canada. Masters thesis, Memorial University of Newfoundland.

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Abstract

Rocks of the Eokuk Uplift have been mapped in detail along the coast of Coronation Gulf and 10 key units have been dated by both conventional Thermal Ionization Mass Spectrometry (TIMS) and Laser Ablation Microprobe Inductively Coupled Plasma Mass Spectrometry (LAM-ICP-MS) U-Pb analysis. Zircon, monazite, and titanite from the various units were dated and the combined data indicate that this inlier of the Slave province has a >3.2 Ga crustal component, evidence of a granulite grade orogenic event pre-dating 2.8 Ga (possibly ca. 2.9 Ga) and a lack of evidence for any significant orogenic activity corresponding with the 2.7-2.6 Ga events common in the rest of the Slave Province. The oldest rocks in the study area are a succession of granitoid and supracrustal gneisses that have been metamorphosed to upper amphibolite to granulite facies. From field relationships, the oldest unit is a granodiorite to tonalite orthogneiss, with a zircon crystallization age of 3254 +13/-6 Ma. A granite gneiss, which may be a small felsic intrusion or an anatectic melt of the tonalite gneiss, yields a zircon age of 3216 +14/-13 Ma. A K-feldspar megacrystic monzogranite gneiss contains old, discordant, possibly inherited zircons with ²⁰⁷Pb/²⁰⁶Pb ages ranging from 3039-3103 Ma, together with coexisting 2879 +/-3 Ma zircon and monazite. These high-grade gneisses are intruded by two megacrystic granite plutons, dated at 2887 +/-2 and 2881+4/-3 Ma. LAM-ICP-MS analyses of zoned zircons from the 2887 Ma granite reveal the presence of xenocrystic zircon with ages of 3165 +57/-34 and 3363 +68/-53 Ma. This implies that the older granites were intruded into, and possibly derived from, still older crust. The absence of extensive recrystallization and complex structures in these plutons indicates that this igneous event post-dated the high-grade metamorphism. An amphibolite-grade synplutonic metamorphic event is dated at ∼2880 Ma by metamorphic monazite in the older gneiss units. A series of variably deformed mafic to felsic dykes and pegmatites intrude both the granites and gneisses and constrain the end of penetrative deformation in the area to ca. 2850 Ma. This contrasts with data from the rest of the Slave Province, where the main phases of deformation, metamorphism and synmetamorphic plutonism have been dated at ca. 2.62-2.59 Ga. Metamorphic titanite ages from the diorite and granodiorite dykes indicate two lower amphibolite to greenschist-grade metamorphic events: one at ca. 2705 Ma and one at ca. 2646 Ma. The youngest Archean magmatic event in the area is represented by granite intrusion at 2594 +3/-2 Ma, coeval with crystallization of titanite at greenschist-grade conditions in some of the older gneissic and intrusive rocks.

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