Geology and U-Pb geochronology of rocks of the Eokuk Uplift: a pre-2.8 Ga basement inlier in the northwestern Slave Province, Nunavut, Canada

摘要

Rocks of the Eokuk Uplift have been mapped indetail along the coast of Coronation Gulf and 10 key units havebeen dated by U-Pb analysis of zircon, monazite, and titanite. Thecombined data indicate that this inlier of the Slave Province has a>3.2 Ga crustal component, evidence of a granulite-grade orogenicevent predating 2.8 Ga and a lack of evidence for any significantorogenic activity corresponding to the 2.7-2.6 Ga events commonin the rest of the Slave Province. The oldest rocks in the study areaare a succession of granitoid and supracrustal gneisses that havebeen metamorphosed to amphibolite to granulite facies. From fieldrelationships, the oldest rock is a granodiorite to tonaliteorthogneiss, with a zircon crystallization age of 3254_-6~+13 Ma.A granite gneiss, which may be a small felsic intrusion or ananatectic melt of the tonalite gneiss, yields a zircon age of 3216_-13~+14 Ma. A K-feldspar megacrystic monzogranite gneisscontains old, discordant, possibly inherited zircons with~207Pb/~206Pb ages ranging from 3103 to 3039 Ma, togetherwith coexisting 2879 +- 3 Ma zircon and monazite. These high-grade gneisses are intruded by two megacrystic granite plutons,dated at 2887 +- 2 and 2881_-3~+4Ma. The absence of extensiverecrystallization and complex structures in these plutons indicatesthat this igneous event postdated the high-grade metamorphism.An amphibolitegrade synplutonic metamorphic event is dated at2880 Ma by new monazite in the older gneiss units. A series ofvariably deformed mafic to felsic dykes and pegmatites intrudeboth the granites and gneisses and constrain the end of penetrativedeformation in the area. Of these, a boudinaged diorite dyke, witha strong internal foliation parallel to the regional fabric, is dated at2877 +- 3 Ma. A younger granodiorite dyke that crosscuts theregional fabric at a high angle and has only a weak internalfoliation yields an age of 2864_-9~+3~ Ma. An undeformedsyenogranite pegmatite, which represents a suite that intrudes ~lother units in the study area, has a combined zircon-monazite ageof 2852 +- 3 Ma. The varying degrees of deformation in theseminor intrusive rocks constrains the end of deformation in thestudy area to ca. 2850 Ma. This contrasts with data from the rest ofthe Slave Province, where the main phases of deformation,metamorphism, and synmetamorphic plutonism have been dated atca. 2.62-2.59 Ga. Metamorphic titanite ages from the diorite andgranodiorite dykes indicate two lower amphibolite to greenschistfacies metamorphic events: one at ca. 2705 Ma and one at ca.2646 Ma. The youngest Archean magmatic event in the area isrepresented by granite intrusions at 2594_-2~+3 Ma, coeval withcrystallization of titanite at greenschist-grade conditions in someof the older gneissic and intrusive rocks.