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Geochronological and geochemical constraints on the origin of the Paleoproterozoic Union Island Group mafic magmatism, East Arm Basin, N.W.T. Open Access


Other title
East Arm basin
Igneous geochemistry
Continental rift
Tracer isotope geochemistry
Type of item
Degree grantor
University of Alberta
Author or creator
Sheen, Alex I
Supervisor and department
Heaman, Larry (Earth and Atmospheric Sciences)
Examining committee member and department
Chacko, Tom (Earth and Atmospheric Sciences)
Pearson, D. Graham (Earth and Atmospheric Sciences)
Department of Earth and Atmospheric Sciences

Date accepted
Graduation date
2017-11:Fall 2017
Master of Science
Degree level
The Slave craton underwent widespread extension during the Paleoproterozoic. In the southern Slave craton, this extension is manifest in multiple periods of continental mafic magmatism emplaced between 2.3–2.2 Ga and prior to 2.0–1.9 Ga calc-alkaline magmatism of the Taltson and Thelon zones along the Slave-Rae boundary. During this time the East Arm basin of Great Slave Lake was formed and now preserves a protracted sedimentary and volcanic record along the southeastern margin of the Slave craton. Within the East Arm basin package, the Union Island Group represents voluminous mafic volcanism with subordinate interbedded carbonate/shale sedimentary strata. Previous stratigraphic interpretations posit that the ca. 1928 Ma Wilson Island Group is the base of the East Arm basin stratigraphy, and that the Union Island Group is younger. This relationship is however equivocal; the Wilson Island Group is structurally isolated, and new field observations indicate that the Union Island Group was deposited unconformably on Archean granitic basement, the latter considered to be derived from the nearby Slave craton. We report the first U–Pb baddeleyite crystallization age, 2042.7±3.0 Ma, for a diabase body which intrudes volcaniclastic horizons belonging to the Union Island Group lower basalt unit; the diabase intrusion is geochemically identical to flows of the lower basalt unit and is therefore interpreted as a feeder to these flows. The diabase crystallization age demonstrates that the Union Island Group is the oldest identified supracrustal package in the East Arm basin and that the East Arm basin is ~115 Myr older than previously thought. Our new stratigraphic interpretation is supported by detrital zircon provenance age distribution in two Union Island Group sedimentary samples. These detrital zircons are dominated by 2.76–2.56 Ga ages reflecting prominent input from the Archean basement. The youngest detrital zircons have ages indistinguishable from the emplacement age of the diabase. Zircon ages from the 1.9–2.0 Ga Taltson and Thelon magmatism, are absent from the Union Island Group sedimentary rocks while being prominent in all other strata of the East Arm basin; further confirming that the Union Island Group is the oldest strata in the East Arm basin. The Union Island Group contains two stratigraphically and geochemically distinct mafic volcanic packages. The lower magmatism is predominantly alkaline, is characterized by high enrichment levels of incompatible elements and a large compositional range (108–438 ppm Zr, 13–62 ppm Nb), and preserves an OIB-like chemical signature. The upper magmatism, in contrast, is tholeiitic, displays much lower levels of incompatible elements with a uniform composition (83–101 ppm Zr, 2–4 ppm Nb), and has a DM-like chemical signature. Both packages display overlapping depleted time-integrated εNd(i) values (+1.1 – +3.2). Petrological modeling suggests that the lower magmatism originated from interaction between upwelling asthenosphere and a depleted mantle reservoir; the upper magmatism was produced by decompression melting of the shallow mantle consisting mostly of the depleted reservoir. The petrogenesis established in this study is consistent with a continental rift origin for the Union Island Group magmatism and is further supported by geochemical similarities with Proterozoic and Phanerozoic rift successions. Combing geochronological and geochemical findings, we propose that the Union Island Group represents an incipient rift sequence during the ca. 2043 Ma rifting at the southern margin of the Slave craton. The proposed tectonic model is consistent with the break-up of the Slave craton from a pre-Laurentia supercontinent during the 2.23–2.01 Ga period. Our conclusions further suggest an intraplate origin for the Taltson and Thelon magmatism, and that the Slave craton and the Rae domain were a contiguous crustal unit as early as ca. 2043 Ma.
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