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Paleomagnetism and U–Pb Geochronology of Franklin Dykes In 689 Paleomagnetism and U–Pb geochronology of Franklin dykes in High Arctic Canada and Greenland: a revised age and paleomagnetic pole constraining block rotations in the Nares Strait region1 Steven W. Denyszyn, Henry C. Halls, Don W. Davis, and David A.D. Evans Abstract: U–Pb baddeleyite ages and paleomagnetic poles obtained for dykes on Devon Island and Ellesmere Island in the Canadian Arctic and the Thule region of Greenland show that they are associated with the Franklin magmatic event. This study is the only one devoted to Franklin igneous rocks where a primary paleomagnetic remanence and U–Pb age have been obtained from the same rocks. Ages from this study range from 721 to 712 Ma, but paleomagnetic directional data show no clear age progression. The paleomagnetic poles from each of the two regional subsets are significantly differ- ent at the 95% confidence level from paleomagnetic results previously published for the Franklin event in the Canadian Shield. The difference in the pole locations can be accounted for, to first approximation, by a simple model of early Ceno- zoic block rotations among the North American plate, Greenland, and a hypothesized ancient microplate comprising Elles- mere, Devon, Cornwallis, and perhaps Somerset islands. A new grand-mean paleopole for the Franklin event, including restoration of Greenland and the proposed ‘‘Ellesmere microplate’’ to North America, is located at (8.48N, 163.88E, A95 = 2.88, N = 78 sites) and is a key pole for Neoproterozoic supercontinent reconstructions. Re´sume´ : Des aˆges U–Pb, de´termine´s sur de la baddeleyite, et des poˆles pale´omagne´tiques obtenus de dykes desˆ ıles de Devon et d’Ellesmere, dans l’Arctique canadien, et de la re´gion de Thule´ au Groenland, montrent que ces dykes sont ass- ocie´sa` l’e´ve´nement magmatique de Franklin. La pre´sente e´tude est la seule consacre´e aux roches igne´es de Franklin ou` une re´manence pale´omagne´tique primaire et des aˆges U–Pb ont e´te´ obtenus des meˆmes roches. Les aˆges dans cette e´tude varient de 721 a` 712 Ma, mais les donne´es sur les directions pale´omagne´tiques ne montrent aucune progression claire de l’aˆge. Les poˆles pale´omagne´tiques de chacun des deux sous-ensembles re´gionaux diffe`rent de manie`re significative, au ni- veau de confiance 95 %, des re´sultats pale´omagne´tiques publie´s ante´rieurement pour l’e´ve´nement de Franklin dans le Bouclier canadien. La diffe´rence d’emplacement des poˆles peut eˆtre explique´e, en une premie`re approximation, par un mode`le simple de rotations de blocs, au Ce´nozoı¨que pre´coce, entre la plaque Nord-ame´ricaine, le Groenland et une anci- enne plaque hypothe´tique qui comprenait lesˆ ıles d’Ellesmere, de Devon, de Cornwallis et peut-eˆtre de Somerset. Un nou- veau pale´opoˆle de grande moyenne pour l’e´ve´nement Franklin, incluant la restoration du Groenland et de la « micro- plaque d’Ellesmere » propose´ea` l’Ame´rique du Nord, est situe´ a` (8,48N, 163,88E, A95 = 2,88, N = 78 sites) et constitue le poˆle cle´ pour les reconstructions du super continent au Ne´oprote´rozoı¨que. [Traduit par la Re´daction] Introduction for reconstructing the past positions of continents and corre- lating what are today the dispersed fragments of superconti- Precisely dated rocks with well-defined paleomagnetic nents (e.g., Buchan and Halls 1990). The Neoproterozoic of poles are scarce in the Proterozoic record, yet are critical North America is currently represented by only a few widely Received 3 September 2008. Accepted 19 August 2009. Published on the NRC Research Press Web site at cjes.nrc.ca on 25 September 2009. Paper handled by Associate Editor L. Corriveau. S.W. Denyszyn.2 University of Toronto, Department of Geology, 22 Russell St., Toronto, ON M5S.3B1, Canada; Present address: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, U.S.A. H.C. Halls. University of Toronto, Department of Geology, 22 Russell St., Toronto, ON M5S.3B1, Canada. D.W. Davis. Jack Satterly Geochronology Lab, University of Toronto, Department of Geology, 22 Russell St., Toronto, ON M5S 3B1, Canada. D.A.D. Evans. Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, U.S.A. 1This is a companion paper to Denyszyn, S.W., Davis, D.W., and Halls, H.C. Paleomagnetism and U–Pb geochronology of the Clarence Head dykes, Arctic Canada: orthogonal emplacement of mafic dykes in a large igneous province. Canadian Journal of Earth Sciences, 46(3): 155–167. 2Corresponding author (e-mail: [email protected]). Can. J. Earth Sci. 46: 689–705 (2009) doi:10.1139/E09-042 Published by NRC Research Press 690 Can. J. Earth Sci. Vol. 46, 2009 Fig. 1. Distribution of intrusions or lava flows associated with the Franklin magmatic event (modified after Fahrig (1987), with dyke loca- tions also from Frisch (1984a, 1984b, 1984c) and Dawes (1991)). CS, Coronation sills; DI, Devon Island dykes; NB, Natkusiak basalts; SS, ,Strathcona Sound dykes; TD, Thule dykes; TS, Thule sills; placement of these labels indicates the locations of paleomagnetic studies referred to in text. Star denotes proposed plume source location based on radiating geometry of the dykes. Solid circles denote sills; v sym- bols denote volcanics. spaced paleopoles with both high-precision U–Pb ages and of Greenland (Fahrig et al. 1971; Frisch 1984a, 1984b, well-characterized primary magnetizations (Buchan et al. 1984c; Dawes 1991; Buchan and Ernst 2004; Shellnutt et 2000). The Franklin magmatic event of Arctic Canada al. 2004). In our study area (Fig. 2), the basement rocks (Fig. 1) is one such case. The currently accepted age of that host the intrusions comprise high-grade granitic gneiss þ4 723À2 Ma (Heaman et al. 1992) and grand-mean paleomag- of Archean to Paleoproterozoic age, overlain in western and netic pole at (8.08N, 1638E, A95 = 4.08; Buchan et al. 2000) central Devon Island and western Ellesmere Island by flat- have been used extensively in Neoproterozoic plate recon- lying Paleozoic carbonate and mudstone that overlie the structions (Buchan et al. 2001; Li et al. 2008; Evans in dykes, and in Greenland by flat-lying Mesoproterozoic sand- press). The Franklin pole is particularly important as the stone of the Thule basin, which is cut by the dykes (Frisch Franklin magmatic event is related to a mantle plume north 1988; Dawes 2004). The dykes are diabase with abundant of Victoria Island (Fig. 1) that has been tied to the incipient clinopyroxene and lath-shaped plagioclase, minor biotite, breakup of the Neoproterozoic supercontinent Rodinia hornblende, and oxide phases, as well as rare titanite and (Heaman et al. 1992; Shellnutt et al. 2004). Sturtian global apatite. Unlike the Franklin dykes of similar trend on Baffin glaciation is proposed to have occurred during this time pe- Island (Jefferson et al. 1994), olivine is absent. The dykes riod, and accurate knowledge of plate reconstruction from are, on average, 30 m wide with well-preserved chilled mar- paleomagnetism may prove important for testing the gins and appear unaltered, except for moderate sericitization ‘‘Snowball Earth’’ hypothesis (Hoffman et al. 1998). This of plagioclase and the occasional presence of chlorite. As study of the Franklin dyke swarm presents new paleomag- they are all near vertical, are overlain by flat-lying strata in netic and U–Pb geochronological results, as well as a rein- Canada, and cut flat-lying strata in Greenland, tectonic ac- terpretation of published data from the Franklin magmatic tivity that might have remagnetized the rocks or locally ro- event. This forms the basis for a new key paleomagnetic tated magnetization directions is likely minimal. pole for Rodinia reconstructions, for a first-order tectonic re- construction of the pre-Cenozoic Laurentian Arctic margin Methodology and for the demonstration that an extinct plate boundary ex- ists between Ellesmere Island and Greenland. Nineteen east–west-trending dykes in the High Arctic of Canada and 10 from northwest Greenland, as well as two Geological setting sills, were sampled with 7–15 samples taken from each for The Franklin igneous event emplaced sills and dykes paleomagnetic analysis. Blocks for geochronological analysis across more than 2500 km of lateral distance in the Cana- were taken from the interior of most dykes because coarser dian Arctic and Greenland (Fig. 1), making it one of the grained interiors are considered more favourable for the re- largest dyke swarms on Earth. It includes large (‡20 m covery of baddeleyite grains large enough for single-grain wide) diabase dykes in mainland Canada, Baffin Island, and U–Pb analysis. The North America and Greenland data sets Greenland. Also associated are the Victoria Island sills and are treated separately because their remanence directions Natkusiak volcanics of Victoria Island, the Coronation sills may have been offset by the Cretaceous movement of Green- of the mainland Northwest Territories, and the Thule sills land relative to Canada (Tessensohn and Piepjohn 1998), Published by NRC Research Press Denyszyn et al. 691 Fig. 2. Map showing the locations of dykes (solid black bars) included in the study and their trends. Circles denote sills. Sample site names in boxes. although they are certainly of the same swarm (Denyszyn et magnetometer with repeatability of results for magnetization al. (2006) includes preliminary results for some of the dykes intensities above *10 –3 Am–1. Measurement procedures in- studied here). cluded an averaging procedure to reduce the dependence of results on the last sample axis to be demagnetized in the sin- Paleomagnetism gle-axis demagnetizer (description in Halls (1986)). Thermal Samples were collected either as field-drilled oriented demagnetization was also used for selected specimens using cores or as oriented block samples.
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