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Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The area northeast of Sudbury, Ontario, is well known for hosting one of the largest unexplained geophysical anomalies in the Canadian Shield, the Temagami Anomaly. In search of a geological explanation for this anomaly, low-grade metamorphic ultrabasic dykes have been discovered in the overlying Huronian Supergroup sedimentary rocks, both in outcrop and in a deep drill core. Here, we report on the first geochemical and geochronological data obtained on these dykes and compare these data with known magmatic units in and around the 1850 Ma impact-generated Sudbury Igneous Complex (SIC). The NW-striking dykes, which cut across sedimentary rocks of the ca. 2.3 Ga Cobalt Group, and which are, in turn, crosscut by pseudotachylitic breccia, are characterized by distinctively high concentrations of Ti, P, Nb, and Zr, highly fractionated rare earth element patterns (La/YbN 7.6–15.5), and a lack of crustal contamination (Nb/Th > 10). Such features are typical of modern ocean island basalts but very different from Palaeoproterozoic rocks previously documented in the wider region. Multigrain U–Pb laser ablation inductively coupled plasma mass spectrometry analyses performed on magmatic titanite and apatite with high Th/U ratios yielded 1876.0 ± 8.7 and 1880.9 ± 8.3 Ma, respectively, which we interpret as the intrusion age of the dykes. This interpretation is supported by similar whole-rock Sm-Nd model ages of 1890–2000 Ma (initial εNd +2.5). This magmatic event in the footwall of the SIC shortly before the impact was coeval with, and likely genetically related to, the 1.88–1.87 Ga Circum-Superior Large Igneous Province. [ABSTRACT FROM AUTHOR]

John Tuzo Wilson coined the term 'plate' in plate tectonics. He is famous for inventing transform boundaries, hot spot tracks, and the Wilson cycle of ocean birth, growth, and decline. Less well remembered is his work in the 1950s on tectonic and radiometric age provinces of the Canadian Shield, as part of which he fathered U/Pb geochronology in Canada. This work gave strong support to the notion of continental growth through accretion of successively younger terranes onto an ancient cratonic core. The present paper reviews how paleomagnetism can trace the motions of continents to test Wilson's ideas. Continental accretion often involves deep burial of one of the colliding elements through subduction or crustal underplating; such was the case with the Grenville orogen and its subprovinces in their Proterozoic accretion onto the Laurentian craton. The resulting heating and metamorphism erases most pre-collisional magnetic information but adds something new: the possibility of following the post-metamorphic uplift and cooling history, in time and space. The time element is provided by a new form of isotopic geochronology, thermochronometry, which provides dates for specific minerals together with the temperatures at which they became closed to isotopic migration. U/Pb dating of sphene is one method used; another is the 40Ar/39Ar variant of K/Ar dating applied to hornblende, micas, and feldspars, which have a wide range of Ar closure temperatures. The two specific Grenville studies described deal with parallel uplift histories determined by 40Ar/39Ar dating and by magnetics for the accreted terranes of the Central Metasedimentary Belt in Ontario and with the paleomagnetic detection of the post-1240 Ma closing of a small ocean between the Elsevir terrane and Laurentia during the Grenvillian orogeny. [ABSTRACT FROM AUTHOR]

The calc-alkaline Great Bear continental arc in the Wopmay Orogen developed after a collision ca. 1890 Ma of the Archean Slave craton with the Paleoproterozoic Hottah terrane to the west. U–Pb zircon dating of three volcanic and six intrusive rocks from the southern part of the arc shows four stages of development: (i) intrusion of a few small sodic leucogranite plutons at 1873 ± 2 Ma into a previously folded metasedimentary sequence; (ii) abundant calc-alkaline felsic volcanism and subvolcanic intrusions during the period 1870–1866 Ma, bracketed by five ages; (iii) intrusion of large calc-alkaline granitic plutons, including the Marian River batholith, dated by zircon and titanite from two samples at 1866 [sup +2] [sub –3] Ma; and (iv) emplacement of the potassic Faber Lake rapakivi granite at 1856 [sup +2] [sub –3] Ma. The arc was developed on the Hottah terrane due to easterly subduction of an oceanic plate under the amalgamated Slave craton – Hottah terrane. The oldest exposed rocks in the southern part of the arc are remnants of a Paleoproterozoic platformal sequence. They were regarded previously as equivalents of the initial passive margin sequence on the Slave craton, but are interpreted here as part of the Hottah terrane. The ages reported here are comparable with earlier data from the northern part of the arc, which show an age range from 1875 to 1840 Ma and also identify two suites of compositionally and temporally distinct granites. The age constraints show that the Great Bear arc evolved rapidly in time from sodic through calc-alkaline, and then, with a pause, to potassic composition.L'arc continental calco-alcalin de Great Bear dans l'orogène Wopmay s'est développé après une collision entre le craton de Slave, de l'Archéen et, à l'ouest, le terrane de Hottah, du Paléoprotérozoïque, il y a environ 1980 Ma. Des datations U–Pb sur des zircons de trois roches volcaniques et de six roches intrusives provenant de la partie sud de l'arc montrent quatre stages de développement : (i) l'intrusion de quelques petits plutons leucogranitiques sodiques (1873 ± 2 Ma) dans une séquence métasédimentaire plissée antérieurement; (ii) beaucoup de volcanisme calco-alcalin felsique et des intrusions subvolcaniques au cours de la période 1870-1866 Ma, encadré par cinq âges; (iii) intrusion de grands plutons calco-alcalins granitiques, incluant le batholite de Marian River, daté par du zircon et de la titanite provenant de deux échantillons à 1866 [sup +2] [sub –3] Ma, et (iv) la mise en place du granite rapakivi potassique de Faber Lake à 1856 [sup +2] [sub –3] Ma. L'arc s'est développé sur le terrane de Hottah en raison de la subduction vers l'est d'une plaque océanique sous l'amalgamation du craton de Slave et du terrane de Hottah. Les roches les plus anciennes qui affleurent dans la partie sud de l'arc sont des lambeaux d'une séquence de plate-forme datant du Paléoprotérozoïque. Auparavant, elles étaient considérées comme les équivalents de la séquence initiale de marge passive du craton de Slave, mais on les interprète ici comme faisant partie du terrane de Hottah. Les âges rapportés ici sont comparables aux données antérieures de la partie nord de l'arc, qui montent une plage d'âges de 1875 à 1840 Ma et identifient aussi deux suites de granites distincts en composition et dans le temps. Les limites d'âge montrent une évolution temporelle rapide de l'arc de Great Bear de sodique à calco-alcalin et, après une pause, à potassique. [Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)