Kenneth L. Buchan

A proposed giant radiating dyke swarm fragmented by the separation of Laurentia and Australia based on paleomagnetism of ca. 780 Ma mafic intrusions in western North America

Abstract We propose that mafic dykes and sheets, recently dated at 780 Ma, in three widely separated areas of western North America (the Canadian Shield, the Mackenzie Mountains in the northern Cordillera, and the Wyoming Province) may represent subswarms of a giant radiating dyke swarm. Paleomagnetic data from these intrusions demonstrate the general tectonic integrity of the whole region, which further suggests that relative strikes of the subswarms are primary. The roughly radial pattern of the subswarms is focused off the western coast of North America and may define the ancient location, relative to North America, of a mantle plume responsible for the giant radiating dyke swarm. We suggest that the recently proposed ca. 800 Ma plume centre defined by the Willouran volcanic rocks in central-southern Australia and the associated Gairdner dyke swarm could represent, respectively, the missing plume centre and a missing subswarm of our proposed giant radiating dyke swarm. If so, the giant radiating dyke swarm was fragmented sometime after 780 Ma by the breakup of the supercontinent which included Laurentia and Australia. Indeed, the 780 Ma magmatism may have been a precursor to the breakup.

Maximum size and distribution in time and space of mantle plumes: evidence from large igneous provinces

Abstract Our recently published database of 304 possible large igneous provinces (LIPs) extending back to 3.8 Ga is used to explore the maximum size and distribution in time and space of large mantle plume heads. Maximum plume head size has been estimated using various techniques for two of the largest continental LIPs, the 1.27 Ga Mackenzie, and 0.20 Ga CAMP events, as well as two of the largest oceanic LIPs, Ontong Java and Manihiki. Each is consistent with an underlying flattened plume head radius of about 1000 km, a value considered normal for a plume rising from the deep mantle, and suggesting that even plumes associated with major mantle upwellings are normal size. The LIP record is semi-continuous back to at least 3.0 Ga, and there is no obvious periodicity, indicating that plume activity is probably not cyclic. Apparent increases in plume activity between 0.15 Ga and present, and perhaps, between 2.8 and 2.7 Ga, can be explained by increased preservation of oceanic LIPs. Clusters of plumes in time and space are common in the LIP record. At 6 times since 1.3 Ga, multiple LIPs whose ages span 10 Myr or less are linked to more than one plume centre. At an additional 37 times since 3.0 Ga, LIPs are found on more than one continental block and may represent multiple plumes. However, better-constrained continental reconstructions are required in order to determine which of these are due to a single plume and which require more than one plume. Some synchronous plume centers are clustered regionally suggesting a common mantle perturbation at depth while others are widely separated and, if genetically linked, would imply a global core–mantle event. In addition, some regional clusters of LIPs show a range in age greater than 10 Myr. However, in most such cases it is unknown whether these represent linked events or whether they are random associations in a dense age spectrum of LIPs.

Integrated Paleomagnetism and U‐Pb Geochronology of Mafic Dikes of the Eastern Anabar Shield Region, Siberia: Implications for Mesoproterozoic Paleolatitude of Siberia and Comparison with Laurentia

This article reports the first joint paleomagnetic and U‐Pb geochronologic study of Precambrian diabase dikes in the Anabar Shield and adjacent Riphean cover of Siberia. It was undertaken to allow comparison with similar published studies in Laurentia and to test Proterozoic reconstructions of Siberia and Laurentia. An east‐trending Kuonamka dike yielded a provisional U‐Pb baddeleyite emplacement age of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Time-series analysis of large igneous provinces: 3500 Ma to present

1503\pm 5

Uplift deduced from remanent magnetization: Sudbury area since 1250 Ma ago

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Baked contact test demonstrates primary nature of dominant (N1) magnetisation of Nipissing intrusions in Southern Province, Canadian Shield

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