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Lying at the large-scale tectonic boundary between the Yukon Tanana and northern extremity of the Stikinia and Quesnellia terranes, mafic eruptions have occurred over the past 4 million years in the Fort Selkirk area. Eruptions have included effusive (lava), explosive activity, and subglacial and subaqueous eruptions that produced hyaloclastite complexes. Eruptions have dammed or disrupted Yukon River at least five times. These volcanic rocks and interstratified sediments are collectively referred to as the Fort Selkirk Volcanic Group. Eruptions occurred from seven centers with activity progressively moving north. An explanation for the northern migration of volcanism is elusive at present. [ABSTRACT FROM AUTHOR]

We have estimated the geochemical equilibration depth and temperature of the widespread Neogene alkaline basalts in the Cordillera of Alaska, Northwest Canada, and in Mexico using geobarometry on bulk compositions that have been minimally differentiated in upward transit. The method has uncertainties of about ±10 km and <70 °C. The regional averages of geochemical equilibration depth for 12 sites in Alaska vary from 50 ± 10 to 84 ± 2 km, somewhat broader than those from the Cordillera in western Canada, western USA, and Mexico. There are no associations of depth with terranes or geological provinces. The final equilibration depth of lavas with the surrounding mantle is concluded to be where partial melt percolating from greater depths' ponds at the lithosphere–asthenosphere boundary (LAB) until it becomes gravitationally unstable and moves upward in conduits. The top of the low velocity zone from seismic receiver functions, taken to be the LAB in regions of Alaska where Neogene volcanism occurs, varies from 60 to 85 km, covering the range of geochemical equilibration depths of the alkaline lavas. A mean lava equilibration depth of 65 ± 10 km occurs in 24 of 36 alkaline volcanic centers from Alaska to Mexico, and several other global locations, suggesting the LAB may be controlled to a first order by the change in H2O storage capacity and viscosity across the garnet–spinel peridotite phase change at this depth. The scatter and variation in equilibration depths and temperatures are a factor of 2 greater than the recognized uncertainties, and are not yet explained. [ABSTRACT FROM AUTHOR]

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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.)

Concentrations of trace elements with bulk distribution coefficients < 1 have been utilized to discriminate between mafic magmas erupted in different tectonic settings. Herein, the Nb–Y–Zr contents of mafic lavas will be utilized to pinpoint derivation from a deep-mantle source. Magmas derived from the Icelandic plume exhibit ΔNb > 0, where ΔNb = log (Nb/Y) + 1.74 – 1.92 log (Zr/Y) (Fitton et al. 1997), whereas melts derived from depleted sections of the mantle and the crust show ΔNb < 0. Regardless of the nature of the melting event, it was suggested the ΔNb value of magmas were essentially identical to that of the source material. Theoretical modeling of a garnet lherzolite source suggests very small partial melts may show elevated ΔNb values, yielding a spurious plumelike signature. For mafic rocks from different areas, with hypothesized hot spot derivation, ΔNb is calculated to search for a deep-mantle signature (ΔNb > 0). The signature of the Réunion hot spot, from ~65–0 Ma, is seen in the Deccan Traps, India, lavas from the Mascarene Plateau, and Réunion Island. The signature of the Kerguélen plume is detected in rocks from Kerguélen Island and the Ninetyeast and Broken Ridges, Indian Ocean, a few lavas from the Rajmahal province, India, but none in the Bunbury Basalt, Australia. Some rocks from continental flood basalt provinces show a hot spot component, in agreement with conclusions based on [sup 3] He/[sup 4] He studies, others show ΔNb < 0, due to considerable subcontinental contamination. For lavas from Heard Island and Grand Comore, Indian Ocean, the technique pinpoints those showing a plume component. In an area of plate convergence in the Pacific Ocean, lavas with very low (<1 ppm) Nb contents, serve as a critical test for this method. Only rocks close to the Samoan hot spot show ΔNb > 0, whereas the others show the Nb depletion typical of arc-related material. In conjunction with trace-element and isotopic discriminants, this appears to be a useful tool in identifying a deep-mantle component in mafic lavas from a variety of plate tectonic settings.Des concentrations d'éléments traces avec des coefficients de distribution massique <1 ont été utilisées pour faire la distinction entre diverses laves mafiques qui ont fait éruption dans différents environnements tectoniques. Dans le présent article, le contenu Nb–Y–Zr des roches mafiques sera utilisé pour cerner avec précision la provenance d'une origine mantellique profonde. Les magmas provenant du panache islandais présentent ΔNb > 0, où ΔNb = log (Nb/Y) + 1,74 – 1,92 log (Zr/Y) (Fitton et al. 1997) alors que des matériaux en fusion provenant de sections appauvries du manteau et de la croûte montrent ΔNb < 0. Peu importe la nature de l'événement de fusion, il a été suggéré que la valeur ΔNb des magmas était essentiellement identique à celle du matériau source. La modélisation théorique d'une source de lherzolite à grenat suggère que de très petites fusions partielles peuvent présenter des valeurs ΔNb élevées, donnant une fausse signature de type panache. En ce qui concerne les roches mafiques de régions différentes, et avec une dérivation hypothétique du point chaud, le calcul de ΔNb est effectué de manière à rechercher une signature mantellique profonde (ΔNb > 0). La signature du point chaud de La Reunion, à partir de ~65–0 Ma est observée dans le plateau de Deccan, en Inde, dans les laves du plateau de Mascarene et à l'île de La Réunion. La signature du panache de Kerguelen est détectée dans les roches de l'île de Kerguelen, dans celles des crêtes de Ninetyest et de Broken, de l'océan indien, dans quelques laves de la province de Rajmahal, en Inde, mais aucune dans le basalte de Bunbury en Australie. Quelques roches provenant de provinces basaltiques des plateaux continentaux montrent une composante de point chaud, en accord avec les conclusions basées sur des études [sup 3] He/[sup 4] He; d'autres montrent ΔNb < 0 en raison d'une grande contamination sous-continentale. En ce qui concerne les laves de l'ile Heard et de Grand Comore, dans l'océan Indien, la technique montre précisément celles qui ont une composante de panache. Dans un endroit de convergence des plaques dans l'océan Pacifique, les laves à très faible teneur en Nb (<1 ppm) servent de test critique pour cette méthode. Seules les roches à proximité du point chaud de Samoa montrent ΔNb > 0, alors que les autres roches montrent un épuisement de Nb typique de matériel associé aux arcs. Utilisé avec les éléments traces et les discriminants isotopiques, cela semble être un outil utile pour identifier une composante profonde du manteau dans des roches mafiques provenant d'une variété d'environnements de tectonique des plaques.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]