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Igneous Rock Associations 11. the Geology and Petrology of Seafloor Volcanic Rocks of the Northeastern Pacific Ocean, Offshore Canada Brian Cousens

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Igneous Rock Associations 11. the Geology and Petrology of Seafloor Volcanic Rocks of the Northeastern Pacific Ocean, Offshore Canada Brian Cousens Document généré le 26 sept. 2021 08:35 Geoscience Canada Igneous Rock Associations 11. The Geology and Petrology of Seafloor Volcanic Rocks of the Northeastern Pacific Ocean, Offshore Canada Brian Cousens Volume 37, numéro 2, june 2010 Résumé de l'article Les fonds océaniques de la Zone économique exclusive du Canada de la région URI : https://id.erudit.org/iderudit/geocan37_2ser01 nord-est du Pacifique montrent des exemples de trois types d’activité volcanique : volcanisme de dorsale médio-océanique, de monts sousmarins de Aller au sommaire du numéro dorsale, et de monts sousmarins d’intra-plaque. Le volcanisme de la dorsale Explorer et de la portion nord de la dorsale de Juan de Fuca, au droit des rifts d’extension de failles inter-transformantes, et non loin des monts sous-marins Éditeur(s) jouxtant la dorsale Explorer et celle de Juan de Fuca produisent des laves de composition géochimique anormalement variable. Les laves renferment des The Geological Association of Canada quantités variables de matériaux du manteau supérieur appauvris en éléments traces mais aussi de filons enrichis en éléments traces, en amas ou traînées ISSN ennoyés dans le manteau supérieur. Ces derniers peuvent provenir de reliquats d’anciens panaches mantelliques similaires aux panaches modernes 0315-0941 (imprimé) à l’origine de la formation du mont sous-marin de Bowie. 1911-4850 (numérique) Découvrir la revue Citer cet article Cousens, B. (2010). Igneous Rock Associations 11. The Geology and Petrology of Seafloor Volcanic Rocks of the Northeastern Pacific Ocean, Offshore Canada. Geoscience Canada, 37(2), 49–64. All rights reserved © The Geological Association of Canada, 2010 Ce document est protégé par la loi sur le droit d’auteur. L’utilisation des services d’Érudit (y compris la reproduction) est assujettie à sa politique d’utilisation que vous pouvez consulter en ligne. https://apropos.erudit.org/fr/usagers/politique-dutilisation/ Cet article est diffusé et préservé par Érudit. Érudit est un consortium interuniversitaire sans but lucratif composé de l’Université de Montréal, l’Université Laval et l’Université du Québec à Montréal. Il a pour mission la promotion et la valorisation de la recherche. https://www.erudit.org/fr/ GEOSCIENCE CANADA Volume 37 Number 2 June 2010 49 SERIES element-enriched veins, blobs or ridges but are not part of the mid- streaks embedded in the depleted ocean ridge system itself; and intraplate upper mantle. The latter may have seamounts that form well away from originated as dispersed parts of ancient plate boundaries, such as the mantle plumes similar to a modern Hawaii–Emperor seamount chain (Fig. plume responsible for the formation of 1). The floor of the NE Pacific the intraplate Bowie Seamount. includes one of the best-studied mid- ocean ridge segments in the world, the SOMMAIRE Juan de Fuca Ridge. However, most Les fonds océaniques de la Zone other volcanic features in the NE économique exclusive du Canada de la Pacific are poorly understood petrolog- Igneous Rock Associations région nord-est du Pacifique montrent ically. des exemples de trois types d’activité The offshore regions of 11. volcanique : volcanisme de dorsale British Columbia, Washington, Ore- The Geology and Petrology médio-océanique, de monts sous- gon, and northern California have marins de dorsale, et de monts sous- played an important role in the devel- of Seafloor Volcanic Rocks marins d’intra-plaque. Le volcanisme opment of the theory of plate tecton- of the Northeastern Pacific de la dorsale Explorer et de la portion ics and our understanding of plate nord de la dorsale de Juan de Fuca, au motion and evolution. Some examples Ocean, Offshore Canada droit des rifts d’extension de failles will illustrate the diversity of research inter-transformantes, et non loin des in the region. First, in 1955 the United Brian Cousens monts sous-marins jouxtant la dorsale States Coast and Geodetic Survey ship Ottawa Carleton Geoscience Centre Explorer et celle de Juan de Fuca pro- Pioneer performed one of the first Isotope Geology and Geochronology Research duisent des laves de composition large-scale marine magnetic surveys, Facility géochimique anormalement variable. encompassing the Juan de Fuca and Department of Earth Sciences, Carleton Les laves renferment des quantités Explorer ridges and demonstrating the University variables de matériaux du manteau existence of magnetic stripes on the 1125 Colonel By Drive supérieur appauvris en éléments traces seafloor (Raff and Mason 1961; Vine Ottawa, ON, Canada, K1S 5B6 mais aussi de filons enrichis en élé- and Wilson 1965). The seafloor record E-mail: [email protected] ments traces, en amas ou traînées was then used to reconstruct the tec- ennoyés dans le manteau supérieur. tonic history of western North Ameri- SUMMARY Ces derniers peuvent provenir de reli- ca (Atwater 1970; Atwater and Stock The seafloor within Canada’s Exclusive quats d’anciens panaches mantelliques 1998). Second, when non-transform Economic Zone in the northeastern similaires aux panaches modernes à l’o- offsets of mid-ocean ridge segments Pacific Ocean features examples of rigine de la formation du mont sous- were first recognized along the Galapa- three kinds of volcanic activity: mid- marin de Bowie. gos Spreading Centre and East Pacific ocean ridge, near-ridge seamount, and Rise, application of the concepts intraplate seamount volcanism. Vol- INTRODUCTION learned there were key to reconstruct- canism on the northern Juan de Fuca The seafloor of the northeastern Pacif- ing the history of the Juan de Fuca and Explorer ridges, at inter-transform ic Ocean is typical of most oceanic Plate (Riddihough 1977, 1984; Riddi- pull-apart rifts, and at near-ridge regions in that it features three differ- hough et al. 1983; Wilson 1988). seamounts close to the Juan de Fuca ent kinds of volcanic edifices that pro- Third, the discovery of massive sul- and Explorer ridges, produces lavas duce primarily basaltic volcanic rocks phide deposits at the Middle Valley with unusually variable geochemical and their intrusive equivalents: mid- (e.g. Goodfellow and Blaise 1988; compositions. Lavas incorporate vari- ocean ridges, where new oceanic crust Davis and Villinger 1992; Bjerkgard et able contributions from both trace-ele- is produced; near-ridge seamounts, al. 2000) and Endeavour (e.g. Tivey ment-depleted upper mantle and trace- which form adjacent to mid-ocean and Delaney 1986; Delaney et al. 1992; 50 McDonough 1989). Although many subdivisions of mid-ocean ridge basalt (MORB) chemical types have been proposed, ratios of incompatible trace elements are commonly used to classify MORB as depleted or normal (N), transitional (T), and enriched (E) types. Typically, primitive mantle-normalized values for either La/Sm (La/Smpmn) or Nb/Zr (Nb/Zrpmn) are <0.8, 0.8 to 1.2, and >1.2 in N-, T-, and E-MORB, respectively. Additionally, radiogenic isotope ratios tell us about the time- integrated parent-daughter ratio in mantle sources for oceanic volcanism, and can establish whether mantle enrichment events are recent or ancient (e.g. Hofmann 1997). The geochem- istry of volcanic rocks from the NE Pacific demonstrates that enriched components (veins, blobs, mantle plumes) in the upper mantle are a major contributor to intraplate, near- Figure 1. Geological features of the northeastern Pacific Ocean, modified from ridge, and ocean ridge volcanism to a Dalrymple et al. (1987). Numbers in brackets are K-Ar ages of lavas in Ma. Grey degree not commonly seen in other contours are water depths in metres. The lined pattern indicates a zone of defor- seafloor regions. mation and seismicity, proposed to be a new transform boundary (Dziak 2006). Near-ridge seamounts include the Heck (HKS), Heckle HKLS), Springfield (SFS), NORTHERN JUAN DE FUCA RIDGE Dellwood (DS), Graham, Drifters, Oshawa and Union seamounts. Intraplate AND EXPLORER RIDGE seamounts include Bowie, Dickins, Hodgkins, Davidson, and Denson (the latter Tectonic Evolution three also include an older, near-ridge phase), as well as many edifices in the Cobb- The Juan de Fuca and Explorer plates Eickelberg seamount chain. The short dash line is the limit of Canada’s Exclusive are the remnants of the Farallon Plate, Economic Zone (EEZ). CO: Cobb Offset; DK: Dellwood Knolls; ED: Explorer most of which has been consumed by Deep; ER: Explorer Rift; RD FZ/TF: Revere−Dellwood Fracture Zone/Trans- subduction beneath the North Ameri- form Fault; SER: Southern Explorer Ridge; SFZ: Sovanco Fracture Zone; W/MV: can Plate over the past 150 Ma (Riddi- West/Middle Valley; NF: Nootka Fault. hough 1984). As the area of the plate has diminished and the Farallon–Pacif- Kelley et al. 2001) segments of the geology, petrology and geochemistry of ic spreading centre has approached the northern Juan de Fuca Ridge (Fig. 1) volcanic features within Canadian North American margin, the Farallon led to vigorous research efforts, includ- northern Pacific waters (i.e. Canada’s Plate has broken up into the Gorda, ing basalt petrology, in both areas (e.g. Exclusive Economic Zone (EEZ); Fig. Juan de Fuca, and Explorer plates Karsten et al. 1990; Stakes and 1) will be reviewed, including the (Riddihough 1984; Wilson et al. 1984). Franklin 1994; Cousens et al. 2002; northern segments of the northern The Sovanco fracture zone, which links Teagle and Alt 2004; Woodcock et al. Juan de Fuca Ridge and Explorer the northern segments of the Juan de 2006). Fourth, the breakup of oceanic Ridge, the Dellwood Knolls and Tuzo Fuca Ridge to Explorer Ridge, formed microplates and the evolution of plate Wilson Volcanic Field, the at approximately 7.4 Ma and length- boundaries are beautifully exemplified Heck/Heckle and Dellwood near-ridge ened rapidly as Explorer Ridge shifted by the Dellwood Knolls and Tuzo Wil- seamount chains, and finally the south- to the northwest (Botros and Johnson son Volcanic Field (Chase 1977; Riddi- ern Pratt–Welker seamount chain.
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