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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

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)

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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.

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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. Par- 1988; Fig. 2). However, the Explorer hough et al. 1980; Carbotte et al. 1989; ticular emphasis is placed on incompat- Plate has only been independent from Allan et al. 1993). Fifth, the ible minor and trace elements as well the Juan de Fuca Plate since ca. 4 Ma, Pratt–Welker (or Bowie–Kodiak) as radiogenic isotope ratios (Sr, Nd, when resistance to subduction reached seamount chain in the Gulf of Alaska Pb), which serve to distinguish differ- a critical value and the Nootka Fault is recognized to be partly of intraplate ent mantle sources for basalts in the was initiated (Riddihough 1984; Botros origin, and geochronological data from NE Pacific. The relative abundance of and Johnson 1988; Fig. 2). Subse- the chain were used to constrain the incompatible elements, particularly the quently, the orientation of Explorer motion of the Pacific Plate in the rare earth elements (REE), is a func- Ridge rotated clockwise via several rift hotspot reference frame (Turner et al. tion of source abundances at the time propagation events and an eastward 1980; Wessel and Kroenke 1998). of melting and the degree of partial ridge jump during the last 0.7 Ma. In the following sections, the melting of the source (e.g. Sun and Thus, the modern Explorer Ridge con- GEOSCIENCE CANADA Volume 37 Number 2 June 2010 51

Figure 2. Tectonic evolution model for the northern Juan de Fuca and Explorer ridges (modified from Botros and Johnson 1988). Solid arrows indicate direction of spreading; size of arrow is proportional to spreading rate. a) The Sovanco frac- ture zone (SFZ) is initiated at ~7.4 Ma. b) Explorer Plate (EXPL) separates from Juan de Fuca Plate (JDF) at 3.4 Ma as resistance to subduction initiates the Nootka Fault Zone (NFZ); breakup and clockwise rotation of the Explorer Ridge occurs by ridge propagation (V-shaped dotted lines labeled ‘PR’, V points in direction of ridge propagation). c) By 1 Ma, rotation of Explorer Plate has widened the SFZ into a broad zone of deformation which now rotates counterclockwise (dashed arrow) and results in abandonment of a spreading centre (thick dashed line labelled ASC, now called Explorer Seamount). Over the last 1 Ma, northwesterly ridge jumps have occurred at the north end of both the Juan de Fuca and Explorer ridges (Fig. 1). PAC: Pacific Plate.

sists of three segments, the more Rohr and Furlong 1995; Dziak 2006). robust Southern Explorer Ridge (SER), If so, then at some point in the future the Explorer Deep rift to the north- all spreading on Explorer Ridge will east, and a complex set of en-echelon cease and most of the Explorer Plate rifts, termed the Explorer Rift, to the will merge with the North American north (Botros and Johnson 1988: Fig. Plate. 1). The latter two rifts terminate along the Revere–Dellwood fracture zone Northern Juan de Fuca Ridge (Figs. 1 and 3a). The first regional studies of the north- Multibeam bathymetry and ern Juan de Fuca Ridge began in the sidescan sonar surveys in the early late 1960s at the University of British 1980s demonstrated that SER is in a Columbia and the University of Wash- period of sustained high magma sup- ington (McManus et al. 1972; Barr ply, having a minimum depth of only 1974; Barr and Chase 1974). Based on 1750 m near its north end, tapering to widely spaced, single-beam echo- a depth of >2400 m at its south end sounder, seismic reflection and mag- (Kappel and Ryan 1986). The SER is netic data, Barr and Chase (1974) pro- split by a narrow central rift that disap- posed that active spreading centres pears at the shallowest part of the were located along the Endeavour ridge (Fig. 3b). In 2002, the National Trough, West Valley, and Middle Ridge, Oceanographic and Atmospheric but Middle Valley was not considered Administration (NOAA) conducted an an active spreading centre because of EM300 and deep-tow Autonomous the thick sediments filling the valley. Benthic Explorer survey of the north- Dredging from bathymetric highs ern SER as well as Explorer Rift and recovered pillow lavas, pillow frag- Explorer Deep (Embley 2002; NOAA ments, and holocrystalline rocks, all 2002). basaltic in composition. The Explorer–Pacific–North A revolution in understanding America triple junction has jumped the geology and segmentation of the northward over the last 700 000 years, northern Juan de Fuca Ridge began manifested in pull-apart regions at the with the first multibeam bathymetric Dellwood Knolls and the Tuzo Wilson and sidescan sonar surveys in 1983 Volcanic Field (Riddihough et al. 1980; (Johnson et al. 1983; Karsten et al. Carbotte et al. 1989; Davis and Currie 1986; Fig. 3). Discontinuities in mag- 1993; Figs. 1 and 3a). Recent structural netic anomaly stripes, first identified by and seismicity studies suggest that Raff and Mason (1961), were recog- Explorer Plate is unstable, and plate nized to result from migrating non- deformation is consistent with the ini- transform offsets (i.e. propagating rifts tiation of a new, broad Pacific–Juan de and overlapping spreading centres; Hey Fuca transform boundary that cuts 1977; Macdonald and Fox 1983). across the Explorer and Pacific plates Spreading segments were accurately from the southern end of the Queen located, and these were named, from Charlotte Islands to the northern Juan south to north, the Endeavour, West de Fuca Ridge (lined pattern, Fig. 1; Valley, and Middle Valley segments 52

Figure 3. Geological interpretations of multibeam bathymetry and sidescan sonar imagery for, a) the Dellwood Knolls and Tuzo Wilson Volcanic Field; b) the Explorer Ridge; and c) the Endeavour segment of the northern Juan de Fuca Ridge. Modi- fied from Davis and Currie (1993). See Figure 1 for locations.

(Figs. 1 and 3c). West Valley and Mid- of the Endeavour Segment (ES), based Le Bas et al. 1986) and CaO/Al2O3 vs. dle Valley are parallel to one another, on 56 dredges and submersible dives Mg/(Mg+Fe2+) comparisons of south- and West Valley is considered to repre- performed between 1971 and 1984. ern Juan de Fuca and ES lavas sent a ‛ridge jump’ from Middle Valley Compared to lavas from the southern (GEOROC 2009). The range of ES during the Brunhes Normal Epoch Juan de Fuca Ridge, lavas from the compositions is as large as that of the (e.g. Davis and Currie 1993). Shallow Endeavour Ridge axis are commonly southern Juan de Fuca, with the excep- and deep-towed sidescan sonar systems higher in Na2O and K2O, but lower in tion that some southern lavas extend clearly identified young, sediment-free FeOt, and are enriched in incompatible to lower Mg#s (Mg/[Mg+Fe2+]) and crust at the spreading segments, out- trace elements. Endeavour Segment are therefore more highly fractionated. lined faults, and, when combined with lavas are also generally enriched in But figures 6a and 7a show the sea-bottom photographs, formed the H2O (0.33 to 0.48 wt%) compared to extraordinary range of K2O/TiO2 and basis of a geological map of the ridge southern Juan de Fuca Ridge basalts Nb/Zrpmn in ES lavas, including N-, T-, system (Fig. 3c). As a result, rock sam- (0.15 to 0.36 wt%) (Dixon et al. 1988). and E-MORB, compared to the south- pling could now be aimed at specific In contrast, off-axis lavas are chemical- ern Juan de Fuca Ridge (N-, T-MORB geological targets, as opposed to ran- ly similar to southern Juan de Fuca only). Like K2O/TiO2, there is a differ- dom dredging aimed primarily at talus Ridge basalts. Both enriched and ence of nearly an order of magnitude slopes (e.g. Woodcock et al. 2006; depleted lavas were commonly found in Nb/Zrpmn among ES lavas, but a Caress et al. 2008). in adjacent (or even the same) dredge variation of only a factor of 3 along hauls, indicating that the mantle source the southern Juan de Fuca Ridge. Endeavour Segment is heterogeneous even at a local scale Small variations in the degree of man- Karsten et al. (1990) reported a major- (recently demonstrated by Woodcock tle partial melting and subsequent crys- and trace-element study of lavas from et al. 2006). Figures 4a and 5a show tallization processes in seafloor magma the ridge axis and adjacent abyssal hills up-to-date total alkalis vs. silica (TAS; reservoirs are likely responsible for the GEOSCIENCE CANADA Volume 37 Number 2 June 2010 53 range of southern Juan de Fuca Ridge compositions, whereas the order of magnitude difference in K2O/TiO2 at a near constant Mg# of 0.65 in ES lavas requires a different explanation. Based on limited isotopic analyses, there is lit- tle difference in Sr and Nd isotope ratios between ES and southern Juan de Fuca Ridge basalts (Eaby et al. 1984; Rhodes et al. 1990; Fig. 8a). Therefore, the generally higher Rb/Sr and lower Sm/Nd ratios in ES axial lavas do not appear to be the result of an ancient enrichment event (> a few hundred Ma) or else the isotopic signa- ture of ES lavas would reflect this. Karsten et al. (1990) conclud- ed that extreme variability in trace-ele- ment ratios requires a heterogeneous source, feeding multiple parental melts to even the shortest ridge segments. Seismic reflection data indicate that a magma lens exists beneath the ES (Rohr et al. 1988), but this magma lens has not eliminated, by mixing, the het- erogeneity of primary magmas. Karsten et al. (1990) also noted no cor- Figure 4. Total alkali vs. SiO2 diagram for northeast Pacific volcanic rocks, with relation between chemistry and ridge fields defined by Le Bas et al. (1986), and alkaline–tholeiitic boundary from Mac- axis morphology, concluding that donald and Katsura (1964). a) Mid-ocean ridges; b) Dellwood Knolls and Tuzo magma supply to the ridge and the Wilson Volcanic Field; c) near-ridge seamounts; and d) intraplate seamount lavas. extent of partial melting were less Data from GEOROC (2009) and PetDB (2009), except for b), from B. Cousens important parameters than source het- (unpublished data, 2010). erogeneity. Although E-MORB com- monly erupts where a ridge axis is near 0.2 Ma, thermal conditions were nor- <40 ppm, Nb <5 ppm, Rb <5 ppm). a hotspot (e.g. Schilling et al. 1982), mal, and therefore off-axis lavas are N- Middle Ridge basalts have low there is no tectonic evidence for a MORB similar to, or even more deplet- K2O/TiO2 (<0.15) and La/Smpmn hotspot in the region. The nearby ed than, the southern Juan de Fuca (<0.7) (Van Wagoner and Leybourne Heck and Heckle seamount chains Ridge. 1991; Stakes and Franklin 1994); might be viewed as evidence of 87Sr/86Sr ranges from 0.70232 to hotspot activity, but the chemistry of Middle Valley 0.70239, 143Nd/144Nd from 0.51314 to the lavas dredged from these seamount Middle Valley, north of the Endeavour 0.51321, and 206Pb/204Pb from 18.30 to chains argue strongly against a hotspot Segment (Figs. 1 and 3c), is a sedi- 18.40, all falling in a very restricted origin (Barr 1974; see ‛Near-Ridge ment-filled rift graben where the range (Cousens, unpublished data, Seamounts’ section). These constraints youngest volcanism appears to be 2010). Like off-axis basalts from the led Karsten et al. (1990) to suggest that restricted to sill complexes within the Endeavour Segment, Middle Ridge recent tectonic changes along the sediments (Stakes and Franklin 1994). basalts are N-MORB and are similar to northern Juan de Fuca Ridge have Prior to 10 000 years ago, Middle Val- basalts of the southern Juan de Fuca played a role in magmatic evolution. ley and the Endeavour Segment were Ridge. As the ridge has re-oriented itself over probably a single continuous spreading The ODP Leg 139 drilling the past 0.2 Ma by ridge propagation, a centre (Davis and Villinger 1992). (Site 856) beneath the Bent Hill topo- cooler adiabatic gradient beneath the Older basaltic lavas have been dredged graphic high in Middle Valley (Fig. 3c) ridge has resulted in smaller-volume and drilled (Ocean Drilling Program intersected relatively fresh picritic sills partial melts that are chemically diverse (ODP) Site 855) along the eastern mar- at depths less than 200 m (Stakes and and have short residence times in axial gin (Middle Ridge) of the rift valley Franklin 1994), and further drilling magma reservoirs. Melting of enriched (Barr and Chase 1974; Stakes and during ODP Leg 169 sampled sills and domains embedded in the upper man- Franklin 1994; Fig. 3c). The older basement lavas just south of Bent Hill tle may be enhanced under these con- lavas are moderately evolved tholeiitic at depths >430 m (Bjerkgard et al. ditions, and these enriched melts have basalts (MgO <8%, Ni <150 ppm in 2000; Teagle and Alt 2004). The sills made a greater contribution to axial non-porphyritic rocks) with low abun- are among the most primitive (Mg# lavas during the last 0.2 Ma. Prior to dances of incompatible elements (Ba >0.74) basaltic rocks recovered from 54

Valley are commonly N-MORB from a depleted mantle source, similar to lavas from the southern Juan de Fuca Ridge. The picritic sills at Site 856 are the youngest magmas recovered from this segment of the ridge and display some of the most primitive compositions reported for the Juan de Fuca Ridge, suggesting that they postdate the exis- tence of any robust magma chamber at Middle Valley (Stakes and Franklin 1994). Site 856 sills are similar chemi- cally to lavas from the nearby Heck and Heckle near-ridge seamounts (Ley- bourne and Van Wagoner 1991). Sills at Site 857 have multiple magma sources with distinct REE patterns, including one having N-type patterns like Site 855 lavas and another more like T-type lavas from Site 858. It is possible that Site 858 lavas represent a shallow axial seamount that formed late in the magmatic history of Middle Valley, and that this seamount fed some of the sills at Site 857 (Stakes and Franklin 1994).

West Valley Figure 5. CaO/Al O vs. atomic Mg/(Mg+Fe2+) diagrams for northeast Pacific vol- 2 3 West Valley is an incipient spreading canic rocks, assuming Fe2+ = 0.9*Fetotal. a) Mid-ocean ridges. Arrows indicate centre, where rifting commenced after schematic trends of residual liquids during crystallization of olivine (ol), olivine a ridge jump from Middle Valley dur- plus plagioclase (ol+pl), or plagioclase plus clinopyroxene (pl+cpx) at 1 kbar pres- ing the Holocene (Davis and Currie sure, as well as the trend of liquids produced by decreasing degrees of partial melt- 1993). Young volcanism is restricted ing of peridotite (from Niu et al. 2002); b) Dellwood Knolls and Tuzo Wilson Vol- canic Field; c) near-ridge seamounts; and d) intraplate seamount lavas. Data sources to the southern half of the segment as in Figure 4. and comprises small volcanoes on the rift valley floor, 60–250 m in height, that are easily visible in sidescan sonar the Juan de Fuca Ridge; they have very Ni content <100 ppm, the magmas are images (Davis et al. 1984; Karsten et al. low concentrations of incompatible moderately evolved. The sills can be 1986; Fig. 3c). Lava flows on the rift 87 86 elements, La/Smpmn <0.3, and Sr/ Sr split into two groups based on normal- valley floor are extremely plagioclase- and 143Nd/144Nd range from 0.70236 to ized REE patterns, one that is light- phyric, suggesting plagioclase concen- 0.70258 and 0.51318 to 0.51329, REE depleted (N-type) similar to lavas tration, and the complexly zoned respectively. The least altered basement drilled at Site 855, and another that is feldspars indicate multiple magma mix- lava is more evolved (<7% MgO) but slightly light-REE enriched (T-type). ing events (Van Wagoner and Ley- remains light-REE-depleted (La/Smpmn The two sill types alternate with depth. bourne 1991; Cousens et al. 1995). ~0.7), much like the basalts dredged La/Smpmn does not correlate with Flows with high backscatter (little to and drilled from Middle Ridge. Leg 143Nd/144Nd, the most reliable isotopic no sediment cover) are also present on 169 sills and lavas have 143Nd/144Nd indicator in these sills, which ranges the west flank of West Valley, and between 0.51318 and 0.51323 (Bjerk- from 0.51312 to 0.51324 (Cousens, emanate from a small off-axis cone gard et al. 2000). unpublished data, 2010). Drilling at southwest of the rift valley. The latter Sill complexes drilled at ODP nearby Site 858 penetrated basement vent, termed Southwest Seamount (Fig. Site 857 (Fig. 3c) in central Middle Val- flows that are less altered than the sills 3c), spills lava flows into the southern ley are intensely hydrothermally altered, at Site 857. The lavas are remarkably end of West Valley. the original igneous geochemistry has homogeneous in composition, with The lavas from West Valley are been obscured, and only the most normalized Ce/Yb >1.3 (T-type; diverse chemically, ranging from alkali- immobile elements are useful (such as Stakes and Franklin 1994) and poor olivine-hypersthene tholeiites to the REE, Nb, Zr; Stakes and Franklin 143Nd/144Nd between 0.51312 and near nepheline-normative tholeiite at 1994). The sills contain small pseudo- 0.51320 (Cousens, unpublished data, Southwest Seamount (Fig. 4a). Chemi- morphs of olivine, plus plagioclase and 2010). cally, these lavas span the range of clinopyroxene phenocrysts. Based on Lavas and sills from Middle compositions for the entire Juan de GEOSCIENCE CANADA Volume 37 Number 2 June 2010 55

Fuca system, ranging from highly light- REE-depleted N-MORB to the most enriched E-MORB. K2O/TiO2 ranges from 0.01 to 0.35, Nb/Zrpmn from 0.01 to 2.3, and La/Smpmn from 0.5 to 2.2 (Figs. 6a, 7a and 9). The lavas also dis- play a considerable range in Sr, Nd and especially Pb isotopic compositions (206Pb/204Pb = 18.2-19.2; Fig. 8) that correlate positively with increases in incompatible element ratios such as Nb/Zr (Fig. 7a) and La/Sm (Fig. 9), indicating that they are mixtures of melts from depleted and enriched mantle sources. Southwest Seamount, perched on the south end of the Heck seamounts (Fig. 3c), emits lavas domi- nated by the light-REE-enriched com- ponent with the most radiogenic Sr and Pb isotope ratios. These lavas have low CaO/Al2O3 (<0.7) given their intermediate Mg# (0.61–0.55, Fig. 5a), which could be explained by ~20% clinopyroxene fractionation from other West Valley magmas but at pressures much higher than would be expected 2+ for an incipient rift in thin crust Figure 6. K2O/TiO2 vs. Mg/(Mg+Fe ) diagrams for northeast Pacific volcanic (Cousens et al. 1995). Alternatively, rocks. a) Mid-ocean ridges; b) Dellwood Knolls and Tuzo Wilson Volcanic Field, c) 5–10% partial melting of mantle peri- near-ridge seamounts, and d) intraplate seamount lavas. Arrows indicate trajectory dotite at pressures of 10–20 kbar can for crystal fractionation and off-scale position of Horton and Pathfinder seamount trachytes. Data sources as in Figure 4. yield primary magmas with CaO/Al2O3 <0.7 (e.g. Falloon and Green 1988), which could then evolve by low-pres- 3b), Explorer Rift, Explorer Deep, and tional to alkaline basalt. Like the sure fractionation of olivine to the northern half of the Southern Endeavour and West Valley segments, decrease the Mg# of the erupted mag- Explorer (SER) segment. With rare Explorer Ridge basalts span a large mas. Finally, low CaO/Al2O3 and Mg# exception, dredge hauls recovered range of K2O/TiO2 and Nb/Zr values may be consistent with a source rich in young glassy pillow lavas with no fer- and extend to much higher ratios than pyroxenite (e.g. Pertermann and romanganese crust or sediment cover basalts from the southern Juan de Fuca Hirschmann 2003). Cousens et al. (Cousens 1982). Some dredge hauls Ridge (Figs. 6a and 7a). Remarkably, (1995) propose that the enriched man- also sampled blocky lavas with no glass virtually the whole range of Nb/Zr is tle component also includes a hydrous surface. Michael et al. (1989) followed seen in lavas from the shallowest part phase, possibly amphibole. Consistent with a more detailed study of SER of SER, within a small area centred at with the ridge morphology, the com- using samples collected with multi- 49.75°N (Michael et al. 1989; Fig. 3b). plex petrogenesis and small volume of beam bathymetric information. Lavas The La/Smpmn ranges from 0.5 to 1.4 erupted lavas indicate a relatively cool from SER contain variable abundances at Explorer Rift, 1.6 to 1.85 at Explor- ridge environment (Cousens et al. of plagioclase, clinopyroxene and er Deep, and 0.7 to 2.3 along SER (Fig. 1995). olivine phenocrysts, commonly in 9). Few isotopic data are available for glomeroporphyritic clusters. The ubiq- Explorer basalts, but the ranges of Explorer Ridge uitous occurrence of clinopyroxene is 87Sr/86Sr and 143Nd/144Nd are greater

Cousens et al. (1984) published the likely related to the high H2O content than those at West Valley (Fig. 8). first petrological and geochemical data of SER basalts (Michael and Chase The 3He/4He ratios in Juan de from Explorer Ridge, based on dredge 1987; Michael et al. 1989). Fuca Ridge basalts (not shown) range hauls from the early 1970s, 1977 and All three segments of Explor- from 7.8 to 8.8 RA (times the modern 1979, all prior to multibeam bathymet- er Ridge (Explorer Rift, Explorer atmospheric ratio; Lupton et al. 1993). ric surveys (Davis et al. 1984, 1985). Deep, and South Explorer Ridge) are Helium isotopic ratios in Explorer N-

Most of the samples were from the chemically heterogeneous, and exhibit MORB and E-MORB are 8.3 RA and northern half of the ridge system, a wide range of SiO2 and total alkali 7.6 RA, respectively (Carbotte 1987). including Paul Revere Ridge contents (Fig. 4a). Some lavas from Lupton et al. (1993) noted a negative (Revere–Dellwood transform wall; Fig. both Explorer Rift and SER are transi- correlation between 3He/4He and 56

age depth than the depleted compo- nent.

THE PACIFIC─EXPLORER─NORTH AMERICA TRIPLE JUNCTION: DELLWOOD KNOLLS AND TUZO WILSON VOLCANIC FIELD Dellwood Knolls The Dellwood Knolls are two volcanic edifices on the east side of the north- western end of the Revere–Dellwood fracture zone and west of the Queen Charlotte Fault (Bertrand 1972; Riddi- hough et al. 1980; Davis 1982; Davis and Currie 1993; Rohr and Furlong 1995; Figs. 1 and 3a). The two seamounts are separated by the Dell- wood Valley, a zone of microseismicity, faulting, and anomalously thin sedi- ment cover that is interpreted to repre- sent a spreading centre that has split an original single edifice in two (Rohr and Furlong 1995). This is a very young feature, however, and shallow reflec- tors in the valley are only offset by a

2+ few tens of metres (Rohr and Furlong Figure 7. Nb/Zrpmn vs. Mg/(Mg+Fe ) diagrams for northeast Pacific volcanic rocks. a) Mid-ocean ridges; note division into N-, T-, and E-type MORB on left 1995). SeaMARC II sidescan sonar side, b) Dellwood Knolls and Tuzo Wilson Volcanic Field (TWVF); c) near-ridge images show that young lava flows seamounts; and d) intraplate seamount lavas. Data sources as in Figure 4. Primi- cover much of the surface of the tive mantle normalization values from Sun and McDonough (1989). northwestern knoll and spill across the Revere–Dellwood fault where it inter- sects the southwestern side of the 87Sr/86Sr for Juan de Fuca Ridge basalts shallow culmination of SER into the knoll (Davis et al. 1984; Davis and (Eaby et al. 1984). Given 87Sr/86Sr of rift valleys to the southeast, and per- Currie 1993; Fig. 3). A hard reflector 0.70233 and 0.70252 for the N- and E- haps as far as the dredge locations in also fills part of a submarine channel MORB chosen for He analysis, the Explorer Deep. If so, the fresh, E- north of the northwestern knoll. The same relationship appears to hold for type lavas dredged from Explorer southeastern knoll is mostly sediment- these two Explorer Ridge basalts. This Deep in 1979 did not have a source covered and only two hard reflectors negative correlation is unusual for within the rift, and thus Explorer Deep are visible along the southwestern edge MORB in general (Lupton 1983), and may not be volcanically active as has of the seamount, both of which cross indicates that the high-87Sr/86Sr compo- been assumed. In any case, the the Revere–Dellwood fracture zone nent in Explorer and Juan de Fuca enriched component in the sub-SER (Fig. 3a). Bulk densities for both Ridge lavas does not originate in deep, mantle may be present as streaks, veins seamounts, calculated from gravity un-degassed mantle. or blobs in a depleted mantle matrix, data, are only 2.5 g/cm3, suggesting Like the Endeavour and West or is contained within a mantle hotspot that the edifices are composed of a Valley segments, small-scale chemical centred beneath the shallowest point of mixture of basalt and lower density heterogeneity argues against the pres- SER (i.e. the point of greatest magma sediment (Riddihough et al. 1980). ence of a persistent magma reservoir supply; Michael et al. 1989). Major-ele- Dredging, in 1970, recovered beneath Explorer Ridge, even in the ment data were used by Michael et al. young (<1 Ma) vesicular pillow basalts magmatically robust shallow region of (1989) to constrain depth and percent- from the northwestern knoll and high- SER. However, smaller magma reser- age of melting for Explorer N- and E- ly altered, manganese-encrusted pillow voirs must exist at various points along MORB, and to determine that the per- basalt from the southeastern knoll the ridge to explain the abundance of cent melting of the enriched mantle (Bertrand 1972). A 1994 cruise (PGC- evolved magmas. Lavas with enriched component is higher than that of the 94-04) recovered glassy pillow lavas mantle sources are common along depleted component. This is corrobo- from a flow just south of the 1970 SER, and are also found in Explorer rated by the observation that middle to dredge on the northwestern knoll. Rift and Explorer Deep. SeaMARC II heavy REE abundances are higher in Lavas from the northwestern knoll are sidescan sonar (Davis et al. 1984) and the N-MORB than in the E-MORB fresh, slightly olivine-phyric lavas with EM300 reflectance (Embley 2002) data lavas. In addition, the enriched mantle abundant vesicles and rare, partially show lava flows extending from the component melts at a shallower aver- resorbed plagioclase xenocrysts. The GEOSCIENCE CANADA Volume 37 Number 2 June 2010 57

1970 sample from the northwestern knoll is transitional basalt, plotting on the Hawaiian tholeiite-alkalic basalt boundary, whereas the 1994 lava is tholeiitic basalt with a higher SiO2 con- tent (Fig. 4b). Both lavas have low

K2O/TiO2, (Fig. 6b), Nb/Zrpmn, (Fig. 7b), and La/Smpmn (0.66–1.15; Fig. 9), and overlap with N-type MORB of the Juan de Fuca Ridge. Northwestern knoll basalts have 87Sr/86Sr between 0.70240 and 0.70245, 143Nd/144Nd of 0.51315, and 206Pb/204Pb between 18.70 and 18.75 (Cousens, unpublished data, Figure 8. a) 87Sr/86Sr vs. 143Nd/144Nd and b) 207Pb/204Pb vs. 206Pb/204Pb diagrams for 2010; Cousens et al. 1984), all typical all northeastern Pacific oceanic lavas. NHRL: Northern Hemisphere Reference of N-MORB from the Juan de Fuca Line, from Hart (1984); Northern Juan de Fuca: Endeavour, Middle, and West Val- and Explorer ridges (Fig. 8). The ley segments; NRS: near-ridge seamounts; TWVF: Tuzo Wilson Volcanic Field. altered basalt from the southeastern Data sources as in Figure 4. knoll has a higher K2O content (alter- ation?), higher K2O/TiO2 and La/Smpmn (1.26) than lavas from the northwestern knoll. The measured 87Sr/86Sr of 0.70279 to 0.70290 (Cousens et al. 1984) is higher than most MORB from the Juan de Fuca and Explorer ridges, and may partly be due to seafloor alteration. In summary, extremely sparse sampling of the Dell- wood Knolls has recovered both N- type and E-type MORB. The lavas fall within the compositional range of northern Juan de Fuca and Explorer Ridge basalts.

Tuzo Wilson Volcanic Field Northwest of the Dellwood Knolls is another broad zone of high reflectivity in sidescan sonar data, originally called the J. Tuzo Wilson Knolls after the famous University of Toronto geo- physicist (Chase 1977), later termed the Tuzo Wilson Seamounts (Cousens et 143 144 al. 1985; Carbotte et al. 1989), and Figure 9. The La/Smpmn vs. Nd/ Nd diagram for all northeastern Pacific more recently the Tuzo Wilson Vol- oceanic lavas. MORB from ridge segments shows good negative correlation canic Field (TWVF; Allan et al. 1993; between trace element enrichment (La/Sm) and 143Nd/144Nd, whereas most alkalic Rohr and Furlong 1995; Fig. 3a). Dis- lavas from the Tuzo Wilson Volcanic Field and intraplate seamounts have variable covered in 1973, the field consists of La/Sm over a small range of 143Nd/144Nd. Data sources as in Figure 4. two ridge-shaped edifices oriented northeast–southwest and having Chase (1977), based on the major-ele- a dark reflector lies to the northwest of heights of 700 and 500 m above the ment chemistry of the lavas recovered the TWVF, hinting that there is some seafloor. The two edifices are sur- in 1973, considered the TWVF to be a recent volcanic activity even farther to rounded by numerous (~75) lava cones hotspot. Carbotte et al. (1989) noted the north (Davis and Currie 1993). and volcanic ridges, usually <100 m in that the Revere–Dellwood fracture Carbotte et al. (1989) proposed that height, that cover an area of at least 20 zone continues northwest of the Dell- spreading was occurring at both the by 30 km (Allan et al. 1993). The lavas wood Knolls and appears to terminate Dellwood Knolls and TWVF, although have no sediment cover and a very at the south end of the TWVF (Fig. the duration and total amount of high magnetization intensity; a single 3a). Both the Dellwood Knolls and spreading that has occurred at each of attempt to date a Tuzo Wilson lava by TWVF are therefore located between these features is difficult to constrain. K-Ar, yielded an age indistinguishable the Revere–Dellwood and Queen They finally note that widely distrib- from zero (Cousens et al. 1985). Charlotte transform faults. Curiously, uted microseismicity on Explorer Plate, 58 the Dellwood Knolls and TWVF sug- lavas also overlap isotopically with pair (Barr 1974; Leybourne and Van gests that a rigid-plate assumption intraplate alkaline rocks of the Wagoner 1991; Van Wagoner et al. “may not be appropriate”, and that the Pratt–Welker seamount chain to the 1995) and the Dellwood Seamounts Pacific–North America–Explorer triple northwest; in particular, they share a (Cousens et al. 1984) (Figs. 1 and 3). junction may not be a discrete point low 207Pb/204Pb at a given 206Pb/204Pb These seamount chains formed imme- but rather a diffuse zone between the (below the Northern Hemisphere Ref- diately adjacent to the ridge, and north end of Explorer Ridge and the erence Line of Hart 1984), which is seamounts within each chain are pro- TWVF. Cousens et al. (1985), Allan et uncharacteristic of Juan de Fuca gressively older with increasing dis- al. (1993) and Rohr and Furlong (1995) MORB (Fig. 8). tance from the ridge. These propose that the TWVF and the Dell- Low Cr, Sc, FeO and seamounts generally lack evidence for a wood Knolls represent ‛leaky trans- CaO/Al2O3 are indicative of extensive hotspot source, and differ in origin form’ or pull-apart rift volcanism with- clinopyroxene fractionation. Thermo- from volcanoes produced where in the crustal block bounded by the dynamic modelling shows that frac- hotspots meet spreading ridges, such as Revere–Dellwood and Queen Char- tionation must have occurred at pres- Axial Seamount on the southern Juan lotte transforms faults, and that they sures >2 kbar, consistent with the evo- de Fuca Ridge (Desonie and Duncan are not true spreading centres. lution of TWVF magmas beneath 1990; Rhodes et al. 1990). Petrographically, all samples oceanic crust >6 km in thickness recovered from the TWVF are fresh, (Allan et al. 1993). High H2O, K 2O, Heck and Heckle Seamount Chains highly vesicular, glassy pillow lavas. P2O5 and light REE contents require The Heck and Heckle seamount chains They commonly contain olivine and that a K-bearing phase (e.g. amphibole extend 70 and 80 km, respectively, plagioclase phenocrysts, and basalts or phlogopite) and apatite are present northwest of the northernmost part of from the northeastern edifice are also in the TWVF mantle source. Howev- the Juan de Fuca Ridge (Barr 1974; Fig. clinopyroxene-bearing and glomero- er, the lack of heavy-REE depletion in 1). Magnetic anomalies of the oceanic porphyritic. The lavas consist of alka- the lavas indicates that garnet is not a crust continue undeflected across the lic basalts, hawaiites, mugearites and residual mantle phase, and melting like- chains, proving their near-ridge origin. benmoreites (Fig. 4b), and are enriched ly occurred in the spinel or plagioclase Based on the thicknesses of man- in the alkalis, P2O5, H2O, the light REE stability field. Assuming an amphibole ganese crusts, the northernmost Heck- and other incompatible elements peridotite source (e.g. Zabargad Island le seamount is ~3.5 Ma in age, which (Cousens et al. 1985; Allan et al. 1993). peridotite), TWVF primary melts were matches its age deduced from magnetic The TWVF lavas also have high derived by 5–10% partial melting. The anomalies (Barr 1974). The southern- Fe3+/Fe2+, and calculated oxygen fugac- source of Explorer Ridge E-MORB is most seamount in the Heck chain, ities are higher than those of Pacific considered to be similar to that of the Endeavour Seamount, is trapped

MORB. Measured H2O contents are TWVF, but the percentage of melting between the northernmost Endeavour commonly >1%, and these are mini- of the Explorer source is higher and southernmost West Valley mum estimates as the lavas have (12–17%; Michael et al. 1989). How- ridge/rift segments (Karsten et al. degassed upon eruption. Lavas from ever, the evidence from Pb, Sr, Nd and 1986; Fig. 3c). the southwestern edifice are more He isotopes indicates that incompatible The Heck and Heckle enriched in incompatible elements element enrichment of the TWVF seamount chains have been dredged at compared to lavas elsewhere in the mantle source must be a recent event, nine localities (Barr 1974; Leybourne field. Both glasses and whole-rock because these isotopic ratios fall within and Van Wagoner 1991). The rocks samples have lower FeO, CaO, and the field for Pacific MORB (Fig. 8). recovered include variably palagoni-

CaO/Al2O3 at a given Mg# compared The lack of a large eruptive tized glassy lavas (pillows, clinker, sheet to Juan de Fuca MORB (Fig. 5b). The volume or a thermal swell argues flows) coated with manganese crusts, lavas are evolved, with MgO contents against a mantle plume origin for the and hyaloclastites. The Endeavour <7% and Ni <100 ppm, but Mg#s are TWVF. Cousens et al. (1985) and Seamount lava is highly plagioclase- relatively high because of the low FeO Allan et al. (1993) conclude that the phyric, but all others are aphyric to content of the lavas. K2O/TiO2, TWVF and Dellwood Knolls are the sparsely phyric (plagioclase +/- olivine, Nb/Zrpmn, and La/Smpmn are distinctly result of leaky transform volcanism rare clinopyroxene). The basalt glasses higher than MORB, and are higher between the Revere–Dellwood and are remarkably primitive, having Mg#s than E-MORB from the northern Juan Queen Charlotte faults, but the magma >0.6 (and many with a Mg# near 0.7), de Fuca and Explorer Ridges (Figs. 6b, types erupted at the two localities are and low total alkalis (Fig. 4c).

7b and 9). Isotopically, TWVF lavas very different. Petrologically, this CaO/Al2O3 ratios are high (Fig. 5c), as fall at the ‛enriched’ end of the Juan de requires dramatically different mantle are Cr (>400 ppm) and Sc (>26 ppm), Fuca MORB spectrum: 87Sr/86Sr ranges sources at a small scale (<100 km). but Ni is <100 ppm, indicating signifi- from 0.70267 to 0.70279, 143Nd/144Nd cant olivine fractionation but no from 0.51310 to 0.51314, and NEAR-RIDGE SEAMOUNT CHAINS clinopyroxene crystallization. K2O 206 204 Pb/ Pb from 18.88 to 19.01 (Fig. 8). Along the northern Juan de Fuca sys- contents, and K2O/TiO2, are very low 3 4 He/ He ranges from 5.3 to 8.9 RA, but tem, two prominent seamount chains (Fig. 6c), as are Nb/Zr (Fig. 7c), and with high uncertainties (Zadnick 1981; extend onto the Pacific Plate from La/Smpmn (<0.5; not shown). Primitive Kyser and Rison 1982). The TWVF spreading centres: the Heck/Heckle mantle-normalized incompatible-ele- GEOSCIENCE CANADA Volume 37 Number 2 June 2010 59 ment patterns show remarkable deple- sample (70-25-7) allegedly from the Seamount on the southern Juan de tion in the light REE, much like picrit- Dellwood seamounts (sample D-1 in Fuca Ridge (Johnson and Embley ic sills at ODP Site 856, and are well Church and Tatsumoto 1975; Hegner 1990; Chadwick et al. 2005). The below levels seen elsewhere on the and Tatsumoto 1989), is probably fresh youngest edifice in the Pratt–Welker northern Juan de Fuca Ridge (Ley- basalt from Explorer Rift. chain is Bowie Seamount, which was bourne and Van Wagoner 1991). Dellwood seamount lavas are recently named as Canada’s second These rocks have among the lowest very different from Heck and Heckle Marine Protected Area (Fisheries and 87Sr/86Sr and 206Pb/204Pb and highest seamount basalts. The Dellwood Oceans Canada 2008). 143Nd/144Nd of seafloor lavas from the seamount lavas are more evolved, with The 1460 km-long northeast Pacific (Hegner and Tat- Mg# <0.62, Ni <80 ppm, but Cr >285 Pratt–Welker chain has been used as a sumoto 1989; Cousens et al. 1995; Fig. ppm and Sc >40 ppm. They have test of the fixed hotspot reference

8). higher total alkalis (Fig. 4c), K2O and frame (Silver et al. 1974; Turner et al. The Heck and Heckle intermediate K2O/TiO2 compared to 1980; Dalrymple et al. 1987). Turner seamount lavas have a source that is most Explorer Ridge MORB (Fig. 6c). et al. (1980) calculated a revised Pacific highly depleted in incompatible ele- Nb/Zrpmn (Fig. 7c) and La/Sm (not Plate pole of rotation utilizing new ments, both at the time of magma gen- shown) are ~1 (Cousens et al. 1984), K–Ar dates from the northernmost eration (low La/Sm) and over geologic and both 87Sr/86Sr and 206Pb/204Pb are and southernmost seamounts in the time (low 87Sr/86Sr); they are therefore higher than values for Heck and Heck- chain. They also recovered both older, unlike adjacent West Valley lavas, both le (Hegner and Tatsumoto 1989; Fig. tholeiitic and younger, alkalic capping petrographically and chemically. The 8). lavas from some of the southern seamounts lack calderas that would be Although sampling is restrict- seamounts. Dalrymple et al. (1987) indicative of longer term magma ed, it appears that Dellwood Seamount determined an 40Ar-39Ar age for Welker chambers (Clague et al. 2000), and the basalts contain an enriched component Seamount in the central part of the primitive compositions of the lavas are that is not found in Heck and Heckle chain that is too old to fit the pole of consistent with rapid migration near-ridge seamounts. This variety in rotation of Turner et al. (1980), and through the crust prior to eruption. seamount chemistry mirrors the results concluded that the Pratt–Welker chain Geochemical modelling suggests that of studies of similar chains adjacent to (and others in the Gulf of Alaska) was the seamounts have tapped an off-axis the East Pacific Rise (e.g. Graham et al. formed by multiple episodes of short- source that has had any enriched com- 1988). Near-ridge seamounts that are lived intraplate volcanism rather than ponents removed by an earlier partial now located off the west coast of the being formed by a single melting melting event, probably beneath the Queen Charlotte Islands (Oshawa, anomaly (e.g. Hawaii–Emperor). A adjacent ridge axis (Leybourne and Van Drifters, and Graham seamounts) but zone of low mantle seismic velocity Wagoner 1991; Cousens et al. 1995). originating at Explorer Ridge between that has been imaged ~150 km north- 11 and 14 Ma, are also composed of east of Bowie Seamount at a depth of Dellwood Seamounts tholeiitic, N- to E-type MORB basalts ~700 km is interpreted as a mantle The Dellwood seamount chain (Figs. 1 (Cousens et al. 1999). The plume (Nataf and VanDecar 1993). and 3a) extends northwesterly from the Oshawa−Drifters−Graham lavas are Little has been published on northern Explorer Ridge and includes similar to MORB from the Explorer the petrology of the Pratt–Welker three edifices, two of which are coa- Ridge and lavas from the Dellwood chain. Turner et al. (1980) briefly out- lesced (Davis and Currie 1993). The Seamounts (Figs. 5c, 6c and 7a), and line five magma types recovered from seamounts rise nearly 1400 m above strongly resemble the older (ca. 14−18 the chain, including 1) tholeiitic basalt the seafloor. They lack calderas, and Ma), tholeiitic phase of volcanism rep- (Macleod and Pratt 1973), 2) transi- late-stage(?) vents pockmark the south- resented by the southern Pratt–Welker tional basalt, 3 and 4) two types of eastern-most edifice, although none of seamounts (Fig. 1; see next section). alkalic basalt, and 5) trachyte (R.B. the vents are young based on the evi- Thus, the enriched mantle component Forbes et al., unpublished data, 2010; dence of sidescan sonar backscatter. in northern Juan de Fuca, Explorer, Forbes and Hoskin 1969; Forbes et al. The seamounts are more massive than and Dellwood seamount sources was 1969). Incompatible element abun- the Heck and Heckle edifices. Where present beneath the northern Juan de dances and La/Smpmn all increase from the southeastern edifice meets Explor- Fuca Ridge at least as long ago as 14 Group 2 to 5 (R.B. Forbes, personal er Rift there is an off-axis lava flow Ma. communication 1981). Lavas recov- associated with two faults (Davis and ered from Welker Seamount include Currie 1993; Fig. 3a). INTRAPLATE MAGMATISM: BOWIE hawaiite, mugearite, and benmoreite, all Three dredge hauls have pre- SEAMOUNT AND THE with low MgO, Ni, Cr and Sc, and viously sampled the Dellwood PRATT─WELKER CHAIN light-REE-enriched trace element pat- seamounts: 71-15-77 crossed the young Two intraplate seamount chains are terns (Dalrymple et al. 1987). The Sr, off-axis flow at the base of the south- found in the NE Pacific, the Nd and Pb isotope ratios have been eastern edifice, while two others sam- Cobb–Eickelberg and Pratt–Welker determined on the Turner et al. (1980) pled the central and southern peaks (Kodiak–Bowie) group (Fig. 1). The samples, and all fall within the range of (Cousens et al. 1984). Based on its focus of modern volcanism of the Pacific E-MORB (Hegner and Tat- reported location, a fourth dredge Cobb–Eickelberg chain is Axial sumoto 1989; Fig. 8). 60

Bowie Seamount from Bowie seamount are enriched in this event occurred during the last 500

Bowie Seamount, located 180 km west alkali elements, have high K2O/TiO2 Ma. Alternatively, Bowie seamount of the Queen Charlotte Islands (Fig. compared to northern Juan de Fuca lavas could represent mixtures of mag- 1), rises to a water depth of only 25 m. Ridge basalts (Fig. 6d), and high mas derived from a depleted mantle The seamount was exposed above sea Nb/Zr and La/Smpmm (Figs. 7d and 9). (DM) and a HIMU-like (high ‘μ’, level during Pleistocene glacial low- Bowie lavas show decreasing where μ is the U/Pb ratio) mantle stands, forming a wave-eroded upper CaO/Al2O3 with decreasing Mg# (Fig. source, in which the percent melting of terrace at 65 to 100 m depth that lacks 5d), indicative of pyroxene or horn- the DM source is low compared to glacially rafted material and includes blende fractionation. All lavas from that of the HIMU source; this model wave-abraded and sorted lapilli and Bowie, from both the normally mag- applies to alkalic lavas from the shell fragments (Herzer 1971). Vol- netized peak and the reversely magnet- Pratt–Welker chain. canic peaks and fresh tephra deposits ized flanks, have steep REE patterns are emplaced on this terrace, and likely from La through Dy, where the slope DISCUSSION: MANTLE SOURCES IN postdate the last glacial maximum. of the pattern flattens out to Lu, THE NORTHEASTERN PACIFIC The bulk of the seamount is reversely exactly like lavas from the TWVF. The Traditionally, alkalic lavas in oceanic magnetized (older than 0.7 Ma) but the REE patterns for the evolved lavas are settings are associated with Hawaiian- main peak is normally magnetized, parallel to those of the basaltic lavas, type mantle plumes that tap enriched consistent with the presence of young, suggesting that they are related by frac- mantle sources (e.g. Greenough et al. fresh lavas at the summit (Michkofsky tional crystallization. 2005a, b), whereas incompatible-ele- 1969). Bowie seamount lavas exhibit ment-depleted tholeiitic lavas are Lavas recovered from Bowie a remarkable range in isotopic compo- ascribed to melting of the depleted seamount range from alkalic basalt and sitions, particularly Pb isotopes, with upper mantle (e.g. Sun et al. 1979; basanite through hawaiite, mugearite 206Pb/204Pb ranging from 18.26 to 19.18 Presnall and Hoover 1984). The out- and benmoreite (Engel et al. 1965; (Cousens, unpublished data, 2010; standing characteristic of seafloor Herzer 1971; Cousens et al. 1985; Cousens 1988; Fig. 8). In a 87Sr/86Sr vs. rocks from the NE Pacific is the inti- Cousens 1988; Channing 2001; Fig. 143Nd/144Nd plot (Fig. 8a), there is some mate coexistence of depleted and 4d). Lavas from the upper part of the overlap with Juan de Fuca and Explor- enriched components in the upper edifice and from two dredge hauls on er Ridge MORB, and the TWVF alkalic mantle (summarized in Cousens 1996). the southeastern flank are fresh, vesic- lava field sits in the centre of the Although the sample density is sparse ular, glassy extrusive basalts. The Bowie data field. The Pratt–Welker (with the exception of new work on basaltic rocks contain plagioclase (usu- alkalic lavas cover the same isotopic the Endeavour Segment; Woodcock et ally labradorite), olivine (Fo80) and range as Bowie seamount. Note that al. 2006; Harris et al. 2008), the accu- clinopyroxene phenocysts in a fine- all NE Pacific volcanic rocks exhibit a mulated data demonstrate the high grained groundmass enclosing intersti- common range in 143Nd/144Nd but a degree of geochemical heterogeneity, tial glass. Hornblende is found in different range in 87Sr/86Sr, although and the small scale of this heterogene- some of the summit samples, as are some overlap exists between lava ity, in mid-ocean ridges and seamounts small gabbroic, plagioclase-magnetite, groups (Fig. 8a). The Pb story is more off Canada’s west coast. Correlations and ultramafic xenoliths that may rep- complicated. Bowie and other between isotopic ratios, trace-element resent disaggregated cumulates (Herzer Pratt–Welker alkalic lavas form a data enrichment, and major-element charac- 1971). The evolved samples are non- array parallel to, but below, the North- teristics indicate that the spectrum of vesicular to finely vesicular, composed ern Hemisphere Reference Line (Hart lava compositions is the result of mix- of flow-oriented plagioclase laths 1984; Fig. 8b). There is very little ing of melts from the enriched mantle

(An30-50), minor olivine, magnetite and overlap between Bowie and component with melts of DM. Two clinopyroxene, rare hornblende and Pratt–Welker alkalic rocks and Juan de questions immediately arise: what is the apatite phenocrysts, and interstitial Fuca or Explorer MORB, but there is enriched component (more than one?), glass. A dredge haul from 1300–1600 some overlap with near-ridge and how is the enriched component m depth on the northeastern ridge seamounts within or southeast of the introduced into the upper mantle? flank of the volcano recovered more Pratt–Welker chain. Characteristics of the enriched altered, ferromanganese-encrusted Cousens (1988) pointed out component in NE Pacific volcanic alkalic basalt and mugearite (Channing that Bowie lavas, like those of the rocks include high total alkalis, H2O, 2001), likely derived from the older, TWVF, are highly enriched in incom- K2O/TiO2, and K2O/Na2O, but low reversely magnetized part of the patible elements but have near-MORB- FeO and CaO/Al2O3. The alkalic lavas seamount. like isotopic compositions (Fig. 8). are enriched in the light-REE, large-ion Bowie glass and whole-rock The incompatible element enrichment lithophile (LIL), and high-field-strength analyses span a wide range of compo- could reflect a recent metasomatic (HFS) elements, but display the same sitions, from basanite to near-trachyte, event in a depleted mantle source, and middle- to heavy-REE abundances as and cover the entire compositional the small difference in 87Sr/86Sr tholeiitic lavas. The enriched compo- range for the Pratt–Welker and other (~0.0005) and Rb/Sr (0.05) between nent has slightly higher 87Sr/86Sr and Gulf of Alaska seamount chains (Fig. Juan de Fuca MORB and Bowie commonly higher 206Pb/204Pb, but lower 4d). Like TWVF lavas, volcanic rocks seamount basaltic rocks indicates that 207Pb/204Pb at a given 206Pb/204Pb, com- GEOSCIENCE CANADA Volume 37 Number 2 June 2010 61 pared to tholeiitic lavas. The 3He/4He chain are consistent with a mantle me to take on this manuscript, to Mike ratios are similar to normal MORB, plume origin (Lambeck et al. 1984; Perfit and Brendan Murphy for sup- but there is a general tendency for He Chapman et al. 1987; Harris and Chap- portive reviews, and to Georgia Pe- isotope ratios to be lower in lavas with man 1989, 1991; Nataf and VanDecar Piper for nurturing this igneous petrol- a greater contribution from the 1993). The proximity of the TWVF to ogy series. enriched mantle source. The La/Smpmn the southern terminus of the correlates positively with 87Sr/86Sr and Pratt–Welker chain suggests that the REFERENCES negatively with 143Nd/144Nd in NE mushroom-shaped top of the plume Allan, J.F., Chase, R.L., Cousens, B.L., Pacific mid-ocean ridge lavas, but little may have expanded southeastward Michael, P.J., Gorton, M.P., and Scott, correlation exists among the alkalic beneath the oceanic lithosphere as far S.D., 1993, The Tuzo Wilson volcanic lavas alone (Fig. 9). Garnet does not as the TWVF but not as far as the field, NE Pacific: Alkaline volcanism appear to be a residual phase in the Dellwood Knolls. Near-ridge at a complex, diffuse, transform- trench-ridge triple junction: Journal of enriched mantle source, as the slope seamounts are either similar chemically Geophysical Research, v. 98, p. 22 from the middle- to heavy-REE is too to the adjacent ridge lavas or are more 367-22 387. low (Allan et al. 1993). 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Botros, M. and Johnson, H.P., 1988, Tec- along the Endeavour and West Valley The Cobb Offset is, therefore, the tonic evolution of the Explorer- segments do not support a hotspot- southern boundary of a distinct north- Northern Juan de Fuca region from 8 type source for the enriched lavas ern Pacific enriched-mantle province Ma to the present: Journal of Geo- (Karsten et al. 1990; Van Wagoner and that imparts a distinctive geochemical physical Research, v. 93, p. 10 421-10 Leybourne 1991). The shallow depth flavour to seafloor volcanic rocks off 438. of the northern end of the SER may the coast of western Canada. Carbotte, S., 1987, Helium isotopic signa- be consistent with a mantle plume, as ture of Explorer Ridge basalts, north- is a possible gradation from mixed E- ACKNOWLEDGEMENTS east Pacific Ocean: Directed Studies and N-MORB mantle at the shallowest This manuscript presents the work of report, University of California, Santa Barbara, 15 p. depths to primarily N-MORB mantle many seafloor researchers in the NE Carbotte, S.M., Dixon, J.M., Farrar, E., farther south (Michael et al. 1989). No Pacific, and thanks are extended to all Davis, E.E., and Riddihough, R.P., evidence for a thermal anomaly exists of them for collaborations and discus- 1989, Geological and geophysical in the TWVF region, but geophysical sions over the past 28 years. Many characteristics of the Tuzo Wilson characteristics of the Pratt–Welker thanks to Jarda Dostal for convincing Seamounts: Implications for plate 62

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