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A Miocene subcontinental plume in the Pacific Northwest: geochemical evidence
Earth and Planetary Science Letters, 88 (1988) 273-283
273
Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
[2]
A Miocene subcontinental plume in the Pacific Northwest: geochemical evidence A l a n D. B r a n d o n * a n d G o r d o n G. G o l e s Center for Volcanology, University of Oregon, Eugene, OR 97403 (U.S.A.) Received November 20, 1986; revised version received February 22, 1988 Most of the recent discussions on the number and kinds of components which can be distinguished in the Columbia River Basalt Group (CRBG) m a g m a s have used arguments developed from isotopic evidence. In this paper, we consider relative contents of excluded trace elements, interpreted by m e a n s of spidergrams and abundance ratios, and compare data from the C R B G and related lavas with those of selected oceanic basalts. Although m a n y recent models for C R B G petrogeneses do not include a plume component, spidergrams for the American Bar flows of the I n m a h a Basalt of the C R B G have broad h u m p s at Ba, Th, and N b (Ta). This feature suggests that an enriched mantle plume component is present in the American Bar magmas, an inference which is weakly confirmed by 87Sr//86 Sr data. This plume m a y have been identical to that presently beneath Yellowstone. The inferred hot spot track lies almost directly east-west, and requires a vector of motion of the Pacific Northwest part of the North American plate significantly different from that inferred by Minster and Jordan (1978) [31]. Our proposed vector for the North American plate agrees, however, with that suggested by the A n a h i m Belt, a volcanic lineament in southern British Columbia that has been proposed as a hot spot track. This proposed vector requires alterations in current models for Pacific and North American plate movement during the Cenozoic.
1. Introduction The Columbia River Basalt Group (CRBG) of the Pacific Northwest (PNW) has been studied in some detail, and it is now known to be very diverse compositionally [1,2]. If it is assumed that the eruptive locations of CRBG flows record a geographic and temporal arrangement of the source components contributing to their compositional diversity, then compositional signatures of these basalts may reflect lateral variation in the mantle a n d / o r crust sampled by the CRBG. As will be discussed below, isotopic signatures indeed show that CRBG variations can be explained via several crustal and mantle packages which may vary laterally, although the exact number and type remain debatable [1-9]. Prior to this study, incompatible trace element (ITE) signatures of flows of the CRBG have not been much used in these debates. We use ITE patterns of coeval
* Present address: Department of Geological Sciences, University of Washington, Seattle, W A 98195, U.S.A. 0012-821X/88/$03.50
© 1988 Elsevier Science Publishers B.V.
CRBG to pinpoint geographically an ITE-enriched mantle plume which contributed to some of the early C R B G magmatism during mid-Miocene times. The inferred location of this plume-related hotspot may have implications for the Cenozoic motion of the North American plate.
2. Geologic setting Flows of the C R B G are found in the Columbia Plateau Province and in parts of the Blue Mountains, and in geologic provinces west of these areas (see locality map, Fig. 1) [10]. Columbia River Basalt flows outcrop over an area of 160,000 km 2. An additional 40,000 km 2 may be hidden under younger units or may have been eroded during the Plio-Pleistocene and Holocene [10]; Marvin Beeson, personal communication). Stratigraphic successions and radiometric age measurements show that these lavas erupted during Miocene time, between about 17 to 6 Ma ago [11]. Based on field and compositional evidence, the C R B G is divided into several formations, including the Im-
274
t
N
Chief Joseph Dike Swarm
Saddle Mtns. basalt "Main series"
Wanapum basalt I~Grande
Ronde basalt
J
lmnaha basalt aenerally '" " ' overla n by C ~ ' b a s a l t . ~ Slide Creek basalt Bear Creek basalt 0
100
I
I
200 km I
P~cture Gorge basalt
Fig. 1. Generalized geologic map of the Columbia River Basalt Group of eastern Oregon and Washington and western Idaho in the U.S.A. (after Gales [10]). The Main Series and Saddle Mtns. basalts are shown in correct stratigraphic position in the legend. Grande Ronde and Imnaha basalts are contemporaneous with the Slide Creek, Bear Creek, and Picture Gorge basalts. The main feeders of the CRBG, the Chief Joseph and M o n u m e n t dike swarms, are also shown.
naha, Grande Ronde, Wanapum, and Saddle Mountains Basalts of the central plateau [10], and the Picture Gorge Basalts of the Blue Mountains Province, as shown in Fig. 1. Areas in which are found the Slide Creek and Bear Creek Basalts, also of mid-Miocene age, are shown in Fig. 1 as well. The Chief Joseph dike swarm comprises eruptive centers for (in stratigraphic order) Imnaha, Grande Ronde, Wanapum, and Saddle Mountains flows with an irregular northerly and westerly temporal progression within the swarm [1]. Fissures now represented by the Monument dike swarm erupted the Picture Gorge flows. Locations of the eruptive centers for the Bear Creek and Slide Creek flows are unknown at present, although they must be close to or within their respective outcrop areas. Stratigraphic relationships of the Bear Creek flows [12] and the Slide Creek flows [13] indicate that these magmas were erupted in mid-Miocene times and are approximately contemporaneous with Picture Gorge and Imnaha basalts. Thus, these four basaltic units can yield information about source diversity or homogeneity over the wide geographic area in which
C R B G magmas were erupted during the midMiocene. 3. Isotopic and trace element evidence for CRBG source components
3.1. Isotopic constraints The isotopic data on the C R B G flows are extensive [3-9]. The most comprehensive model based on these data is that of Carlson [8]. This model proposes that there are at least three isotopically distinct components for the Main Series of the C R B G (Imnaha, Grande Ronde, and W a n a p u m flows, [1]) and the Picture Gorge magmas (Fig. 2). A depleted mantle source ("C1") with 87Sr/S6Sr=0.7035 and ENd = +6.5 represents a mantle reservoir with time-integrated ITE depletion, probably owing to repetitive extraction of ITE-enriched magmas. Carlson also identified what he interpreted to be a subducted lithospheric component (" C2") w i t h 878r/86 Sr = 0.704 and ENd = +4.5, suggested by a bend in his plot for the isotopic data from the Main Series and the Picture Gorge specimens. The inferred isotopic features of
275
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-12 -16 I
I
0.704
I
I
0.706
I
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0.708
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Fig. 2. CNa versus 87Sr/86Sr diagram for the Columbia River Basalt Group (PG= Picture Gorge, I=Imnaha, GR-W= Grande Ronde-Wanapum) [8,15], and Snake River Plain-Yellowstone National Park tholeiites (SRP-YNP) [35]. Fields for MORB and Hawaii (HI) after Carlson [8], and Iceland (IC) after Norry and Fitton [36], are shown. The individual points shown are C1 (*) and C2 (triangle) [8], samples AB-2 and AB-7 (open squares) [15], and Bulk Silicate Earth (hexagon) [14]. Vectors to components EMII [14] and C4 [8] are also illustrated. C2 best fit a depleted mantle contaminated with oceanic sediment, subducted along with a downgoing slab (presumably, the Juan de Fuca plate). A third component ("C4"), representing a crustal contaminant with 87Sr/86Sr=0.715 and £Nd= --30, was inferred to explain the trend of data points from C2 to beyond the current bulk silicate earth (BSE) values (87Sr/86 Sr = 0.7045-0.7050 and 143Nd/a44 N d = 0.512638 or end = 0 [8,14]). DePaolo [6] first suggested that the concentration of Nd isotopic values for the C R B G flows around end = 0 was evidence for a primitive plume component contributing to these magmas rather than a crustal contaminant. Additional data [8,15] extend the C R B G data slightly beyond BSE values to 87Sr/S6Sr=0.7055 and ENd = --1.0. Mixing with C2 either a plume component with isotopic ratios of close to BSE values (i.e., a component located about where the C R B G data end in Fig. 2) or a small amount of a crustal component similar to Carlson's C4 could produce the trend observed in the Main Series data. However, the existence of a mantle source component which has remained "undifferentiated" through Earth's history, and thus is now represented by BSE isotopic values is controversial [14]. The grouping of hotspot magmas such as Iceland and Hawaii and the most
depleted continental flood basalts on the mantle array between MORB and BSE (Fig. 2), has led Zindler and Hart [14] to propose a prevalent mantle composition (PREMA), which represents a mixture of a depleted MORB source, enriched mantle, and other mantle sources that may exist. The proposed P R E M A isotopic values are: 87Sr/ 86Sr = 0.7035 and 143Nd/144 Nd = 0.5130 or gNd = +7.0, and are very similar to Carlson's C1 depleted mantle component. As will be shown below, ITE trace element patterns of Cl-type CRBG and I c e ! a n d / H a w a i i hotspot magmas are different, suggesting that these two sets of magmas represent mantle sources that must be distinct. However, mixing between a P R E M A component and an enriched component such as C4 or an enriched mantle lithospheric component such as EMII (very large 878r/86 Sr > 0.710 and very small eNa < - 1 2 , [14]) could also produce the trend observed in the C R B G data, and this model may be viable for the compositional types which have ITE patterns and ratios similar to Hawaii and Iceland magmas. It is apparent that Sr and Nd isotopic data alone cannot resolve the problem of which component contributed to which magma or if a crustal component, an enriched mantle component, or a plume component ( P R E M A or BSE) were contributors, because a range of kinds of mixing between these and other sources may produce similar Sr and Nd isotopic results. Other constraints are needed to limit plausible sources for C R B G magmas. 3.2. Trace element constraints Fig. 3a shows spidergram plots (i.e., Wood et al. [16] comparison plots) for selected C R B G flows. Two different patterns are observed. Picture Gorge (average of three Monument Mountain flows [10]), Slide Creek (average of 15 flows [13]), and Bear Creek (one sample representative of the Bear Creek suite [12]) types represented in Fig. 3a show a narrow spike at Ba and a kink at Th. In addition, the Bear Creek sample has a marked negative Nb anomaly. All of these features are characteristic of island-arc tholeiites (see Pearce [17] for examples) and may in part be evidence for a subducted lithosphere component (Carlson's C2). The spike over Ba would be the result of the slab yielding an alkali and alkaline-earth enriched fluid or melt which was partly derived from subducted pelagic
276
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Fig. 3. (a) Selected spidergrams of Columbia River Basalt Group samples. The two inferred American Bar end-members are represented by the solid squares (AB-2) and solid diamonds (AB-7) from Hooper and Goles (unpublished data), and Hooper and Swanson [15], and the range of American Bar types is represented by vertical lines between end-members. The Slide Creek (solid triangles) pattern is the average for the Slide Creek flows [13]. Picture Gorge (open circles) is the average of the Monument Mountain type [10], and the Bear Creek sample (solid circles) is the most primitive sample of the Bear Creek suite [12]. (Ta): Ta is plotted in place of Nb for the Picture Gorge pattern owing to lack of Nb data. (b) Spidergrams of Iceland basalts [26]. Squares = ISL 66, circles = ISL 20, and triangles = ISL 10. Lined pattern is the range of the American Bar type. (c) Spidergrams of Loihi basalts [24]. Squares = 21-2, triangles = 18-8, and circles = 29-10. Lined pattern is the range of the American Bar type.
s e d i m e n t [18-20]. T h i s p r o c e s s yields o n l y a slight e n r i c h m e n t in t h e light r a r e e a r t h a n d l a r g e i o n l i t h o p h i l e e l e m e n t s a n d h e n c e w o u l d also a c c o u n t for the c h a n g e to a s h a l l o w e r s l o p e in t h e spiderg r a m to the r i g h t of the alkalies a n d Ba.
T h e n e g a t i v e N b a n o m a l y in t h e B e a r C r e e k b a s a l t s p i d e r g r a m c a n also b e e x p l a i n e d b y the hydrous nature of fluid or melt derived from a s u b d u c t e d slab. N b a n d o t h e r h i g h - f i e l d s t r e n g t h e l e m e n t s ( H F S E ) s u c h as Ta, H f , a n d Zr, h a v e
277
ionic self-potentials very much larger than those of their neighboring ITE and would be less mobile in a hydrous environment, where they would be left behind during dehydration or melting. This immobihty would yield a melt or fluid depleted in HFSE. The depletion is most prominent in Nb and Ta, which have the largest self-ionic potentials as a result of having the highest valence state of + 5, which results in the greatest relative immobility when water is present. This contrast has been demonstrated experimentally for dehydration of serpentine under the physical conditions present during subduction [19]. Small amounts of crustal contamination may also have contributed to the Picture Gorge, Shde Creek, and Bear Creek magmas. The rock yielding the pattern shown for the Bear Creek sample in Fig. 3a has an M g # of 71 and was the most primitive compositional type in the Bear Creek series. The more evolved basaltic andesites in this magmatic series have the same spidergram pattern, only displaced upwards. Their enrichments over the primitive sample exceed what could have occurred by simple fractional crystallization, and may require addition of an ITE-enriched component being assimilated during shallow level fractionation [21]. Inverse calculations show that this component is most similar in trace and major element compositions to continental greywackes which reside in the shallow crust. A similar component has been identified in the Medicine Lake Highlands volcanic rocks, and it has been proposed that it is a shallow crustal sedimentary signature [22]. It is noteworthy that both subducted lithosphere (C2) and shallow crustal components known to be present in back-arc magmas produce the ITE patterns observed in the Picture Gorge, Slide Creek, and Bear Creek basalts. Also shown in Fig. 3a are spidergrams for two end-member compositions for the American Bar member of the Imnaha Basalt (AB-2 and AB-7; [15] and Hooper and Goles, unpublished data). The Nd and Sr isotopic values of these two flows plot in the Hawaiian field in Fig. 2. All of the other American Bar flows (there are at least 7 compositional types with an undetermined number of individual flows, Hooper et al. [23]) fall within this range of values. These American Bar patterns are different from those of the Picture Gorge type in that they are humped with a peak
over Ba, Th, and Nb and a kink at La and Ce. These magmas must have incorporated a distinct component in addition to those in the Picture Gorge, Slide Creek, and Bear Creek magmas. In order to characterize the nature of the additional component in the American Bar magmas, spidergrams and ITE ratios from basalts with similar source region histories and characteristics can be compared. C R B G basalt spidergrams are compared to those of Icelandic and Hawaiian tholeiites where a well-documented " h o t spot" exists and a mantle plume component is well constrained. Fig. 3b and c show spidergrams for three Icelandic tholeiites from a suite of lavas where it has been suggested that these samples reflect mixing of variable amounts of P-MORB (plume source) and N - M O R B (normal MORB source) [16], and three tholeiites from Loihi seamount [24] (the newest eruptive center along the Hawaiian chain). These Iceland and Loihi samples are ideal in showing the nature of hotspot compositional signatures because their relationship to plumes has been demonstrated from studies of He isotopes [25,26]. The spidergrams look much like those for American Bar flows. Note especially the hump showing an excess of Th and Nb in the Icelandic flows, and the excess of Nb in the Loihi basalts. These features are not seen in the Picture Gorge, Slide Creek, and Bear Creek spidergrams; if this "Nb-rich" component does exist in these magmas, it is masked by the subducted slab and crustal sediment components, and therefore must have been a minor constituent of these magmas. The striking resemblance of the spidergram patterns, the Nb enrichment, and the Sr and N d isotopic values of the American Bar samples to those of Iceland and Hawaii which have a plume component leads us to conclude that a plume component is present in the American Bar flows. An excess or deficiency of Nb is also shown by the ( N b / Y ) n and ( N b / Z r ) n ratios. Table 1 gives these ratios for the Bouvet triplejunction basalts [27], selected C R B G flows, Icelandic basalts, and Loihi basalts plotted in Fig. 3. Le Roex et al. [27] have used these ratios as an indication of a plume component in the Bouvet magmas. Basalts with a large inferred plume component have ( N b / Y ) n and ( N b / Z r ) n larger than 4.5 and 3.1, respectively. Those with a less-prominent plume compo-
278 TABLE 1 (Nb/Y)n and (Nb/Zr)n values for the Bouvet plume [27], CRBG ([10,12,13,15], and Hooper and Goles, unpublished data), Iceland [16], and Loihi [24]. N-type, T-type, and P-type refer to normal type, transitional type, and plume type basalts, respectively [27]. Also shown are Sr isotopic data for CRBG, Bear Creek [12], Picture Gorge [1,8,15], Slide Creek [28], and American Bar [15]. Ta and Yb values replace Nb and Y, respectively, in the Picture Gorge set because these data are not available (Nb/Y)n (Nb/Zr), S7Sr/86Sr Bouvet plume
N-type T-type P-type
0.4-1.9 2.0-4.5 9.5
0.3-1.5 1.7-3.1 4.1
0.31 2.6 6.2 6.9 7.7
0.39 3.2 5.4 4.1
2.8 5.5 7.1
2.8 3.6 3.7
4.8 7.2 6.4
2.3 3.1 2.8
Columbia River basalts
Bear Creek Picture Gorge Slide Creek American Bar (AB-2) American Bar (AB-7)
0.7037-0.7041 0.7035-0.7039 0.7037-0.7041 0.7042 0.7040
Iceland plume
ISL 10 ISL 20 ISL 66 Loihi
29-10 18-8 21-2
nent have characteristically smaller ratios (Table 1). The ratios for the CRBG flows suggest that a plume component was absent in the Picture Gorge and Bear Creek magmas. The values for the American Bar magmas fall within the ranges expected for a plume component, as do the Iceland (ISL-20 and ISL-66) and Loihi samples which have similar spidergram patterns. Note that there are no simple correlations among the various potential indicators of a plume component. For instance, although the ratios (including Sr isotopic ratios) for Slide Creek and American Bar flows in Table 1 are similar, the spidergrams have contrasting shapes (Fig. 3a), Th and Nb are depleted relative to Ba in the Slide Creek flows, and hence their spidergram does not have a hump over Ba, Th, and Nb as observed in American Bar flows. In addition, the more primitive Slide Creek flows show negative anomalies over Ta and Nb (not shown--sample 3-2 of Robyn [13]) similar to that of the Bear Creek sample in
Fig. 3a. These features suggest the absence of a plume component in the Slide Creek magnas. The Sr isotopic ratios for all of the C R B G and related flows are strikingly similar (Table 1), although the Picture Gorge, Slide Creek, and Bear Creek flows show slightly wider ranges and smaller minimum values than do the American Bar flows. This comparison demonstrates the ambiguity which may exist in the isotopic data and ITE ratios (that may reflect variable degrees of differentiation processes), hence the need to use ITE patterns as well as isotopic data to constrain the number and types of source components in these magmas. The presence of a plume component in the American Bar flows appears to be unique among the mid-Miocene flows of C R B G and related basalts. Not only is a plume component lacking in the Picture Gorge, Slide Creek, and Bear Creek flows, it is also not apparent in other mid-Miocene basaltic units of C R B G and related basalts. For instance, flows and dikes of the mid-Miocene Owyhee basalts, which lie just south of the Imnaha region of Fig. 1, have spidergrams with a Ba spike and a negative Ta anomaly much like those of the Bear Creek specimen of Fig. 3a [28], and hence lack any indication of a plume component. Patterns for the Grande Ronde basalts, which comprise 87% of the C R B G [29], show weak Nb and Ta anomalies (fig. 2E of Thompson et al. [30]). If a plume component does exist in the Grande Ronde magmas, it is partially masked by additional source components and cannot be discerned on the basis of trace element patterns. Thus a plume-related hotspot could not have been the major contributor to Grande Ronde volcanism, but possibly may be one factor of several. We therefore conclude that the plume material which contributed to the American Bar magmas was restricted laterally and temporally, and pinpoints a mantle plume lying underneath the Imnaha region (Fig. 1) during the time of their eruption. 4. Plume tracks in the Pacific Northwest 4.1. Tracing a p l u m e track
If a plume did exist underneath the Imnaha region at 17 Ma, its signature should be discernible along its trace earlier (if it existed before 17 Ma) and later than mid-Miocene times. It is known qualitatively that the North American plate has
279
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,
3 '
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Fig. 4. Inferred plume tracks in the Pacific Northwest. The Yellowstone track is defined schematically by Yellowstone (hexagon), Trans-Idaho Cenozoic volcanic belt (vertical lines), Imnaha region (large circle), and the Oregon and Washington Coast range with the location of Cascade Head (square) as an eruptive center in the late Eocene. The age progression observed in the submarine volcanic rocks of the Oregon and Washington Coast Range is plotted (in Ma) to the left [38]. The Roseburg Volcanics fie in the southern end of the Coast Range and have an age of 62 Ma. The Anahim belt is defined by its vent locations (small circles) and age progression shown in Ma [45,46]. The Idaho batholith (stipples) and the Columbia River Basalt province (horizontal fines) are also shown.
moved slightly south of west during the Neogene [31]. Volcanism associated with a plume beneath this part of the North American plate should have a trace generally WSW to ENE. The well-documented Yellowstone hotspot [32,33] is due east of the Imnaha region (Fig. 4). The Snake River Plain (SRP) is in plan view a V-shaped volcanic terrane which links the Columbia River Plateau and its basalts to Yellowstone. An eastward progression of maximum ages in the volcanic units from midto late-Miocene to Recent along the Snake River Plain has been demonstrated [34], suggesting a genetic link among the CRBG, the Snake River Plain volcanic terrane, and the Yellowstone plume-related hotspot. Fig. 5 shows a spidergram plot of an average of 200 analyses of SRP-Yellowstone National Park (YNP) tholeiites [35]. A comparison with the American Bar spidergrams shows that the SRP-YNP tholeiites are strikingly similar (Fig. 5). Both show the characteristic Nb (Ta is plotted in the SRP pattern) enrichment and overall pattern indicative of a plume component. Menzies et al. [35] suggest that the SRP-YNP tholeiites
reflect partial melting of enriched subcontinental lithosphere about 2.5 Ga old, based on Sr and Nd isotopic ratios (Fig. 1) and their similarity to Etype MORB. E-type MORB is enriched in Ta and Nb and represents the plume component in hotspots such as Iceland [36]. The Sr and Nd isotopic ratios for the SRP-YNP samples are much greater and smaller, respectively, than those of Icelandic and Hawaiian hotspot basalts (Fig. 2), and clearly reflect an ancient enriched lithospheric component as at least one source for the magmas represented by these basalts. However, the range of the Sr and N d isotopic data imply that at least two components were involved in the genesis of the SRP tholeiites. Oxygen isotope ratios for the SRP tholeiites show that these basalts have retained mantle signatures and that the enriched chemical signatures cannot plausibly be thought to be crustal in origin [37]. These features suggest that the SRP tholeiites may have resulted from interaction between a hotspot (mantle plume) component with BSE or P R E M A isotopic ratios and HFSE 50
10-
(If OC 3O ~E
0.1
t R'b = T'h t L'a I j~ I H' t I1.' i Iy'b K
Ba
Nb (Ta)
Ce
Zr
Sm
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Fig. 5. Spidergrams for basalts of probable Yellowstone hotspot origin normalized to N-MORB [17]. The triangles are for an average of 200 analyses of Snake River Plain-Yellowstone National Park tholeiites [35]. The circles are for a sample from the late Eocene Cascade Head basalts of the Oregon Coast Range [42], and the diamonds are for sample DA 221a (Pyle and Duncan, unpublished data) of the late Paleocene/early Eocene Roseburg Volcanics of the Oregon Coast Range. The range of the American Bar type is shown as vertical fines. (Ta): Ta is plotted instead of N b for the SRP-YNP and Roseburg spidergrams owing to lack of N b data.
280
enrichment, and an isotopically enriched subcontinental mantle component similar to EMII. Thus, it is possible that there is a genetic link between the American Bar and SRP-Yellowstone magmas, because all seem to have a plume component present. It may also be possible to trace the hotspot to earlier times than the mid-Miocene. It has been suggested that late P a l e o c e n e / e a r l y Eocene seamount volcanic rocks of the Oregon and Washington Coast Range were erupted from the Yellowstone hotspot [38], and that subaerial late Eocene volcanism was produced by this hotspot [39,40]. The Cascade Head alkali basalts (Fig. 4) are late Eocene submarine and subaerial flows which were thought to have erupted in a nonoceanic, within-plate environment, based on geochemical discrimination diagrams [41]. Fig. 5 shows a spidergram representative of the Cascade Head "Class A" alkali basalts [42]. The pattern is remarkably similar to American Bar and SRP patterns, and it has a Nb and Ta enrichment inferred to be of mantle plume origin. This pattern suggests that at least some late Eocene basalts of the Oregon Coast Range were erupted from a mantle plume with compositional characteristics similar to those of the American Bar and Snake River Plain magmas. A representative tholeiite from the late Paleoc e n e / e a r l y Eocene Coast Range volcanic rocks is also plotted in Fig. 5 (Pyle and Duncan, unpublished data). The sample comes from the Roseburg Volcanics (Fig. 4) which have been dated by K-Ar techniques [38]. Ages for the Roseburg Volcanics range from 56 to 65 Ma with a best average of 62 Ma, and they thus represent the oldest volcanic unit known in the Coast Range seamount chain. This tholeiite has a humped pattern characteristic of a plume component, which agrees with earlier conclusions and those of Pyle and Duncan (written communication) that the late Paleocene/early Eocene Coast Range volcanic rocks were associated with a mantle plume. 4.2. Late Cenozoic tectonics in the Pacific Northwest
The above inferences have important implications for late Cenozoic plate motion of North America. About 17 Ma ago a mantle plume may have resided under the southeastern part of the
present Columbia Plateau (Fig. 4). As the North American plate moved in a westerly direction, the surface expression of this plume moved eastwards to what is its present location at Yellowstone. Past models incorporating the Yellowstone plume-related hotspot have proposed two different tracks. One group [31,43] suggests that this hotspot migrated from a WSW direction relative to the overriding North American plate, as recorded by the linear trend of eastern Snake River Plain volcanism (Fig. 4). However, the absolute pole of rotation for the North American plate is poorly constrained [31], and there seems to be little or no evidence in this proposed direction for the track of the Yellowstone hotspot beyond the eastern Snake River Plain. An alternative hypothesis [33,38] has the Yellowstone hotspot track migrating in a due east-west direction. Morgan [33] concluded that several hotspot tracks are associated with flood basalt volcanism, and that a clear age progression in volcanism leads away from many flood basalt provinces (e.g., Deccan and Seral Geral) towards modern-day hotspots. He suggested that the Yellowstone plume may have initiated C R B G volcanism and that the Snake River Plain reflects the migration of the hotspot to its present day location under Yellowstone. Duncan [38] further proposed that late P a l e o c e n e / e a r l y Eocene volcanic rocks in the Coast Range of Oregon and Washington were formed by the Yellowstone plume when it resided under a spreading ridge (between the Kula and Farallon plates) in oceanic crust. Duncan notes that the K-Ar ages of these rocks progress from 49 Ma in the center to 62 Ma in the south and to 57 Ma in the north (Fig. 4). He proposed a model of hotspot volcanism on a spreading ridge much like the situation that exists at Iceland, and which created the submarine Greenland-Iceland-Faeroes volcanic ridge. This seamount chain was later added to the North American plate as a result of accretion during subduction of the ocean plates and clockwise rotation of the seamount package, and was still later uplifted in what is now the Oregon and Washington Coast Range [38]. Late Eocene/Oligocene subaerial volcanism in the Oregon Coast Range may also have been produced by the Yellowstone hotspot [38-40]. These data support an east-west hotspot track, and this model effectively explains
281
the ITE and isotopic signatures used above to identify a plume component. In addition, the plume component in the CRBG is most prominent in one subset of flows of the earliest formation, the Imnaha Basalt, and weak or entirely absent for the rest of the magma types erupted later. This change accords with Morgan's conclusions that a plume-related hotspot may have initiated CRBG volcanism, and suggests that the plate then moved the main eruptive centers away from the plume. The Snake River Plain is a V-shaped feature and hence does not coincide exactly to an east-west hotspot trace. Continental lithosphere is relatively thick and cool, and has complicated structure. Thus, volcanism associated with the Yellowstone plume would occur where it could penetrate the lithosphere and not necessarily in a linear track, as in the instance of a plume beneath oceanic lithosphere. The Idaho batholith (see Fig. 4) may have impeded surficial volcanism along the track of the Yellowstone plume that in reality trended east-west. Instead such volcanism was manifested along the Trans-Idaho Cenozoic volcanic belt (SRP and its margins). Eaton et al. [44] pointed out that the eastern part of this belt lies on an extension of an aeromagnetic anomaly that stretches northeast of Yellowstone across much on Montana, and suggest that the whole anomaly "is related to a fundamental, reactivated Precambrian structure" (p. 795), as illustrated in Fig. 4. Their interpretation makes it unlikely that the eastern Snake River Plain is simply a hotspot track defined by a WSW-ESE trend. Rather, it seems to be a surficial expression of a reactivated zone of weakness, which via extension provided a less-resistant zone of upward movement for the Snake River Plain magmas. It is not plausible to suggest that a hotspot precedes itself by causing large-scale structural modifications several hundred kilometres in front of the plume, and it would be a remarkable coincidence if t h e North American plate vector of motion should coincide exactly with this linear feature deep in the crust. The Anahim Belt of central British Columbia, Canada, has also been proposed as a hot spot track [45-47]. The age progression from 24 Ma in the peralkaline plutonic rocks of the Queen Charlotte Islands, to a Miocene/Pliocene east-west track of peralkaline volcanics, to the latest
volcanism on the eastern end (the Clearwater basalt) is well established [45] and shown in Fig. 4. If the Yellowstone hotspot track also lies approximately east-west, then at least two hotspot tracks lying on the North American do not conform to the Minster and Jordan [31] model which proposes a WSW-ENE track for PNW plumes. This discrepancy suggests that a modification of the vector of motion for this part of the North American plate may be in order, using trends and age progressions for the Anahim trace and the east-west inferred direction of the Yellowstone hotspot track. 5. Conclusions
Mid-Miocene C R B G flows have isotopic and trace element features which indicate a complex petrogenetic history. Samples from the American Bar member of the Imnaha Basalt, CRBG are unique in that they show evidence of an enriched mantle plume component, similar in ITE characteristics to samples from Iceland and Hawaii where chemical features of a plume component are well constrained. The existence of this plume component suggests that a mantle hotspot existed under the Imnaha region 17 Ma ago. Analysis of ITE data from Snake River Plain-Yellowstone National Park tholeiites and Oregon and Washington Coast Range basalts suggest a genetic link between these and the American Bar magmas--all of these samples have similar spidergram patterns reflecting the presence of a plume component. This inference suggests an east-west age progression of plume-related hotspot volcanism from late Eocene to Recent across the North American plate to what is now Yellowstone. In addition, a plume signature is observed in late Paleocene/early Eocene accreted seamount volcanic rocks of the Oregon and Washington Coast Range, which may reflect volcanism induced by the Yellowstone hotspot when it transited oceanic crust. Thus, the record of Yellowstone hotspot activity may be recorded from as early as 62 Ma to Recent in the rocks of the Pacific Northwest. These conclusions do not agree with a WSW vector of motion for the Pacifc Northwest part of the North American plate, but instead accord with models that argue for an east-west motion. The
282
Anahim Belt of central British Columbia has an Early Miocene to Recent age progression in hotspot volcanism that is also east-west, and is thus in agreement with these models.
Acknowledgements The paper benefited greatly from discussions a n d r e v i e w s f r o m R. St J. L a m b e r t , D a n a J o h n ston, and Sara Hoffman. In addition, two anonym o u s reviewers are t h a n k e d for their c o m m e n t s . D o u g P y l e is t h a n k e d f o r u s e o f his u n p u b l i s h e d data.
References 1 P.R. Hooper, Physical and chemical constraints on the evolution of the Columbia River basalt, Geology 12, 495-499, 1984. 2 T. Prestvik and G.G. Goles, Comments on petrogeneses and the tectonic setting of Columbia River basalts, Earth Planet. Sci. Lett. 72, 65-73, 1985. 3 D.O. Nelson, Strontium isotopic and trace element geochemistry of the Saddle Mountains and Grande Ronde basalts of the Columbia River Group, 224 pp., Ph.D. Dissertation, Oregon State University, Corvallis, Oreg., 1980. 4 R.W. Carlson, G.W. Lugmair and J.D. Macdougall, Columbia River volcanism: the question of mantle heterogeneity or crustal contamination, Geochim. Cosmochim. Acta 45, 2483-2499, 1981. 5 D.O. Nelson, Implications of oxygen isotopic data and trace-element modeling for a large scale mixing model for the Columbia River basalt, Geology 11, 248-251, 1983. 6 D.J. DePaolo, Comment on "Columbia River volcanism: the question of mantle heterogeneity or crustal contamination" by R.W. Carlson, G.W. Lugmair and J.D. Macdougall, Geochim. Cosmochim. Acta 47, 841-844, 1983. 7 R.W. Carlson, C.W. Lugmair and J.D. Macdougall, "Columbia River volcanism: the question of mantle heterogeneity or crustal contamination"--reply to a comment by D.J. DePaolo, Geochim. Cosmochim. Acta 47, 845-846. 8 R.W. Carlson, Isotopic constraints on Columbia River basalt genesis and the nature of the subcontinental mantle, Geochim. Cosmochim. Acta 48, 2357-2372, 1984. 9 S.E. Church, Genetic interpretation of lead-isotopic data from the Columbia River Basalt Group, Oregon, Washington, and Idaho, Geol. Soc. Am. Bull. 96, 676-690, 1985. 10 G.G. Goles, Miocene basalts of the Blue Mountains Province in Oregon I. Compositional types and their geological settings, J. Petrol. 27, 495-520, 1986. 11 D.A. Swanson, T.L. Wright, P.R. Hooper and R.D. Bentley, Revisions in stratigraphic nomenclature of the Columbia River Basalt Group, U.S. Geol. Surv. Bull. 1457-G, 1-59, 1979. 12 A.D. Brandon, Geochemical features of the Bear Creek
basalts, Deschutes and Crook Counties, central Oregon, 122 pp., M.S. Thesis, University of Oregon, Eugene, Oreg., 1987. 13 T.L. Robyn, Geology and petrology of the Strawberry Volcanics, northeast Oregon, 197 pp., Ph.D. Dissertation, University of Oregon, Eugene, Oreg., 1977. 14 A. Zindler and S. Hart, Chemical geodynamics, Annu. Rev. Earth Sci. 14, 493-571, 1986. 15 P.R. Hooper and D.A. Swanson, Columbia River Basalt Group and associated volcanic rocks in the Blue Mountains Province, in: G. Walker, ed., U.S. Geol. Surv., Prof. Pap. (in press). 16 D.A. Wood, J.L. Joron, M. Treuil, M. Norry and J. Tarney, Elemental and Sr isotopic variations in basic lavas from Iceland and the surrounding ocean floor, Contrib. Mineral. Petrol. 70, 319-339, 1979. 17 J.A. Pearce, Role of the sub-continental lithosphere in magma genesis at active continental margins, in: Continental Basalts and Mantle Xenoliths, C.J. Hawkesworth and M.J. Norry eds., pp. 230-249, Shiva Publ., Cheshire, 1983. 18 C.J. Hawkesworth, 143Nd/144Nd, 87Sr//86Sr and trace element characteristics of magmas along destructive plate margins, in: Origin or Granitic Batholiths, M.P. Atherton and J. Tarney, eds., pp. 76-89, Shiva Publ., Cheshire, 1979. 19 Y. Tatsumi, D.L. Hamilton and R.W. Nesbitt, Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc magmas: evidence from highpressure experiments and natural rocks, J. Volcanol. Geotherm. Res. 29, 293-309, 1986. 20 M. Sakuyama and R.W. Nesbitt, Geochemistry of the Quaternary volcanic rocks of the northeast Japan arc, J. Volcanol. Geotherm. Res. 29, 413-450, 1986. 21 A.D. Brandon and G.G. Goles, Constraints on magma genesis behind the Neogene Cascade arc: evidence from lavas in the Bear Creek area in central Oregon, EOS 66, 1533, 1987. 22 T.L. Grove, D.C. Gerlach and T.W. Sando, Origin of calc-alkaline series lavas at Medicine Lake volcano by fractionation, assimilation and mixing, Contrib. Mineral. Petrol. 80, 160-182, 1982. 23 P.R. Hooper, W.D. Kleck, C.A. Knowles, S.P. Reidel and R.L. Thiessen, Imnaha basalt, Columbia River Basalt Group, J. Petrol. 25,473-500, 1984. 24 F.A. Frey and D.A. Clague, Geochemistry of diverse basalt types from Loihi seamount, Hawaii: petrogenetic implications, Earth Planet. Sci. Lett. 66, 337-355, 1983. 25 R. Poreda, J.G. Schilling and H. Craig, Helium and hydrogen isotopes in ocean-ridge basalts north and south of Iceland, Earth Planet. Sci. Lett. 78, 1-17, 1986. 26 W. Rison and H. Craig, Helium isotopes and mantle volatiles in Loihi seamount and Hawaiian island basalts and xenoliths, Earth Planet. Sci. Lett. 66, 407-426, 1983. 27 A.P. le Roex, H. Dick, A.M. Reid, F.A. Frey, A.J. Erlank and S.R. Hart, Petrology and geochemistry of basalts from the American-Antarctic ridge, southern ocean: implications for the westward influence of the Bouvet mantle plume, Contrib. Mineral. Petrol. 90, 367-380, 1985. 28 G.G. Goles, A.D. Brandon and R.St J. Lambert, Miocene basalts of the Blue Mountains province in Oregon, II. Trace element and isotopic features of little-known Miocene
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basalts of central and eastern Oregon, Geol. Soc. Am. Special Paper on the IV Columbia River Basalt Symp. (in press). T.L. Tolan, M.H. Beeson, J.L. Anderson and D.A. Swanson, Columbia River Basalt Group (CRBG): new estimates of its extent and volume, EOS 68, 1815, 1987. R.N. Thompson, M.A. Morrison, A.P. Dickin and G.L. Hendry, Continental flood basalts ... arachnids rule OK? in: Continental Basalts and Mantle Xenoliths, C.J. Hawkesworth and M.J. Non-y, eds., pp. 158-185, Shiva Publ., Cheshire, 1983. J.B. Minster and T.H. Jordan, Present day plate motions, J. Geophys. Res. 83, 5331-5354, 1978. H. Craig, J.E. Lupton, J.A. Welhan and R. Poreda, Helium isotope ratios in Yellowstone and Lassen Park volcanic gases, Geophys. Res. Lett. 5, 897-900, 1978. W.J. Morgan, Hotspot tracks and the opening of the Atlantic and Indian Oceans, in: The Sea, Vol. 7. The Oceanic Lithosphere, C. Emiliani, ed., Chapter 13, pp. 443-487, J. Wiley and Sons, New York, N.Y., 1981. R.L. Armstrong, W.P. Leeman and H.E. Malde, K-Ar dating, Quaternary and Neogene volcanic rocks of the Snake River Plain, Idaho, Am. J. Sci. 275, 225-251, 1975. M.A. Menzies, W.P. Leeman and C.J. Hawkesworth, Isotope geochemistry of Cenozoic volcanic rocks reveals mantle heterogeneity below western USA, Nature 303, 205-209, 1983. M.J. Norry and J.G. Fitton, Compositional differences between oceanic and continental basic lavas and their significance, in: Continental Basalts and Mantle Xenoliths, C.J. Hawkesworth and M.J. Norry, eds., pp. 5-19, Shiva Publ., Cheshire, 1983. W.P. Leeman and J.F. Whelan, Oxygen and strontium isotopic studies of basaltic lavas from the Snake River plain, Idaho, U.S. Geol. Surv., Open-File Rep. 1978, 35 pp., 1983.
38 R.A. Duncan, A captured island chain in the Coast Range of Oregon and Washington, J. Geophys. Res. 87, 10827-10837, 1982. 39 K.R. McElwee and R.A. Duncan, Volcanic episodicity and Tertiary absolute motions in the Pacific Northwest, EOS 63, 914, 1982. 40 K.R. McElwee, R.A. Duncan and W.P. Leeman, Petrogenesis of Middle Tertiary forearc volcanic rocks, Oregon and Washington Coast Range, EOS 66, 1112, 1985. 41 M.A.W. Barnes, Alkali basalts of Cascade Head, Oregon: seamounts or transition zone volcanism? in: Mantle Metasomatism and Alkaline Magrnatism, E.M. Morris and J.D. Pasteris, eds., Geol. Soc. Am. Spec. Pap. 215, 374, 1987. 42 M.A.W. Barnes, The geology of Cascade Head, an Eocene volcanic center, 94 pp., M.S. Thesis, University of Oregon, Eugene, Oreg., 1981. 43 J. Suppe, C. Powell and R. Berry, Regional topography, seismicity, Quaternary volcanism, and the present-day tectonics of the western United States, Am. J. Sci. 275-A, 397-436, 1975. 44 G.P. Eaton, R.L. Christiansen, H.M. Iyer, A.M. Pitt, D.R. Mabey, H.R. Blank, Jr., I. Zietz and M.E. Gettings, Magma beneath Yellowstone National Park, Science 188, 787-796, 1975. 45 M.L. Bevier, R.L. Armstrong and J.G. Souther, Miocene peralkaline volcanism in west-central British Columbia--its temporal and plate-tectonics setting, Geology 7, 389-392, 1979. 46 G.C. Rogers and J.G. Souther, Hotspots trace plate movements, GEOS 12, 10-13, 1983. 47 J.G. Souther, The western Anahim Belt: root zone of a peralkaline magma system, Can. J. Earth Sci. 23, 895-908, 1986.
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[2]
A Miocene subcontinental plume in the Pacific Northwest: geochemical evidence A l a n D. B r a n d o n * a n d G o r d o n G. G o l e s Center for Volcanology, University of Oregon, Eugene, OR 97403 (U.S.A.) Received November 20, 1986; revised version received February 22, 1988 Most of the recent discussions on the number and kinds of components which can be distinguished in the Columbia River Basalt Group (CRBG) m a g m a s have used arguments developed from isotopic evidence. In this paper, we consider relative contents of excluded trace elements, interpreted by m e a n s of spidergrams and abundance ratios, and compare data from the C R B G and related lavas with those of selected oceanic basalts. Although m a n y recent models for C R B G petrogeneses do not include a plume component, spidergrams for the American Bar flows of the I n m a h a Basalt of the C R B G have broad h u m p s at Ba, Th, and N b (Ta). This feature suggests that an enriched mantle plume component is present in the American Bar magmas, an inference which is weakly confirmed by 87Sr//86 Sr data. This plume m a y have been identical to that presently beneath Yellowstone. The inferred hot spot track lies almost directly east-west, and requires a vector of motion of the Pacific Northwest part of the North American plate significantly different from that inferred by Minster and Jordan (1978) [31]. Our proposed vector for the North American plate agrees, however, with that suggested by the A n a h i m Belt, a volcanic lineament in southern British Columbia that has been proposed as a hot spot track. This proposed vector requires alterations in current models for Pacific and North American plate movement during the Cenozoic.
1. Introduction The Columbia River Basalt Group (CRBG) of the Pacific Northwest (PNW) has been studied in some detail, and it is now known to be very diverse compositionally [1,2]. If it is assumed that the eruptive locations of CRBG flows record a geographic and temporal arrangement of the source components contributing to their compositional diversity, then compositional signatures of these basalts may reflect lateral variation in the mantle a n d / o r crust sampled by the CRBG. As will be discussed below, isotopic signatures indeed show that CRBG variations can be explained via several crustal and mantle packages which may vary laterally, although the exact number and type remain debatable [1-9]. Prior to this study, incompatible trace element (ITE) signatures of flows of the CRBG have not been much used in these debates. We use ITE patterns of coeval
* Present address: Department of Geological Sciences, University of Washington, Seattle, W A 98195, U.S.A. 0012-821X/88/$03.50
© 1988 Elsevier Science Publishers B.V.
CRBG to pinpoint geographically an ITE-enriched mantle plume which contributed to some of the early C R B G magmatism during mid-Miocene times. The inferred location of this plume-related hotspot may have implications for the Cenozoic motion of the North American plate.
2. Geologic setting Flows of the C R B G are found in the Columbia Plateau Province and in parts of the Blue Mountains, and in geologic provinces west of these areas (see locality map, Fig. 1) [10]. Columbia River Basalt flows outcrop over an area of 160,000 km 2. An additional 40,000 km 2 may be hidden under younger units or may have been eroded during the Plio-Pleistocene and Holocene [10]; Marvin Beeson, personal communication). Stratigraphic successions and radiometric age measurements show that these lavas erupted during Miocene time, between about 17 to 6 Ma ago [11]. Based on field and compositional evidence, the C R B G is divided into several formations, including the Im-
274
t
N
Chief Joseph Dike Swarm
Saddle Mtns. basalt "Main series"
Wanapum basalt I~Grande
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lmnaha basalt aenerally '" " ' overla n by C ~ ' b a s a l t . ~ Slide Creek basalt Bear Creek basalt 0
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Fig. 1. Generalized geologic map of the Columbia River Basalt Group of eastern Oregon and Washington and western Idaho in the U.S.A. (after Gales [10]). The Main Series and Saddle Mtns. basalts are shown in correct stratigraphic position in the legend. Grande Ronde and Imnaha basalts are contemporaneous with the Slide Creek, Bear Creek, and Picture Gorge basalts. The main feeders of the CRBG, the Chief Joseph and M o n u m e n t dike swarms, are also shown.
naha, Grande Ronde, Wanapum, and Saddle Mountains Basalts of the central plateau [10], and the Picture Gorge Basalts of the Blue Mountains Province, as shown in Fig. 1. Areas in which are found the Slide Creek and Bear Creek Basalts, also of mid-Miocene age, are shown in Fig. 1 as well. The Chief Joseph dike swarm comprises eruptive centers for (in stratigraphic order) Imnaha, Grande Ronde, Wanapum, and Saddle Mountains flows with an irregular northerly and westerly temporal progression within the swarm [1]. Fissures now represented by the Monument dike swarm erupted the Picture Gorge flows. Locations of the eruptive centers for the Bear Creek and Slide Creek flows are unknown at present, although they must be close to or within their respective outcrop areas. Stratigraphic relationships of the Bear Creek flows [12] and the Slide Creek flows [13] indicate that these magmas were erupted in mid-Miocene times and are approximately contemporaneous with Picture Gorge and Imnaha basalts. Thus, these four basaltic units can yield information about source diversity or homogeneity over the wide geographic area in which
C R B G magmas were erupted during the midMiocene. 3. Isotopic and trace element evidence for CRBG source components
3.1. Isotopic constraints The isotopic data on the C R B G flows are extensive [3-9]. The most comprehensive model based on these data is that of Carlson [8]. This model proposes that there are at least three isotopically distinct components for the Main Series of the C R B G (Imnaha, Grande Ronde, and W a n a p u m flows, [1]) and the Picture Gorge magmas (Fig. 2). A depleted mantle source ("C1") with 87Sr/S6Sr=0.7035 and ENd = +6.5 represents a mantle reservoir with time-integrated ITE depletion, probably owing to repetitive extraction of ITE-enriched magmas. Carlson also identified what he interpreted to be a subducted lithospheric component (" C2") w i t h 878r/86 Sr = 0.704 and ENd = +4.5, suggested by a bend in his plot for the isotopic data from the Main Series and the Picture Gorge specimens. The inferred isotopic features of
275
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I
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Fig. 2. CNa versus 87Sr/86Sr diagram for the Columbia River Basalt Group (PG= Picture Gorge, I=Imnaha, GR-W= Grande Ronde-Wanapum) [8,15], and Snake River Plain-Yellowstone National Park tholeiites (SRP-YNP) [35]. Fields for MORB and Hawaii (HI) after Carlson [8], and Iceland (IC) after Norry and Fitton [36], are shown. The individual points shown are C1 (*) and C2 (triangle) [8], samples AB-2 and AB-7 (open squares) [15], and Bulk Silicate Earth (hexagon) [14]. Vectors to components EMII [14] and C4 [8] are also illustrated. C2 best fit a depleted mantle contaminated with oceanic sediment, subducted along with a downgoing slab (presumably, the Juan de Fuca plate). A third component ("C4"), representing a crustal contaminant with 87Sr/86Sr=0.715 and £Nd= --30, was inferred to explain the trend of data points from C2 to beyond the current bulk silicate earth (BSE) values (87Sr/86 Sr = 0.7045-0.7050 and 143Nd/a44 N d = 0.512638 or end = 0 [8,14]). DePaolo [6] first suggested that the concentration of Nd isotopic values for the C R B G flows around end = 0 was evidence for a primitive plume component contributing to these magmas rather than a crustal contaminant. Additional data [8,15] extend the C R B G data slightly beyond BSE values to 87Sr/S6Sr=0.7055 and ENd = --1.0. Mixing with C2 either a plume component with isotopic ratios of close to BSE values (i.e., a component located about where the C R B G data end in Fig. 2) or a small amount of a crustal component similar to Carlson's C4 could produce the trend observed in the Main Series data. However, the existence of a mantle source component which has remained "undifferentiated" through Earth's history, and thus is now represented by BSE isotopic values is controversial [14]. The grouping of hotspot magmas such as Iceland and Hawaii and the most
depleted continental flood basalts on the mantle array between MORB and BSE (Fig. 2), has led Zindler and Hart [14] to propose a prevalent mantle composition (PREMA), which represents a mixture of a depleted MORB source, enriched mantle, and other mantle sources that may exist. The proposed P R E M A isotopic values are: 87Sr/ 86Sr = 0.7035 and 143Nd/144 Nd = 0.5130 or gNd = +7.0, and are very similar to Carlson's C1 depleted mantle component. As will be shown below, ITE trace element patterns of Cl-type CRBG and I c e ! a n d / H a w a i i hotspot magmas are different, suggesting that these two sets of magmas represent mantle sources that must be distinct. However, mixing between a P R E M A component and an enriched component such as C4 or an enriched mantle lithospheric component such as EMII (very large 878r/86 Sr > 0.710 and very small eNa < - 1 2 , [14]) could also produce the trend observed in the C R B G data, and this model may be viable for the compositional types which have ITE patterns and ratios similar to Hawaii and Iceland magmas. It is apparent that Sr and Nd isotopic data alone cannot resolve the problem of which component contributed to which magma or if a crustal component, an enriched mantle component, or a plume component ( P R E M A or BSE) were contributors, because a range of kinds of mixing between these and other sources may produce similar Sr and Nd isotopic results. Other constraints are needed to limit plausible sources for C R B G magmas. 3.2. Trace element constraints Fig. 3a shows spidergram plots (i.e., Wood et al. [16] comparison plots) for selected C R B G flows. Two different patterns are observed. Picture Gorge (average of three Monument Mountain flows [10]), Slide Creek (average of 15 flows [13]), and Bear Creek (one sample representative of the Bear Creek suite [12]) types represented in Fig. 3a show a narrow spike at Ba and a kink at Th. In addition, the Bear Creek sample has a marked negative Nb anomaly. All of these features are characteristic of island-arc tholeiites (see Pearce [17] for examples) and may in part be evidence for a subducted lithosphere component (Carlson's C2). The spike over Ba would be the result of the slab yielding an alkali and alkaline-earth enriched fluid or melt which was partly derived from subducted pelagic
276
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Fig. 3. (a) Selected spidergrams of Columbia River Basalt Group samples. The two inferred American Bar end-members are represented by the solid squares (AB-2) and solid diamonds (AB-7) from Hooper and Goles (unpublished data), and Hooper and Swanson [15], and the range of American Bar types is represented by vertical lines between end-members. The Slide Creek (solid triangles) pattern is the average for the Slide Creek flows [13]. Picture Gorge (open circles) is the average of the Monument Mountain type [10], and the Bear Creek sample (solid circles) is the most primitive sample of the Bear Creek suite [12]. (Ta): Ta is plotted in place of Nb for the Picture Gorge pattern owing to lack of Nb data. (b) Spidergrams of Iceland basalts [26]. Squares = ISL 66, circles = ISL 20, and triangles = ISL 10. Lined pattern is the range of the American Bar type. (c) Spidergrams of Loihi basalts [24]. Squares = 21-2, triangles = 18-8, and circles = 29-10. Lined pattern is the range of the American Bar type.
s e d i m e n t [18-20]. T h i s p r o c e s s yields o n l y a slight e n r i c h m e n t in t h e light r a r e e a r t h a n d l a r g e i o n l i t h o p h i l e e l e m e n t s a n d h e n c e w o u l d also a c c o u n t for the c h a n g e to a s h a l l o w e r s l o p e in t h e spiderg r a m to the r i g h t of the alkalies a n d Ba.
T h e n e g a t i v e N b a n o m a l y in t h e B e a r C r e e k b a s a l t s p i d e r g r a m c a n also b e e x p l a i n e d b y the hydrous nature of fluid or melt derived from a s u b d u c t e d slab. N b a n d o t h e r h i g h - f i e l d s t r e n g t h e l e m e n t s ( H F S E ) s u c h as Ta, H f , a n d Zr, h a v e
277
ionic self-potentials very much larger than those of their neighboring ITE and would be less mobile in a hydrous environment, where they would be left behind during dehydration or melting. This immobihty would yield a melt or fluid depleted in HFSE. The depletion is most prominent in Nb and Ta, which have the largest self-ionic potentials as a result of having the highest valence state of + 5, which results in the greatest relative immobility when water is present. This contrast has been demonstrated experimentally for dehydration of serpentine under the physical conditions present during subduction [19]. Small amounts of crustal contamination may also have contributed to the Picture Gorge, Shde Creek, and Bear Creek magmas. The rock yielding the pattern shown for the Bear Creek sample in Fig. 3a has an M g # of 71 and was the most primitive compositional type in the Bear Creek series. The more evolved basaltic andesites in this magmatic series have the same spidergram pattern, only displaced upwards. Their enrichments over the primitive sample exceed what could have occurred by simple fractional crystallization, and may require addition of an ITE-enriched component being assimilated during shallow level fractionation [21]. Inverse calculations show that this component is most similar in trace and major element compositions to continental greywackes which reside in the shallow crust. A similar component has been identified in the Medicine Lake Highlands volcanic rocks, and it has been proposed that it is a shallow crustal sedimentary signature [22]. It is noteworthy that both subducted lithosphere (C2) and shallow crustal components known to be present in back-arc magmas produce the ITE patterns observed in the Picture Gorge, Slide Creek, and Bear Creek basalts. Also shown in Fig. 3a are spidergrams for two end-member compositions for the American Bar member of the Imnaha Basalt (AB-2 and AB-7; [15] and Hooper and Goles, unpublished data). The Nd and Sr isotopic values of these two flows plot in the Hawaiian field in Fig. 2. All of the other American Bar flows (there are at least 7 compositional types with an undetermined number of individual flows, Hooper et al. [23]) fall within this range of values. These American Bar patterns are different from those of the Picture Gorge type in that they are humped with a peak
over Ba, Th, and Nb and a kink at La and Ce. These magmas must have incorporated a distinct component in addition to those in the Picture Gorge, Slide Creek, and Bear Creek magmas. In order to characterize the nature of the additional component in the American Bar magmas, spidergrams and ITE ratios from basalts with similar source region histories and characteristics can be compared. C R B G basalt spidergrams are compared to those of Icelandic and Hawaiian tholeiites where a well-documented " h o t spot" exists and a mantle plume component is well constrained. Fig. 3b and c show spidergrams for three Icelandic tholeiites from a suite of lavas where it has been suggested that these samples reflect mixing of variable amounts of P-MORB (plume source) and N - M O R B (normal MORB source) [16], and three tholeiites from Loihi seamount [24] (the newest eruptive center along the Hawaiian chain). These Iceland and Loihi samples are ideal in showing the nature of hotspot compositional signatures because their relationship to plumes has been demonstrated from studies of He isotopes [25,26]. The spidergrams look much like those for American Bar flows. Note especially the hump showing an excess of Th and Nb in the Icelandic flows, and the excess of Nb in the Loihi basalts. These features are not seen in the Picture Gorge, Slide Creek, and Bear Creek spidergrams; if this "Nb-rich" component does exist in these magmas, it is masked by the subducted slab and crustal sediment components, and therefore must have been a minor constituent of these magmas. The striking resemblance of the spidergram patterns, the Nb enrichment, and the Sr and N d isotopic values of the American Bar samples to those of Iceland and Hawaii which have a plume component leads us to conclude that a plume component is present in the American Bar flows. An excess or deficiency of Nb is also shown by the ( N b / Y ) n and ( N b / Z r ) n ratios. Table 1 gives these ratios for the Bouvet triplejunction basalts [27], selected C R B G flows, Icelandic basalts, and Loihi basalts plotted in Fig. 3. Le Roex et al. [27] have used these ratios as an indication of a plume component in the Bouvet magmas. Basalts with a large inferred plume component have ( N b / Y ) n and ( N b / Z r ) n larger than 4.5 and 3.1, respectively. Those with a less-prominent plume compo-
278 TABLE 1 (Nb/Y)n and (Nb/Zr)n values for the Bouvet plume [27], CRBG ([10,12,13,15], and Hooper and Goles, unpublished data), Iceland [16], and Loihi [24]. N-type, T-type, and P-type refer to normal type, transitional type, and plume type basalts, respectively [27]. Also shown are Sr isotopic data for CRBG, Bear Creek [12], Picture Gorge [1,8,15], Slide Creek [28], and American Bar [15]. Ta and Yb values replace Nb and Y, respectively, in the Picture Gorge set because these data are not available (Nb/Y)n (Nb/Zr), S7Sr/86Sr Bouvet plume
N-type T-type P-type
0.4-1.9 2.0-4.5 9.5
0.3-1.5 1.7-3.1 4.1
0.31 2.6 6.2 6.9 7.7
0.39 3.2 5.4 4.1
2.8 5.5 7.1
2.8 3.6 3.7
4.8 7.2 6.4
2.3 3.1 2.8
Columbia River basalts
Bear Creek Picture Gorge Slide Creek American Bar (AB-2) American Bar (AB-7)
0.7037-0.7041 0.7035-0.7039 0.7037-0.7041 0.7042 0.7040
Iceland plume
ISL 10 ISL 20 ISL 66 Loihi
29-10 18-8 21-2
nent have characteristically smaller ratios (Table 1). The ratios for the CRBG flows suggest that a plume component was absent in the Picture Gorge and Bear Creek magmas. The values for the American Bar magmas fall within the ranges expected for a plume component, as do the Iceland (ISL-20 and ISL-66) and Loihi samples which have similar spidergram patterns. Note that there are no simple correlations among the various potential indicators of a plume component. For instance, although the ratios (including Sr isotopic ratios) for Slide Creek and American Bar flows in Table 1 are similar, the spidergrams have contrasting shapes (Fig. 3a), Th and Nb are depleted relative to Ba in the Slide Creek flows, and hence their spidergram does not have a hump over Ba, Th, and Nb as observed in American Bar flows. In addition, the more primitive Slide Creek flows show negative anomalies over Ta and Nb (not shown--sample 3-2 of Robyn [13]) similar to that of the Bear Creek sample in
Fig. 3a. These features suggest the absence of a plume component in the Slide Creek magnas. The Sr isotopic ratios for all of the C R B G and related flows are strikingly similar (Table 1), although the Picture Gorge, Slide Creek, and Bear Creek flows show slightly wider ranges and smaller minimum values than do the American Bar flows. This comparison demonstrates the ambiguity which may exist in the isotopic data and ITE ratios (that may reflect variable degrees of differentiation processes), hence the need to use ITE patterns as well as isotopic data to constrain the number and types of source components in these magmas. The presence of a plume component in the American Bar flows appears to be unique among the mid-Miocene flows of C R B G and related basalts. Not only is a plume component lacking in the Picture Gorge, Slide Creek, and Bear Creek flows, it is also not apparent in other mid-Miocene basaltic units of C R B G and related basalts. For instance, flows and dikes of the mid-Miocene Owyhee basalts, which lie just south of the Imnaha region of Fig. 1, have spidergrams with a Ba spike and a negative Ta anomaly much like those of the Bear Creek specimen of Fig. 3a [28], and hence lack any indication of a plume component. Patterns for the Grande Ronde basalts, which comprise 87% of the C R B G [29], show weak Nb and Ta anomalies (fig. 2E of Thompson et al. [30]). If a plume component does exist in the Grande Ronde magmas, it is partially masked by additional source components and cannot be discerned on the basis of trace element patterns. Thus a plume-related hotspot could not have been the major contributor to Grande Ronde volcanism, but possibly may be one factor of several. We therefore conclude that the plume material which contributed to the American Bar magmas was restricted laterally and temporally, and pinpoints a mantle plume lying underneath the Imnaha region (Fig. 1) during the time of their eruption. 4. Plume tracks in the Pacific Northwest 4.1. Tracing a p l u m e track
If a plume did exist underneath the Imnaha region at 17 Ma, its signature should be discernible along its trace earlier (if it existed before 17 Ma) and later than mid-Miocene times. It is known qualitatively that the North American plate has
279
a~..~87 .32 / k
IJ o N
\
KM
-I 5oo ,
,
3 '
r I
"2"
6
Fig. 4. Inferred plume tracks in the Pacific Northwest. The Yellowstone track is defined schematically by Yellowstone (hexagon), Trans-Idaho Cenozoic volcanic belt (vertical lines), Imnaha region (large circle), and the Oregon and Washington Coast range with the location of Cascade Head (square) as an eruptive center in the late Eocene. The age progression observed in the submarine volcanic rocks of the Oregon and Washington Coast Range is plotted (in Ma) to the left [38]. The Roseburg Volcanics fie in the southern end of the Coast Range and have an age of 62 Ma. The Anahim belt is defined by its vent locations (small circles) and age progression shown in Ma [45,46]. The Idaho batholith (stipples) and the Columbia River Basalt province (horizontal fines) are also shown.
moved slightly south of west during the Neogene [31]. Volcanism associated with a plume beneath this part of the North American plate should have a trace generally WSW to ENE. The well-documented Yellowstone hotspot [32,33] is due east of the Imnaha region (Fig. 4). The Snake River Plain (SRP) is in plan view a V-shaped volcanic terrane which links the Columbia River Plateau and its basalts to Yellowstone. An eastward progression of maximum ages in the volcanic units from midto late-Miocene to Recent along the Snake River Plain has been demonstrated [34], suggesting a genetic link among the CRBG, the Snake River Plain volcanic terrane, and the Yellowstone plume-related hotspot. Fig. 5 shows a spidergram plot of an average of 200 analyses of SRP-Yellowstone National Park (YNP) tholeiites [35]. A comparison with the American Bar spidergrams shows that the SRP-YNP tholeiites are strikingly similar (Fig. 5). Both show the characteristic Nb (Ta is plotted in the SRP pattern) enrichment and overall pattern indicative of a plume component. Menzies et al. [35] suggest that the SRP-YNP tholeiites
reflect partial melting of enriched subcontinental lithosphere about 2.5 Ga old, based on Sr and Nd isotopic ratios (Fig. 1) and their similarity to Etype MORB. E-type MORB is enriched in Ta and Nb and represents the plume component in hotspots such as Iceland [36]. The Sr and Nd isotopic ratios for the SRP-YNP samples are much greater and smaller, respectively, than those of Icelandic and Hawaiian hotspot basalts (Fig. 2), and clearly reflect an ancient enriched lithospheric component as at least one source for the magmas represented by these basalts. However, the range of the Sr and N d isotopic data imply that at least two components were involved in the genesis of the SRP tholeiites. Oxygen isotope ratios for the SRP tholeiites show that these basalts have retained mantle signatures and that the enriched chemical signatures cannot plausibly be thought to be crustal in origin [37]. These features suggest that the SRP tholeiites may have resulted from interaction between a hotspot (mantle plume) component with BSE or P R E M A isotopic ratios and HFSE 50
10-
(If OC 3O ~E
0.1
t R'b = T'h t L'a I j~ I H' t I1.' i Iy'b K
Ba
Nb (Ta)
Ce
Zr
Sm
Y
1 Lu
Fig. 5. Spidergrams for basalts of probable Yellowstone hotspot origin normalized to N-MORB [17]. The triangles are for an average of 200 analyses of Snake River Plain-Yellowstone National Park tholeiites [35]. The circles are for a sample from the late Eocene Cascade Head basalts of the Oregon Coast Range [42], and the diamonds are for sample DA 221a (Pyle and Duncan, unpublished data) of the late Paleocene/early Eocene Roseburg Volcanics of the Oregon Coast Range. The range of the American Bar type is shown as vertical fines. (Ta): Ta is plotted instead of N b for the SRP-YNP and Roseburg spidergrams owing to lack of N b data.
280
enrichment, and an isotopically enriched subcontinental mantle component similar to EMII. Thus, it is possible that there is a genetic link between the American Bar and SRP-Yellowstone magmas, because all seem to have a plume component present. It may also be possible to trace the hotspot to earlier times than the mid-Miocene. It has been suggested that late P a l e o c e n e / e a r l y Eocene seamount volcanic rocks of the Oregon and Washington Coast Range were erupted from the Yellowstone hotspot [38], and that subaerial late Eocene volcanism was produced by this hotspot [39,40]. The Cascade Head alkali basalts (Fig. 4) are late Eocene submarine and subaerial flows which were thought to have erupted in a nonoceanic, within-plate environment, based on geochemical discrimination diagrams [41]. Fig. 5 shows a spidergram representative of the Cascade Head "Class A" alkali basalts [42]. The pattern is remarkably similar to American Bar and SRP patterns, and it has a Nb and Ta enrichment inferred to be of mantle plume origin. This pattern suggests that at least some late Eocene basalts of the Oregon Coast Range were erupted from a mantle plume with compositional characteristics similar to those of the American Bar and Snake River Plain magmas. A representative tholeiite from the late Paleoc e n e / e a r l y Eocene Coast Range volcanic rocks is also plotted in Fig. 5 (Pyle and Duncan, unpublished data). The sample comes from the Roseburg Volcanics (Fig. 4) which have been dated by K-Ar techniques [38]. Ages for the Roseburg Volcanics range from 56 to 65 Ma with a best average of 62 Ma, and they thus represent the oldest volcanic unit known in the Coast Range seamount chain. This tholeiite has a humped pattern characteristic of a plume component, which agrees with earlier conclusions and those of Pyle and Duncan (written communication) that the late Paleocene/early Eocene Coast Range volcanic rocks were associated with a mantle plume. 4.2. Late Cenozoic tectonics in the Pacific Northwest
The above inferences have important implications for late Cenozoic plate motion of North America. About 17 Ma ago a mantle plume may have resided under the southeastern part of the
present Columbia Plateau (Fig. 4). As the North American plate moved in a westerly direction, the surface expression of this plume moved eastwards to what is its present location at Yellowstone. Past models incorporating the Yellowstone plume-related hotspot have proposed two different tracks. One group [31,43] suggests that this hotspot migrated from a WSW direction relative to the overriding North American plate, as recorded by the linear trend of eastern Snake River Plain volcanism (Fig. 4). However, the absolute pole of rotation for the North American plate is poorly constrained [31], and there seems to be little or no evidence in this proposed direction for the track of the Yellowstone hotspot beyond the eastern Snake River Plain. An alternative hypothesis [33,38] has the Yellowstone hotspot track migrating in a due east-west direction. Morgan [33] concluded that several hotspot tracks are associated with flood basalt volcanism, and that a clear age progression in volcanism leads away from many flood basalt provinces (e.g., Deccan and Seral Geral) towards modern-day hotspots. He suggested that the Yellowstone plume may have initiated C R B G volcanism and that the Snake River Plain reflects the migration of the hotspot to its present day location under Yellowstone. Duncan [38] further proposed that late P a l e o c e n e / e a r l y Eocene volcanic rocks in the Coast Range of Oregon and Washington were formed by the Yellowstone plume when it resided under a spreading ridge (between the Kula and Farallon plates) in oceanic crust. Duncan notes that the K-Ar ages of these rocks progress from 49 Ma in the center to 62 Ma in the south and to 57 Ma in the north (Fig. 4). He proposed a model of hotspot volcanism on a spreading ridge much like the situation that exists at Iceland, and which created the submarine Greenland-Iceland-Faeroes volcanic ridge. This seamount chain was later added to the North American plate as a result of accretion during subduction of the ocean plates and clockwise rotation of the seamount package, and was still later uplifted in what is now the Oregon and Washington Coast Range [38]. Late Eocene/Oligocene subaerial volcanism in the Oregon Coast Range may also have been produced by the Yellowstone hotspot [38-40]. These data support an east-west hotspot track, and this model effectively explains
281
the ITE and isotopic signatures used above to identify a plume component. In addition, the plume component in the CRBG is most prominent in one subset of flows of the earliest formation, the Imnaha Basalt, and weak or entirely absent for the rest of the magma types erupted later. This change accords with Morgan's conclusions that a plume-related hotspot may have initiated CRBG volcanism, and suggests that the plate then moved the main eruptive centers away from the plume. The Snake River Plain is a V-shaped feature and hence does not coincide exactly to an east-west hotspot trace. Continental lithosphere is relatively thick and cool, and has complicated structure. Thus, volcanism associated with the Yellowstone plume would occur where it could penetrate the lithosphere and not necessarily in a linear track, as in the instance of a plume beneath oceanic lithosphere. The Idaho batholith (see Fig. 4) may have impeded surficial volcanism along the track of the Yellowstone plume that in reality trended east-west. Instead such volcanism was manifested along the Trans-Idaho Cenozoic volcanic belt (SRP and its margins). Eaton et al. [44] pointed out that the eastern part of this belt lies on an extension of an aeromagnetic anomaly that stretches northeast of Yellowstone across much on Montana, and suggest that the whole anomaly "is related to a fundamental, reactivated Precambrian structure" (p. 795), as illustrated in Fig. 4. Their interpretation makes it unlikely that the eastern Snake River Plain is simply a hotspot track defined by a WSW-ESE trend. Rather, it seems to be a surficial expression of a reactivated zone of weakness, which via extension provided a less-resistant zone of upward movement for the Snake River Plain magmas. It is not plausible to suggest that a hotspot precedes itself by causing large-scale structural modifications several hundred kilometres in front of the plume, and it would be a remarkable coincidence if t h e North American plate vector of motion should coincide exactly with this linear feature deep in the crust. The Anahim Belt of central British Columbia, Canada, has also been proposed as a hot spot track [45-47]. The age progression from 24 Ma in the peralkaline plutonic rocks of the Queen Charlotte Islands, to a Miocene/Pliocene east-west track of peralkaline volcanics, to the latest
volcanism on the eastern end (the Clearwater basalt) is well established [45] and shown in Fig. 4. If the Yellowstone hotspot track also lies approximately east-west, then at least two hotspot tracks lying on the North American do not conform to the Minster and Jordan [31] model which proposes a WSW-ENE track for PNW plumes. This discrepancy suggests that a modification of the vector of motion for this part of the North American plate may be in order, using trends and age progressions for the Anahim trace and the east-west inferred direction of the Yellowstone hotspot track. 5. Conclusions
Mid-Miocene C R B G flows have isotopic and trace element features which indicate a complex petrogenetic history. Samples from the American Bar member of the Imnaha Basalt, CRBG are unique in that they show evidence of an enriched mantle plume component, similar in ITE characteristics to samples from Iceland and Hawaii where chemical features of a plume component are well constrained. The existence of this plume component suggests that a mantle hotspot existed under the Imnaha region 17 Ma ago. Analysis of ITE data from Snake River Plain-Yellowstone National Park tholeiites and Oregon and Washington Coast Range basalts suggest a genetic link between these and the American Bar magmas--all of these samples have similar spidergram patterns reflecting the presence of a plume component. This inference suggests an east-west age progression of plume-related hotspot volcanism from late Eocene to Recent across the North American plate to what is now Yellowstone. In addition, a plume signature is observed in late Paleocene/early Eocene accreted seamount volcanic rocks of the Oregon and Washington Coast Range, which may reflect volcanism induced by the Yellowstone hotspot when it transited oceanic crust. Thus, the record of Yellowstone hotspot activity may be recorded from as early as 62 Ma to Recent in the rocks of the Pacific Northwest. These conclusions do not agree with a WSW vector of motion for the Pacifc Northwest part of the North American plate, but instead accord with models that argue for an east-west motion. The
282
Anahim Belt of central British Columbia has an Early Miocene to Recent age progression in hotspot volcanism that is also east-west, and is thus in agreement with these models.
Acknowledgements The paper benefited greatly from discussions a n d r e v i e w s f r o m R. St J. L a m b e r t , D a n a J o h n ston, and Sara Hoffman. In addition, two anonym o u s reviewers are t h a n k e d for their c o m m e n t s . D o u g P y l e is t h a n k e d f o r u s e o f his u n p u b l i s h e d data.
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