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Arc-transform magmatism in the Wrangell volcanic belt. Thomas Skulski, Don Francis. Department ot Geological Sciences, McGill University, Montreal, Quebec ...

Arc-transform magmatism in the Wrangell volcanic belt Thomas Skulski, Don Francis Department ot Geological Sciences, McGill University, Montreal, Quebec H3A 2A7, Canada John Ludden Département de Géologie, Université de Montréal, Montréal, Québec H3C 3J7, Canada ABSTRACT The late Cenozoic Wrangell volcanic belt records a transition in magma supply and geochemistry from a subduction to transform margin between the northeastern Pacific and North American plates. The northwestern volcanic belt comprises talc-alkaline lavas that are above the Wrangell-Wadati Benioff zone, whereas the southeastern belt comprises transitional lavas with minor alkaline and calc-alkaline lavas that overlie a leaky transform fault. The subduction-transform transition is marked by an increase in F e / M g and N b / L a ratios and a decrease in B a / L a ratios. Thus, the lavas of the transform regime display a geochemical signature that is intermediate to that of calcalkaline and intraplate alkaline lavas. Whereas the effects of crustal contamination can be recognized in the evolved lavas of all suites, all primitive lavas in the transform regime are mantle derived and reflect the variable melting and mixing of depleted (mid-ocean ridge basalt) upper mantle, enriched mantle (ocean island basalt), and slab-derived components. INTRODUCTION The chemical variations in volcanic arcs are well described; however, their lateral terminations are less frequently examined. The edges of volcanic arcs are an ideal location to study the source of sub-arc magmas in the presence of a diminished slab-derived component. The North American continental margin along the Gulf of Alaska changes from a subduction zone in Alaska to a transform fault along the coast of British Columbia. In this paper we document the nature of the chemical and isotopic compositional variations in upper Cenozoic Wrangell Lava across

this tectonic transition. The Wrangell Lava represents unusual magmas sharing both calc-alkaline and intraplate alkaline geochemical affinities. The chemical and isotopic differences of primitive lavas erupted across the transform-arc transition cannot be attributed to crustal contamination, but must reflect an increased input of slab-derived components and a relative decrease in both enriched (ocean island basalt [OIB]) and depleted (midocean ridge basalt [MORB]) mantle components toward the northwest. TECTONIC SETTING O F T H E W R A N G E L L VOLCANIC BELT The Wrangell volcanic belt extends 500 km from northwestern British Columbia, through the southwestern Yukon, into southern Alaska (Fig. 1). Southeast of the Yukon-Alaska border, the volcanic belt comprises Miocene subaerial volcanic outliers that overlie the Duke River fault. This dextral strike-slip fault is a splay of the Queen Charlotte-Denali transform fault system that juxtaposes late Paleozoic-age rocks of Wrangellia and Paleozoic rocks of the Alexander terrane. Northwest of the Yukon-Alaska border the Wrangell volcanic belt comprises an Oligocene to Holocene volcanic sequence overlying the Wrangell-Wadati Benioff zone (Richter and Smith, 1976; Richter et al, 1979; Nye, 1983; Lowe et al., 1982; Richter et al., 1989; Page et al., 1989; Miller and Richter, 1991). The near-perfect small-circle geometry of the Denali fault segment in the southwestern Yukon (Stout and Chase, 1980) indicates that rightlateral slip along this structure can be interpreted in terms of rigid-plate tectonics. As the Duke River fault deviates northwestward away from the Denali fault, the overlying Wrangell Lava shows an increase in volume, age, and duration of volcanic activity, which suggests a genetic link be-

Figure 1. Wrangell volcanic belt (ruled pattern) and H y.I tectono-stratigraphic terWRANGELL J) PENINSULAR / te Mg ranes of northern Gulf of 0 4 Ma /( " , i ? WRANGELLIA ft Mg Mwes Alaska (simplified from —62 TERRANE Plafker, 1987). Triangles indicate Wrangell eruptive centers (data on Wrangell field from Miller and Richter, 1991; Alsek from CHUGACH TERRANE ALSEK Souther and Stanciu, 108 13 5 Ma fp M o 1975; other fields from this study). Faults: AM = Aleutian megathrust, BRFS = I Si Border Ranges fault sysYUKON _ tem, CFS = Contact fault system, CSFS = ChugachSt. Elias fault system, DFS YAKUTAT = Denali fault system, DRF = Duke River fault, DRZ = 6 [STAHLE j. > " PRINCE WILLIAM J Dangerous River zone, TERRANE T r KIZ = Kayak Island zone, y G U L F O F ALASKA rp.'Mg TERRANE OCF = Queen Charlotte f. J cm y fault, RF = Resurrection fault, TF = Totschunda fault, TFS = Transition fault PACIFIC PLATE system. Alkaline (A), transitional (T), and calc-alkaline (C) Wrangell Lavas are shown in cation per150 km cent on Fe/ Mg vs. Si variation diagram, where total Fe is Fe2+. Compilation of geochemical data from Wrangell volcanic belt: field data from Lerbekmo and Campbell (1969); Richter and Smith (1976); Richter et al. (1979, 1989); Loweet al (1982); Nye (1983); other data from this study. Compilation of ages of Wrangell Lava includes K-Ar ages (Miller and Richter, 1991; Richter et al., 1990; Dodds and Campbell, 1988; R. L. Armstrong, 1988, written commun.) and 39Ar/40Ar data (T. Skulski, unpublished).


GEOLOGY, v. 19, p. lt-14, January 1991


tween volcanism and extension along this fault. The orientation of the Duke River fault relative to the Denali fault (Fig. 1) results in extension that is reflected in the northerly strike of mafic dikes with steep dips (>75°) in the St. Clare Creek volcanic field, and in the north-trending principal axes and east-trending minor-stress axes through most of the Cenozoic in southwestern Yukon (Eisbacher and Hopkins, 1977). The history of subduction beneath the Wrangell volcanic belt is linked to the accretion history of the composite Yakutat terrane (Fig. 1; Lahr and Plafker, 1980; Brans, 1983; Stephens et al., 1984; Plafker, 1987). Plafker (1987) suggested that the Yakutat terrane started moving northwestward along the Queen Charlotte and Transition fault systems - 2 5 Ma and that - 2 2 5 km of the Pacific oceanic plate was subducted prior to the onset of Wrangell volcanism at - 2 0 Ma. Calc-alkaline Wrangell volcanism is now known to have started at - 2 6 Ma (Richter et al., 1990). If it is assumed that the date of this volcanic activity best limits the timing of subduction (cf. Plafker, 1987), then by applying the displacement rates used by Plafker (1987) it can be shown that the northward migration of the Yakutat terrane started as early as 30 Ma. The continental margin of the northeastern Gulf of Alaska is the site of transpression and underthrusting of the buoyant continental part of the Yakutat terrane, which resulted in the active uplift of the Chugach and St. Elias mountains (Fig. 1). The Wrangell-Wadati Benioff zone extends to a depth of 100 km below the northwestern volcanic belt and appears to be continuous with the Aleutian-Wadati Benioff zone (Page et al., 1989). The depth and topography of the Mohorovicic discontinuity beneath the belt is largely unknown; it may be as much as 55 km near the northwestern end of the belt (Goodwin et al., 1989). CHEMICAL, SPATIAL, AND TEMPORAL RELATIONS IN WRANGELL VOLCANIC ROCKS Three main magma types are distinguished in the Wrangell volcanic belt (Figs. 2 and 3; Table 1): (1) alkaline magmas (alkaline basalt to mugearite) with normative ne, Na > K, and high Fe/Mg and Nb/La ratios; (2) transitional magmas (basalt to rhyolite) with normative hy and intermediate Fe/Mg and Nb/La ratios; and (3) calc-alkaline magmas (basalt to rhyolite) with normative qz, modal orthopyroxene, low Fe/Mg and Nb/La ratios, and high Ba/La ratios. The transform regime of the Wrangell belt is composed mainly of transitional lavas, but it includes early-erupted alkaline and later erupted calc-alkaline lavas. The Alsek field (Fig. 1) stands out in this regard in that it is composed largely of calc-alkaline volcanic rocks, and smaller amounts of early-erupted alkaline and transitional lava. Volcanic activity in the Wrangell field in Alaska, associated with subduction in the WrangellWadati Benioff zone, is composed entirely of calc-alkaline magmas that appear to have been erupted in two spatial-temporal phases (Fig. 1; Nye, 1983). The early phase is found east of long 143°W and includes 26 Ma Rocker Creek lavas and 23 to 8 Ma Wrangell Lava (Denton and Armstrong, 1969; Richter et al., 1990). The latest phase of calc-alkaline volcanic activity in the Wrangell belt is confined mostly to the area west of 143°W (with the exception of 3.8 to 1.6 Ma lavas near the Sonya Creek center [Denton and Armstrong, 1969] and the - 1 . 5 ka White River Ash [Lerbekmo and Campbell, 1969]; Fig. 1). This latest phase of Wrangell activity comprises shield volcanoes and less abundant stratovolcanoes that show a westward decrease in age from 4 Ma to Holocene (Nye, 1983; Miller and Richter, 1991; Richter et al., 1990). The majority of calc-alkaline lavas from the St. Clare Creek field in the transform regime have low 8 7 Sr/ 8 6 Sr ratios (-0.7035; Fig. 4), which overlap those of Mt. Wrangell in the subduction regime. Alkaline lavas have 8 7 Sr/ 8 6 Sr ratios that range from 0.70405 in alkaline basalt (e.g., TS-15-85, Table 1) to 0.70448 in mugearite. Transitional lavas in the Stanley Creek and Nines Creek volcanic fields have 8 7 Sr/ 8 6 Sr ratios of -0.7040, and ratios in transitional lavas in the St. Clare Creek field range from -0.7032 to 0.7045. In summary, transitional magmas, with minor alkaline and calc12





Figure 2. (Na + K) vs. Si variation diagram shows whole-rock chemical data from southeastern Wrangell volcanic belt (SE WVB). Tie lines connecting idealized mineral compositions are for reference. Feldspar tie line on alkali-silica plots separates alkaline lavas (above line) from transitional lavas (along line) and calc-alkaline lavas (below line). Tie line with arrows projects toward quartz (lower right) and nepheline (upper left). A: Calc-alkaline lavas of SE WVB (open circles) and hybrid lavas (calc-alkaline-transitional; solid circles), fields of transitional and alkaline SE WVB lavas, and calc-alkaline lavas of northwestern WVB (NW WVB, dotted field; sources of data as in Fig. 1). B: Transitional lavas (crosses), alkaline (solid-line field), and calc-alkaline (dashed-line field) lavas of SE WVB, and calc-alkaline lavas of NW WVB. C: Alkaline lavas of SE WVB (open squares) relative to calc-alkaline and transitional lavas and field of northern Cordilleran Cenozoic intraplate lavas (dated from Eiche et al., 1987; Francis and Ludden, 1990, and unpublished). GEOLOGY, January 1991

alkaline magmas, characterize the transform-related regime, whereas only calc-alkaline lavas have been documented in the subduction-related segment. Furthermore, the calc-alkaline lavas in the transform-related segment have higher Nb/La ratios and lower Ba/La ratios than those in the subduction regime (Fig. 3). DISCUSSION In order to decipher the mantle-derived chemical signature of continental volcanic rocks, it is necessary to evaluate the possible extent to which lavas may have been contaminated by continental crust. An evaluation of existing Sr isotopic data of Paleozoic basement rocks of the Alexander and Wrangellia terranes (Downey et al, 1980; Samson et al., 1989) suggests that the basement to the Wrangell volcanic belt may have been characterized by 8 7 Sr/ 8 6 Sr ratios as high as 0.710 at 15 Ma. In contrast, primary Cenozoic alkaline lavas show that the northern Cordilleran upper mantle has 8 7 Sr/ 8 6 Sr ratios that range from -0.7029 to 0.7039 (Fig. 4; Eiche et al., 1987; Francis and Ludden, 1990). The St. Clare Creek volcanic field (Fig. 1), which includes all three magma types observed in the transform regime of the Wrangell volcanic belt, provides an opportunity to evaluate the involvement of the basement

rocks in the pedogenesis of the belt. Crustal contamination appears to have been involved in the evolution of the alkaline magmas; 8 7 Sr/ 8 6 Sr ratios increase in the alkaline basalt (0.70405 to 0.70448 in mugearites) with increasing fractionation (Fig. 4). The subsequent eruption of Fe-rich transitional lavas and calc-alkaline lavas reflects a systematic increase in silica activity with time. Transitional lavas at this stage have lower 8 7 Sr/ 8 6 Sr ratios of -0.7039 over the complete compositional range from basalt to rhyolite, and calc-alkaline lavas have ratios as low as 0.70365 (e.g., TS238-86, Table 1). These features cannot be reconciled with crustal contamination. The succeeding lavas (transitional basalts) show a large trace element and isotopic diversity, ranging from relatively depleted basalt with 87 Sr/ 8 6 Sr of 0.70319 and (La/Yb) n o f - 1 . 5 (TS-98-85, Table 1), to more enriched basalt with 8 7 Sr/ 8 6 Sr of 0.70442 and (La/Yb) n of 2.7. This isotopic and trace element diversity is decoupled from the major elements, as is evident from the most Mg-rich basalt (9.3% MgO), which is intermediate between the depleted and enriched end members in terms of its isotopic and trace element composition. The assimilation of basement rocks would require a prohibitively high degree of major element enrichment to reconcile the range in trace element and isotopic compositions observed in the basalts. Although the basalts themselves do not appear to TABLE 1 CHEMICAL AND ISOTOPIC COMPOSITION OF END-MEMBER LAVAS IN ST. CLARE CREEK FIELD, SOUTHWESTERN WRANGELL VOLCANIC BELT Alkaline basalt TS-15-85









12 GEOLOGY, January 1991

60.09 0.81 18.19 1.57 5.48 0.14 4.69 4.91 2.03 0.37 0.62 98.90

48.07 1.64 16.66 7.23 8.58 0.14 9.38 3.57 0.92 0.39 3.15 99.73

47.05 1.38 17.45 8.84 8.85 0.14 10.47 2.89 0.26 0.17 2.81 100.31

ne ol hy

1.30 16.15

0.29 19.77



Sc V Cr Ni Rb Sr

24.7 206 203 98 12 546 26 174 16 280 17.2 37.4 19.1 4.99 1.54 0.79 0.96 0.35 2.34 0.35

26.2 184 102 40 2 302 24 120 8 72 5.55 13.8 9.59 3.33 1.15 0.72 0.96 0.34 2.36 0.35 0.30 2.8

Zr Nb Ba La Ce Nd Sm Eu Tb Ho Tm Yb Lu Ta Hf 87

Figure 3. Trace element variation diagrams of Wrangell Lavas in parts per million. Calc-alkaline lavas (open circles), hybrid lavas (calcalkaline-transitional; solid circles), transitional lavas (crosses), and alkaline lavas (open squares) are shown alongside northwestern Wrangell volcanic belt lavas (NW WVB) and northwestern Cordilleran intraplate lavas. Data from southeastern Wrangell volcanic belt (SE WVB) are from this study; data from NW WVB are from Nye (1983); northwestern Cordilleran data are from Francis and Ludden (1990 and unpublished).

Calcalkaline andesite TS-238-86

Si02 Ti0 2 A1 2 0 3 MgO FeO MnO CaO Na 2 0 K20 P205 LOI Total



Transitional basalt TS-98-85

Sr/ 86 Sr

1.00 4.00 0.70405 ± 0.00003

0.70319 ± 0.00004

_ -

1.27 5.7 49 -

32 493 23 229 17 742 24.0 41.2 18.9 4.32 1.30 0.63 1.17 0.35 2.20 0.41 1.20 5.4 0.70365 ± 0.00004

Note: The major elements and Rb, Sr, Zr, Nb, and Y were analyzed by X-ray fluorescence at McGill University. Se, Hf, Ta, and REE were analyzed by instrumental neutron activation analysis at the Université de Montréal. Sr isotope determinations were carried out at the Université de Montréal. For details on the analytical method, accuracy, and precision, see Francis and Ludden (1990).




Figure 4. Chemical and isotopic variations of alkaline, transitional, and calc-alkaline lavas in St. Clare Creek field, southeastern Wrangell volcanic belt (error bar in upper left; calc-alkaline lavas [open circles], hybrid lavas [calc-alkaline-transitional; solid circles], transitional lavas [crosses] and alkaline lavas [open squares]; data from southeastern Wrangell volcanic belt [SE WVB] are from this study). Fields of northern Cordilleran intraplate lavas and arc lavas from northwestern belt (NW WVB) are shown for comparison (data from NW WVB are from Nye [1983]; northwestern Cordilleran data are from Francis and Ludden [1990 and unpublished]). Note that hybrid calc-alkaline lavas lie between transitional and NW WVB calc-alkaline lavas. Numbers are order in which volcanic units erupted in St. Clare Creek field (1 is oldest): 1—alkaline lavas; 2—intermediate and evolved, Fe-rich, and other transitional lavas; 3—calc-alkaline and hybrid lavas; 4—transitional basalts; 5—late intermediate and evolved transitional lavas.

be contaminated, the subsequent evolved transitional and calc-alkaline lavas show a positive correlation between Si content and 8 7 Sr/ 8 6 Sr ratios (dacite: 0.70431; trachyte: 0.70485), which may reflect interaction with crustal melts. A minimum of three mantle source components appear to be required to produce the spectrum of compositions in the transform regime of the volcanic belt. These include (1) an enriched OIB source for the alkaline magmas, (2) a depleted MORB source for the transitional basalts, and (3) a slab-derived component to produce calc-alkaline magmas. The three magma types in the transform regime represent mixing between these components. This is demonstrated by calc-alkaline lavas in the transform regime that are enriched in the OIB component relative to the main Wrangell belt arc lavas; the depleted transitional basalts, which have higher (La/Yb) n and 8 7 Sr/ 8 6 Sr ratios than normal MORB; and the alkaline lavas of the Wrangell belt, which have much lower Nb/La ratios than alkaline intraplate lavas of the northern Cordillera. The coexistence of alkaline, transitional, and calc-alkaline lavas in the transform regime can be explained in terms of leaky transform magmatism at the edge of an active arc. Extension across the Duke River fault may have triggered the adiabatic melting of heterogeneous continental lithosphere, giving rise to alkaline followed by transitional magmas. Superimposed on this heterogeneous mantle were the effects of subduction-related processes at the margin of the Wrangell arc, giving rise to calc-alkaline magmas that preserved chemical compositions transitional between those of arc and intraplate tectonic settings. REFERENCES CITED Bruns, T.R., 1983, Model for the origin of the Yakutat block, an accreting terrane in the northern Gulf of Alaska: Geology, v. 11, p. 718-721. Denton, G.H., and Armstrong, R.L., 1969, Miocene-Pliocene glaciations in southern Alaska: American Journal of Science, v. 267, p. 1121-1142. 14

Dodds, C.J., and Campbell, R.B., 1988, Potassium-argon ages of mainly intrusive rocks in the St. Elias Mountains, Yukon and British Columbia: Geological Survey of Canada Paper 87-16,43 p. Downey, M.E., Armstrong, R.L., and Parrish, R.P., 1980, K-Ar, Rb-Sr and fission track geochronometry of the Bock's Brook stock, Kluane Ranges, southwestern Yukon Territory, in Current research, Part B: Geological Survey of Canada Paper 80-1B, p. 189-193. Eiche, G.E. Francis, D.M., and Ludden, J.N., 1987, Primary alkaline magmas associated with the Quaternary Alligator Lake volcanic complex, Yukon Territory, Canada: Contributions to Mineralogy and Petrology, v. 95, p. 191-201. Eisbacher, G.H., and Hopkins, S.L., 1977, Mid-Cenozoic paleogeomorphology and tectonic setting of the St. Elias Mountains, Yukon Territory, in Report of activities, Part B: Geological Survey of Canada Paper 77-1B, p. 319-335. Francis, D.M., and Ludden, J., 1990, The mantle source for olivine nephelenite, basanite, and alkaline olivine basalt at Fort Selkirk, Yukon, Canada: Journal of Petrology, v. 31, p. 371-400. Goodwin, E.B., Fuis, G.S., Nokleberg, W.J., and Ambos, E., 1989, The crustal structure of the Wrangellia terrane along the East Glenn Highway, easternsouthern Alaska: Journal of Geophysical Research, v. 94, p. 16,037-16,057. Lahr, J.C., and Plafker, G., 1980, Holocene Pacific-North American plate interaction in southern Alaska: Implications for the Yakataga seismic gap: Geology, v. 8, p. 483-486. Lerbekmo, J.F., and Campbell, F.A., 1969, Distribution, composition, and source of the White River Ash, Yukon Territory: Canadian Journal of Earth Sciences, 1 v. 6, p. 109-116. Lowe, P.C., Richter, D.H., Smith, R.L., and Schmoll, H.R., 1982, Geologic map of the Nabesna B-5 quadrangle, Alaska: U.S. Geological Survey Geologic Quadrangle Map GQ-1566, scale 1:63,360. Miller, T.P., and Richter, D.H., 199f, Quaternary volcanism of the Alaska Peninsula and Wrangell Mountains, Alaska, in Plafker, G., and Berg, H.C., eds., The Cordilleran orogeny: Alaska: Boulder, Colorado, Geological Society of America, Geology of North America, v. G-l "(in press). Nye, C.J., 1983, Petrology and geochemistry of Ökmok and Wrangell Volcanoes, Alaska [Ph.D. thesis]: Santa Cruz, University of California, 208 p. Page, R.A., Stephens, C.D., and Lahr, J.C., 1§89, Seismicity of the Wrangell and Aleutian Wadati-Benioff zones and the North American plate along the TransAlaska Crustal Transect, Chugach Mountkigs and Copper River Basin, southern Alaska: Journal of Geophysical Research, v. 94, p. 16,059-16,082. Plafker, G., 1987, Regional geology and petroleum potential of the northern Gulf of Alaska continental margin: Circum-Pacific Council for Energy and Mineral ! Resources Earth Science Series, v. 6, p. 229-268. Richter, D.H., and Smith, R.L., 1976, Geologic map of the Nabesna A-S quadrangle, Alaska: U.S. Geological Survey Geologic Quadrangle Map GQ-1292, scale 1:63,360. Richter, D.H., Smith, R.L., Yehle, L.A., and Miller, T.P., 1979, Geologic map of the Gulkana A-2 quadrangle^ Alaska: U.S. Geological Survey Geologic Quadrangle Map GQ-1520, scale 1:63,360. Richter, D.H., Ratte, J.D., Schmoll, H.R., Leeman, W.P., Smith, J.G., and Yehle, L.A., 1989, Geologic map of the Gulkana B-l quadrangle, Alaska: U.S. Geological Survey Geologic Quadrangle Map GQ-1655, scale 1:63,360. Richter, D.H., Lanphere, M.A., Dalrymple, G.B., Smith, J.G., and Shew, N., 1990, Age of volcanism, Wrangell volcanic field, Alaska: Bulletin of Volcanology. Samson, S.D., Patchett, P.J., Gehreis, G.E., and Anderson, R.G., 1989, Nd and Sr isotopes in the Wrangellia terrane of the North American Cordillera: Implications for Phanerozoic crustal growth: Eos (Transactions, American Geophysical Union), v. 70, p. 1389. Souther, J.G., and Stanciu,'C., 1975, Operation Saint Elias, Yukon Territory: Tertiary volcanic rocks: Geological Survey of Canada Paper 75-J A, p. 63-70. Stephens, C.D., Fogleman, K.A., Lahr, J.C., and Page, R.A., 1984, Wrangell Benioff Zone, southern Alaska: Geology, v. 12, p. 373-376. Stout, J.H., and Chase, C.G., 1980, Plate kinematics of the Denali fault system: Canadian Journal of Earth Sciences, v. 17, p. 1527-1537.

ACKNOWLEDGMENTS Supported by Natural Science and Engineering Research Council grants to Francis and Ludden, by Reinhardt and Leroy scholarships, and by summer field fellowships (Centre for Northern Studies and Research, McGill University) to Skulski. Logistical support was provided by the Department of Indian Affairs and Northern Development (Whitehprse, Yukon). Tariq Ahmedali, Gilles Gauthier, and Brigitte Dionne assisted in analytical work. We thank Jim Clark, Roland Dechesne, and Andrew Hynes for helpful reviews, and Don Richter, George Plafker, and Chris Nye for discussions. Manuscript received April 16, 1990 Revised manuscript received July 24, 1990 Manuscript accepted August 8, 1990

Printed in U.S.A.

GEOLOGY, January 1991