Chronology of the Peralkaline, Late Cenozoic Mount Edziza Volcanic Complex, Northern British Columbia, Canada

Chronology of the peralkaline, late Cenozoic Mount Edziza Volcanic Complex, northern British Columbia, Canada

J. G. SOUTHER Geological Survey of Canada, 100 West Pender Street, Vancouver, British Columbia, V6B 1R8, Canada j R A KA L ^ ^ ^ } Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, V6T 1W5, Canada

ABSTRACT

The late Cenozoic Mount Edziza Volcanic Complex covers an area history than any other Stikine volcano. Understanding the nature and of about 1,000 km2 in north-central British Columbia, 300 km east of the timing of its evolution is essential to interpreting the late Cenozoic tecton- transcurrent boundary between the North American and Pacific plates. It ics of the northern Cordillera. is made up of a group of overlapping basaltic shields and intermediate to Previous discussion of the Complex (Souther and Symons, 1974; salic peralkaline composite domes, flows, and central volcanoes that are Aumento and Souther, 1973) was based on few age determinations and associated with extensional structures in the underlying basement. New limited stratigraphic data. This paper presents 45 new K-Ar and fission- K-Ar and fission-track dates (45) and Rb-Sr and Sr isotope analyses (12) track age dates (Table 1), new Sr isotope data, and extensive stratigraphic from the Mount Edziza Volcanic Complex are reported. The age dates are observations.1 for the most part consistent with the stratigraphy and indicate that frequent eruptive activity occurred during the past 8 m.y. Five major magmatic PHYSIOGRAPHY cycles each began with the eruption of basalt and culminated with the 87 86 eruption of oversaturated peralkaline magma. Low Sr/ Sr initial ratios The Mount Edziza Volcanic Complex forms a rolling upland plateau, (0.7028 ± 0.0001) indicate a mantle source for the basalts. Low Sr con- flanked on the west by rugged granitic peaks of the Coast Mountains and 87 86 tents and high Sr/ Sr ratios in the salic end members suggest that the on the east by the more subdued Skeena Mountains and Klastline Plateau. oversaturated rocks were derived from the basaltic magma by crystal The volcanic plateau is bounded by escarpments and cut by steep canyons fractionation in crustal reservoirs. Rb-Sr isochrons suggest that residence eroded in the four glacier-capped central volcanoes that rise from its crest times for the fractionating magma were about 0.7 to 1 m.y. Early removal (Fig. 4). Between Armadillo Peak and the Spectrum Range the deep of large amounts of plagiociase, followed by fractionation of potash feld- east-west valley of Raspberry Pass separates the northern and southern spar, can account for most of the observed petrological and isotopic portions. Armadillo Peak, near the center of the complex, is the oldest and relationships. A few individual compositions and one suite of mainly most deeply dissected central volcano. Erosion has stripped away most of 87 intermediate samples contain anomalously large amounts of both Sr and the old caldera rim and cut deep, V-shaped valleys into the underlying radiogenic argon. This indicates that contamination with crustal material Mesozoic sediments and volcanics. Farther south, the brightly colored, and possibly mixing of parental basalt with partly fractionated magma felsenmeer-covered slopes of the Spectrum Range have been etched from a from previous events may have produced the relatively small volume of broad edifice of overlapping rhyolite domes. Although younger than Ar- intermediate rocks. madillo Peak, the Spectrum Range domes have also been greatly eroded, and the larger streams have cut into underlying basement rocks. INTRODUCTION The northern end of the Complex is dominated by the symmetrical cone of Mount Edziza, which rests on and covers most of the north flank Mount Edziza, Ice Peak, Armadillo Peak, and the adjacent Spectrum of the older Ice Peak edifice. Ice Peak has been modified by alpine glaciers Range are eroded late Cenozoic composite volcanoes that lie along the into a small pyramidal horn, bounded by active cirques, whereas much of axis of the Stikine Volcanic Belt (Fig. 1). Their products, along with those the original crater rim and conical form of Mount Edziza remains intact. erupted from satellitic centers, have merged to form the Mount Edziza However, both volcanoes have been preferentially dissected on the east Volcanic Complex that covers about 1,000 km2 (Figs. 2, 3). The Stikine side, where erosion by alpine glaciers has exposed their conduits. Volcanic Belt runs approximately north-south through northwestern Brit- Although the major central volcanoes predate the last regional glacia- ish Columbia, between the northern end of the Cascade-Garibaldi Belt and tion, volcanic activity continued from satellitic vents during and after the southeastern end of the Wrangell Belt. It is believed to be a zone of late glaciation. The earlier eruption products of this final stage are subglacial, Cenozoic extension related to transcurrent motion along the adjacent con- tuyalike mounds of pillow lava and tuff breccia, whereas postglacial erup- tinental margin (Souther, 1977). In addition to Mount Edziza, the Stikine tions yielded pristine-looking fields of blocky lava, cinder cones, and Belt includes the shields of Level Mountain (Hamilton, 1981) and Hart ashbeds. Peaks, the composite dome of Hoodoo Mountain, and scores of small, isolated cinder cones. Although smaller in volume than Level Mountain, 'Complete analytical data and locations are available in GSA Supplementary the Mount Edziza Complex has undergone a longer and more varied Data 84-6. Free copies are sent upon request.

Complete analytical data and sample locations are available from the GSA Data Repository. To receive copies free of charge, request Supplementary Material 84-6 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 95, p. 337-349, 9 figs., 2 tables, March 1984.

337

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roxene, amphibole, and glass. In the comenditic and pantelleritic trachytes, anorthoclase and hedenbergite are the principal phenocrysts. and the groundmass includes alkali feldspar and varying proportions of aegirine- augite, sodic hedenbergite, opaque oxides, arfvedsonite, quartz, and aenigmatite. Most comendites have phenocrysts of both anorthoclase and sanidine and lesser amounts of hedenbergite, opaque oxides, quartz, arfvedsonite, aenigmatite, and acmite. The chemistry of the Mount Edziza Complex3 is similar to that of other alkaline-peiralkaline suites in British Columbia (Bevier, 1981; Hamil- ton, 1981). Silica varies from 46 wt % in picrite basalt to 79 wt % in

comendite, agpaitic index (molar Na20 + K2O /A1203) varies from 0.35 to 1.5. Si02 versus total alkalies, and AFM plots (Fig. 5) show extreme alkali enrichment of the salic end members. The complex is undeformed except for minor normal faults that bound north-sou th-trending half-grabens. These are confined to the east and west margins of the pile, where basal units have been displaced as much as 120 m. Smaller displacements of younger units suggest that faulting was contemporaneous with volcanism. The complexity of the Mount Edziza Complex is due to different physical behavior of the basaltic and salic lavas, and to erosion concurrent with effusive activity. Fluid basaltic lavas filliid topographic lows, commonly far removed from their source. In contrast, relatively viscous salic lavas piled up around their vents, forming thick stubby flows, domes, and steep-sided composite cones. Salic pyroclastic flows and air-fall deposits are relatively minor. Rapid erosion continuously modified the terrane between episodes of volcanic activity, leading to a complex cut-and-fill structure characterized by disrupted drainage and topographic inversions. Where sequences of flows have been isolated from one another by erosion or younger cover, isotopic age determinations provide the only reliable means of correlation.

GEOCHRONOMETRY

The K-Ar dates reported in Table 1 were determined over a period of 10 years in the laboratories of the Geological Survey of Canada and at the University of British Columbia. Ar extraction techniques and mass spec- trometry procedures at both labs are similar and did not greatly change over that time interval (see Table 1). Sample preparation techniques evolved somewhat as our experience grew. Where possible, feldspars at 40 to 80 mesh grain size were separated using density and magnetic methods. Figure 1. Index map of late Tertiary (Miocene-Pliocene) These usually gave lower atmospheric Ar yields and, especially when plateau basalt and Quaternary volcanic belts of British Co- K-rich, the feldspars gave more precise dates. Where feldspars are very fine lumbia and northern Washington. grained, we ran whole-rock samples, crushed to about 80 mesh and ultra- sonically washed in deionized water containing a few percent hydrochloric GENERAL GEOLOGY acid. This removed dust and carbonate and yielded minimal trapped atmospheric Ar. Even with this preparation, some samples of older rocks The Mount Edziza Complex is constructed of overlapping basaltic proved unsuitable for dating because of overwhelming atmospheric Ar shields and salic composite cones and domes. The basalts make up about content. During pumpdown before analysis at the University of British equal volumes of hawaiite and alkali olivine basalt2 in which the principal Columbia, all samples were baked at 189 °C for at least 15 hr. minerals are calcic plagioclase, titaniferous augite, forsteritic olivine, and opaque oxides. Many of the basalts are aphyric, but porphyritic varieties Problems are also common. The salic and intermediate rocks are chiefly peralkaline and about equally divided among trachyte, comenditic or pantelleritic Most of the dates reported in this paper fit the observed stra tigraphic trachyte, and comendite.2 The trachytes commonly contain phenocrysts of framework. Significant anomalies (Fig. 6) probably result from excess calcic oligoclase and zoned titanaugite in a matrix of alkali feldspar, py- radiogenic argon, present at the time of eruption, and from isotopic frac-

2Peraluminous rocks are classified according to Irvine and Baragar (1971); peralkaline rocks, according to Macdonald (1974). 3Major- and trace-element chemistry is being published in a separate paper.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/3/337/3444868/i0016-7606-95-3-337.pdf by guest on 25 September 2021 CYCLE 5 SHEEP TRACK: BIG RAVEN Trachyte, Basalt

CYCLE 4 KAKIDDI Pantelleritic trachyte

KLASTLINE: ARCTIC LAKE Basalt

CYCLE 3 J EDZIZA: PILLOW RIDGE Comenditic trachyte basalt

ICE PEAK Basalt, benmoreite, trachyte pantelleritic trachyte

CYCLE 2 PYRAMID Trachyte, comendite S«

+ + + SPECTRUM + + + + + Comenditic trachyte, comendite, minor basalt

NIDO: KOUNUGU Basalt

CYCLE 1 ARMADILLO Comendite, trachyte basalt

LITTLE ISKUT: RASPBERRY Trachybasal t, basalt

BASEMENT PRE EDZIZA Mainly Mesozoic and early Tertiary rocks

0 2 4 6 8 10 KILOMETERS ^r f f f -t- T f -t^

130 30

Figure 2. Generalized geological map of the Mount Edziza Volcanic Complex showing locations of the seven cross sections in Figure 3.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/3/337/3444868/i0016-7606-95-3-337.pdf by guest on 25 September 2021 O.P±0.3(7) 0V41O.12 (16) 0 810 25(7) -0 94Î005U6R

5.9±0.9(8) 4.5-0 3(2 4) . 0jl2t040(4)

r /%

3711 O(l5)Î0Îi

S.Sil.6(2«)

4,4±0.5(25l 6.2 iO. 1(33) 6.IÎ0.M28) 6.110.2(29) 6.1 i 0-4(39) 5.SÌ0.I (38)

ICE PEAK (a) Pantellentic trachyte, ARMADILLO minor co mend ite ¡o) Comendite. trachyte, (b) Basalt sodo granite PYRAMID (b) Basalt KLAST LINE AND ARCTIC LAKE (a) Aphyric comendite LITTLE ISKUT Alkali olivine basalt (b) Porphyritie Trachybasalt howante, minor picrile sanidine trachyie [RASPBERRY E D ZIZ A SPECTRUM Alkali olivine basalt, Ponlellerilic and Comenditic trochyte. howoiite comenditic trachyte Comendite. minor basalt PILLOW R IDGE EDZIZA BASEMENT NIDO AND KOUNUGU Mainly Mesoioic and eorly Alkali olivine basoll Alkali olivine basalt, Tertiary sed vol. ond howaiile. minor piente plutonic rocks

Figure 3. Schematic cross sections with geology projected onto the seven planes located in Figure 2. Relative positions and dates of the 41 dated samples are shown with reference to 12 principal formations. The postglacial Big Raven and Sheep Track Formations have been omitted because they are volumetrically insignificant and could be shown only with great exaggeration.

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TABLE 1 K-Arm AND FISSION TRACK'2' DATES FROM THE MT. EDZIZA VOLCANIC COMPLEX

Sample Formation* Laboratory Metliod Date Ma Sample Formation Laboratory Method Date Ma Lithology Li thology

1 KA.t* U.B C. K-Ar w.r. 0.30 + 0.02 21 SP ,c U.B.C. K-Ar w.r. 3.0 ± 0.1 IF KA,t U.B C. K-Ar 0.29 ± 0.02 22 SP ,c G.S.C. K-Ar w.r. 3.1 t 0.1 Feldspar 2 KA.t U.B C. K-Ar w.r. 0.28 ± 0.2 23 SP.cg U.B.C. K-Ar w.r. 3.4 i 0.1 3 KA.m U.B C. K-Ar w.r. 0.31 ± 0.07 24 NI ,b G.S.C. K-Ar w.r. 4.5 t 0.3 4 KA,b U.B C. K-Ar w.r. 0.62 0.04 25 NI ,b G.S.C. K-Ar w.r. 4.4 ± 0.5

5 KA.b U.B C. K-Ar w.r. 0.71 » 0.05 26 NI ,b U.B.C. K-Ar w.r. 5.5 t 1.6 6 EZ,t U.B C. K-Ar w.r. 0.9 ± 0.3 27 KU ,b U.B.C. K-Ar w.r. 7.8 ±0.3 7 PR,x U.B C. Fisn.Tr.(3) 0.9 ± 0.3 28 AR,c G.S.C. K-Ar w.r. 6.1 ± 0.1 7 PR,x U.B C. Fisn.Tr.(4) 0.8 ± 0.25 29 AR,h G.S.C. K-Ar w.r. 6.1 t 0.2 8 PR,b U.B C. K-Ar w.r. 5.9 ± 0.9 30 AR,eg U.B.C. K-Ar w.r. 6.4 ± 0.2

9 IP,t U.B C. K-Ar w.r. 1.2 ± 0.1 31 AR,h U.B.C. K-Ar w.r. 6.5 ± 0.2 10 IP.t U.B C. K-Ar w.r. 1.5 ± 0.4 32 AR ,c G.S.C. K-Ar w.r. 6.9 ± 0.3 11 IP.t U.B C. K-Ar w.r. 1.5 ± 0.1 33 AR,h G.S.C. K-Ar w.r. 6.2 ± 0.1 12 IP, t U.B C. K-Ar w.r. 1.6 + 0.2 34 AR,h G.S.C. K-Ar w.r. 6.3 ± 0.5 13 IP.t U.B C. K-AR w.r. 2.8 ± 0.1 35 AR,s U.B.C. K-Ar w.r. 7.1 ± 0.3

14 IP,h U.B c. K-Ar w.r. 2.8 0.2 36 AR,c U.B.C. K-Ar w.r. 10.2 ± 1.4 15 IP,h U.B c. K-Ar w.r. 3.7 ± 1.0 37 LI ,r U.B.C. K-Ar w.r. 7.2 ± 0.3 16 PY,t G.S c. K-Ar w.r. 0.94 ± 0.12 38 RA ,b G.S.C. K-Ar w.r. 5.5 ± 0.1 16F PY,t G.S c. K-Ar 0.94 ± 0.05 39 RA.b G.S.C. K-Ar w.r. 6.1 ± 0.4 Feldspar 17 PY.cg G.S c. K-Ar w.r. 1.20 ± 0.03 40 RA,h U.B.C. K-Ar ».4 ± 0.4 Feldspar

18 PY ,cg U.B.C. K-Ar w.r. 1.2 0.4 41F RA,h U.B.C. K-Ar 6.4 ± 0.3 Feldspar 19 SP,b U.B c. K-Ar w.r. 5.9 ± 1.1 41 RA,h LI.B.C. K-Ar w.r. 11.4 ± 1.5 20 SP,eg G.S c. K-Ar w.r. 2.9 0.1

•(Formation: KA.Kakiddi; EZ,Edzi2a; PR,Pillow Ridge; IP,Ice Peak; PY,Pyramid; SP,Spectrum; NI.Nido; KU.Kounugu; AR,Armadillo; LI,Little Iskut; RA,Raspberry)

t(Lithology: b,basalt; h.hawaiite; m.mugearite; t,trachyte; r .trachybasal t; g,glass; x.xenolith)

(1) K-Ar dates were determined by R.K. Wanless at the Geological Survey of Canada laboratory in Ottawa (GSC)and by R.L. Armstrong at the University of British Columbia (U.B.C.). K is determined in duplicate by atomic absorption using a Techtron AA4 spectrophotometer and Ar by isotope dilution using an AEI MS - 10 mass spectrometer and high purity ^8Ar spike. Errors reported are for one standard deviation. The constants used are:

1 KXe = 0.581 x lO-'V. • ''•962 * ltf'V '

40K/K = 0.01167 atom percent.

(2) Fission track ages were determined by R. Parish, University of B.C., following procedures of Parish (1982). Irradiation at TRIGA reactor, U.S. Geological Survey, Oenver Colorado. Xfission = 7.00 x lQ""/year, 9 24 i238 = 0.155125 x 10" /year, U238/235 - 137.88, U235 fission cross-section = 580 x 10" cm'.

(3) 75 grains counted for each induced and natural track density determination; track density (tracks/cm2) and total number of tracks counted (): induced, 42920 (136), natural, 3787 (12); neutron dose, 1.72 x lO" neutrons/cm2, date = 900,000 ± 300,000 years.

(4) 75 grains counted for each induced and natural track density determination; track density and total number of tracks counted (): induced, 44810 (142), natural, 3479 (11); neutron dose 1.72 x 10lfl neutrons/cm2, date = 800,000 ± 250,000 years.

tionation during analysis. Excess argon has been observed in other young volcanic rocks (Dalrymple, 1969; Krummenacher, 1970; Fisher, 1971) and is especially prevalent in xenolith-rich or rapidly cooled basaltic magmas from ice-contact or subaqueous environments (Rutford and others, 1972; Dalrymple and Moore, 1968; Armstrong, 1978). In the Mount Edziza Complex, the obvious examples of excess initial radiogenic argon involve the Ice Peak and Pillow Ridge lavas. The excess is made evident by large discordance with dates for overlying and underlying stratigraphic units and is associated with unusual bulk chemistry and excess radiogenic Sr in the case of Ice Peak, and with partially fused granitic xenoliths and ice-contact features in the case of Pillow Ridge. Confirmation of excess argon in Pillow Ridge lavas is provided by a fission-track date on apatite that is much younger than the observed K-Ar date and is in accord with stratigraphy. Probable isotopic fractionation during analysis is illustrated by other J anomalously old dates and may contribute to the Ice Peak and Pillow Figure 4. View looking north across the eroded remnants of Ridge anomalies. In every case, the rocks are basaltic (therefore relatively Armadillo Caldera (foreground) and Ice Peak (middle background) to low in K) and unusually rich in atmospheric argon. This abundance of the composite cone of Mount Edziza.

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atmospheric argon shows up in Figure 5 as elongated error bars, because pheric argon by diffusion out of sintered aluminum oxide crucibles; the the calculated precision of an analysis is inversely proportional to the process proposed to explain the anomalous dates is consequently experi- observed percentage of radiogenic argon. The correction for atmospheric mentally demonstrable. Any older (> 1 Ma) sample yielding argon that is argon assumes that the ratio of 36Ar to 40Ar is exactly atmospheric. This less than 10% radiogenic is suspect on this account. assumption probably fails for these samples because atmospheric argon is Fractionation of atmospheric argon cannot be predicted and is not fractionated by diffusion during pumpdown (Baksi, 1974). In atmospheri- taken into account in reporting analytical errors, but large errors (lag those cally clean samples, this is not a serious problem, but where really large samples for which the process is likely to be important. amounts of atmospheric argon are trapped (this is favored by incipient 36 alteration) the Ar, which is removed by diffusion out of the sample STRATIGRAPHY during pumpdown, escapes more rapidly than 40Ar. In extreme cases, this fractionation could exceed 5%. The atmospheric argon eventually analyzed Alternation of basic, intermediate, and salic units suggests that the 40 may thus have several percent excess Ar. Where the radiogenic argon is Edziza Complex is the product of five magmatic cycles, each beginning itself only a few percent of total argon extracted, the discrepancy in calcu- with basalt and culminating with oversaturated alkaline or peralkaline lated date can exceed a factor of two. Two extreme, but instructive, cases, magma (Fig. 2). The volcanic pile has been further subdivided into 15 not listed in Table 1, gave dates of 16 ± 6 Ma (2.3% radiogenic) and 22 ± formations (Figs. 2-3), each from a different center or group oF centers. 8 Ma (2.7% radiogenic) for rocks approximately 7 m.y. old. In both cases, Formations are separated by erosion surfaces or by fluvial or glacial depos- the 95% confidence limits would include the correct age, but the errors are its. The stratigraphic continuity of the Mount Edziza Complex is broken so large that the dates are not stratigraphically useful. Only about 2% by the deep, east-west valley of Raspberry Pass. Only two formations, the isotopic fraction by diffusion would explain the systematic overestimate of basal Raspberry basalt and the voluminous Armadillo rhyolite, can be the ages. We have noted as much as 2.5 ± 0.3% fractionation of atmos- projected with confidence across Raspberry Pass. Although some formations of the Spectrum Range are similar in age and lithology to rocks north of Raspberry Pass;, they originated mainly from different centers and ex- hibit sufficient mineralogical differences to warrant Formational status. The names used in this paper have been proposed to the Board of Stra- tigraphic Nomenclature for formal use.

Raspberry Formation

Raspberry basalt is the basal formation of the Mount Edziza Com- plex and the initial material of the first magmatic cycle. It rests on a late Miocene erosion surface that slopes westward into the ancestral valley of Mess Creek (Fig. 2). The basalt originated from vents, now b .tried by younger rocks, north of Raspberry Pass. The lava flowed west and. ponded in the lowlands, where more than 180 m of flows is exposed in I he Mess Creek escarpment. North of Raspberry Pass, the eastern edge of the Rasp- berry pile laps out against the basement surface and there only a few valley-filling flows are preserved. Proximal sections of Raspberry basalt include as many as 25 flows. Individual flow units are 1 to 30 m thick and constitute a central core of crudely columnar, reddish-brown-weathering basalt; a relatively thin basal breccia of ropey clinker; and a variable thickness of flow-top breccia. Intraflow fluvial layers are uncommon, lenticular, and of local origin, suggesting that Raspberry activity was not interrupted by long periods of dormancy. Raspberry basalt includes both alkali olivine basalt and hawaiite. The alkali olivine basalt is commonly aphyric or contains sparse phenocrysts of plagioclase and locally of titanaugite in an ophitic to subophitic groundmass of calcic plagioclase, titaniferous augite, intergranular olivine, and opaques. The hawaiites, which are coarsely and abundantly feldspar- phyric, are most common in the upper part of the Raspberry pile. Most Raspberry basalts contain carbonate-filled amygdules and locally the groundmass shows carbonate alteration. Four samples of Raspberry basalt yielded K-Ar dates ranging from 5.5 to 11.4 Ma (Table 1). The large spread is probably due to pervasive carbonate alteration and relatively large atmospheric contents, w.iich are common to all of the Raspberry rocks. The 11.4 ± 1.5 Ma age, a whole- rock analysis of sample 41, has the greatest error and the highest content of atmospheric argon. The Ar has probably fractionated, so that the deter- Si02 (Wt%) mined date is anomalously old. A feldspar separate from the same sample, 4IF, yielded an age of 6.4 ± 0.3 Ma. This and the other 3 whole-rock ages Figure 5. AFM and alkali-silica diagrams of 96 representative (38, 39, 40) have smaller errors and relatively high radiogenic; argon rocks from the Mount Edziza Volcanic Complex. (>19%). The spread in these 4 better dates (5.5 to 8.4 Ma) is probably still

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• 1 • IF KAKIDDI • 2 • 3

KLASTLINE 4«

ARCTIC LK. 5*

EDZ IZ A 61 0 I

-| | 1 Fission track 1 PILLOW 71 > 1 Fission track 2 RIDGE 1 8-« 1

9 l#t 101 • 1 11 t»4 ICE PEAK 12H»-I 13t* 14t-»-l 1 15—• 1

160 16FO PYRAMID 170 181 • "

I 19^ 1 20 K>t SPECTRUM 21 ••« 22 23»«4

24 1—O—1 N IDO 1 26—® 1

KOUNGU 271 m 1

28 0 29H>I 30 •—•—I 311-CM ARMADILLO 321—O-H 33»CH

341 ° 351 • 1 1 36—• 1

LITTLE ISKUT 37-^-

38 tOl

RASPBERY 41FI • 1 ' * ' 1 41 • 1

AGE Ma 0 1 2 3 4 5 6 7 8 9 10 11 12 13

Figure 6. Plot of K-Ar and fission-track age dates versus stratigraphie position. Error bars are one standard deviation. Open circles, Geological Survey of Canada; solid circles, University of British Columbia.

greater than the actual duration of Raspberry volcanism, which, from the of thermal shock fracturing and phreatic alteration associated with water absence of intraformational erosion surfaces, appears to have been rela- quenching. Above the basal quenched unit, the Little Iskut section is a tively short-lived. One approximation of the age is a weighted average monotonous succession of thick, irregular, black-weathering, randomly (weighting factor 1/1CT) of the four better dates, which places the Rasp- jointed flows and associated blocky, polygonal basal and flow-top brec- berry event at 6.2 Ma. Another would be an average of the two feldspar cias. Most Little Iskut rocks are aphyric or sparsely microporphyritic dates, 7.4 Ma. The samples are for the most part from less altered, more trachybasalts in which the groundmass is a panidiomorphic mosaic of highly feldspar-phyric rocks near the top of the formation. The 6.2 to 7.4 oriented feldspar laths (sodic andesine), ragged interstitial grains of green Ma age is thus a minimum. ferrohedenbergite, and opaques. Phenocrysts, where present, are sodic anorthoclase and ferroaugite zoned to rims of ferrohedenbergite. Little Iskut Formation The single sample from the Little Iskut Formation, dated at 7.2 ± 0.3 Ma (Table 1, no. 37), is within the range of dates from the underlying The Little Iskut trachybasalt, erupted during the first magmatic cycle, Raspberry basalt. The Little Iskut Formation occupies a small part of the is confined to a relatively small area along the eastern edge of the Mount area underlain by Raspberry basalt and no erosion is evident between the Edziza Complex, south of Raspberry Pass. It is made up of the eroded two piles. Little Iskut activity thus may have been coeval with late Rasp- remnant of a small shield with a proximal thickness of as much as 150 m. berry flows from a center north of the Little Iskut shield, or it may have The basal unit (15 to 120 m thick) is brecciated and discolored, suggestive immediately followed Raspberry eruptions.

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Armadillo Formation The Kounugu differs from the Nido in the presence of sparse, lenticular picritic flows. The Armadillo is the oldest salic formation in the complex. It consists The single Kounugu sample (Table 1, no. 27), which yields a date of chiefly of comendite, interlayered with lesser amounts of comenditic and 7.8 ± 0.3 Ma, is from the base of a thick basalt section tha; rests on pantelleritic trachyte and minor basalt. By far the largest volume of mate- basement southeast of the Spectrum Range, beyond the southern limit of rial issued as domes and viscous flows, erupted during the culmination of the Armadillo Formation. If this age is correct, then basalt assig led to the the first rnagmatic cycle. Some of the flows are as much as 100 m thick Kounugu may span an interval from Raspberry to post-Armadi lo time. and more than 10 km long. Moderately to strongly welded pyroclastic The three Nido samples, which yield ages from 5.5 to 4.4 Ma, are all flows and air-fall pumice deposits, although volumetrically small, form a from north of Raspberry Pass, where the Nido flows are underlain by widespread distal fades that serves as an important marker horizon. Armadillo comendite. Sample 26 has a large error and may be too old as a South of Raspberry Pass, distal Armadillo ash flows rest directly on consequence of its high atmospheric argon content. The average of the the unmodified surface of the Little Iskut shield. Elsewhere, Armadillo remaining Nido ages, weighted according to analytical precision (25, 24), rocks rest on moderately eroded Raspberry basalt, from which they are is 4.4 Ma. If sample 26 is included, the average increases only to 4.5 Ma. commonly separated by a colluvial layer. The Nido is overlain locally by Pyramid rhyolite and elsewhere by The Armadillo pile originated from vents on a basement high near the more widespread Ice Peak Formation, whereas the Kounugu is over- the northeastern end of Raspberry Pass. Principal among these was a lain exclusively by Spectrum rhyolite. circular caldera more than 4 km in diameter, of which Armadillo Peak is a remnant. The center of the caldera is occupied by dikes, sills, and irregular Spectrum Formation subvolcanic intrusions of aegirine-aenigmatite soda granite. Major satellitic vents are exposed a few kilometres north and west of Armadillo Peak. The The Spectrum Formation is confined to the Spectrum Ra ige, which overlapping domes and proximal flows of Armadillo comendite were derives its name from the brightly colored rhyolite domes and flows that never buried beneath younger flows. Instead, they were progressively constitute its high central peaks. The formation consists entirely of co- eroded, shedding an apron of clastic debris onto the surrounding plateau, mendite, pantellerite, and trachyte, of which more than 90% was erupted where it was covered by and incorporated within a succession of younger as lava and less than 10% as pyroclastic flows and pumice. Individual flows formations. are as much as 200 m thick, and proximal sections commonly contain 3 to Thin, very fine-grained, aphyric alkali olivine basalt flows are inter- 10 units, with a combined thickness of as much as 400 m. Each flow unit is layered with ash flows and air-fall pumice in the distal facies of the defined by a vitreous base of black to light bluish-gray or green obsidian Armadillo pile. The basalt source is unknown, but the thinness of flows that often serves as the only means of distinguishing successive flows. (± 1 m), great areal extent, and absence of associated flow breccia suggest The Spectrum rhyolite includes the same mineral species as the Ar- highly mobile lava. madillo comendite, but it is richer in quartz and alkali feldspars relative to The Armadillo comendites and trachytes have similar mineralogy the femic constituents. Quartz, commonly pitted and embayed, is a ubiqui- except for more quartz in the comendites. Most flows have a quenched, tous phenocryst phase in the Spectrum rocks, whereas it appears only in vitreous base of flow-banded obsidian from 1 to 3 m thick. Above this, the the groundmass. of most Armadillo comendites. The Spectrum Formation rock is holocrystalline, commonly with 5% to 10% phenocrysts of sanidine is overlain locally by small remnants of basalt (Kitsu member), from and hedenbergite and less commonly of fayalite, arfvedsonite, and aenig- which it is separated by a layer of polymict gravel that includes clasts of matite (Yagi and Souther, 1974). The groundmass minerals, alkali feld- both Spectrum and pre-Spectrum rocks. spars, quartz, acmitic pyroxene, aenigmatite, arfvedsonite, and kataphorite Four dated comendites from the Spectrum Formation (20-23 in commonly occur in radiating spherulitic clusters. Table 1) have small errors and a very narrow range, 2.9 to 3.4 Ma. The Nine whole-rock samples from the Armadillo Formation yielded a weighted average of 3.1 Ma is probably a fairly precise age for the Spec- range of ages from 6.1 to 10.2 Ma (Table 1). The 10.2-Ma age on sample trum activity. 36 is discrepant and has a much higher error (±1.4 Ma) than the other ages The Kitsu date (19) of 5.9 ± 1.1 Ma, from basalt with a large due to its high content of atmospheric argon, which is probably somewhat atmospheric correction, is clearly too old, because of either contamination fractionated. The remaining eight dates fall within a range of 6.1 to 7.1 Ma or isotopic fractionation during extraction of atmospheric argon. and give a weighted average of 6.3 Ma. Pyramid Formation Nido and Kounugu Formations The Pyramid Formation, erupted at the end of the second magmatic The Nido basalt, which is confined to the area north of Raspberry cycle, is confined to the eastern margin of the Mount Edziza Complex, Pass, and the Kounugu basalt, which is restricted to the Spectrum Range, north of Raspberry Pass. It includes two distinct facies, each the product of are remnants of composite shields that were erupted early in the second a separate pulse of activity. The first to erupt was a symmetrical lava dome magmatic cycle. The Nido rests either directly on Armadillo comendite or of highly porphyritic trachyte approximately 2 km in diameter and on gravel containing Armadillo clasts, whereas most of the Kounugu 1,000 m high. The rock contains abundant, 1- to 2-cm phenocrysts of basalt rests directly on pre-Tertiary basement rocks at the southern edge of sanidine and anorthoclase and sparse 1- to 2-mm phenocrysts of sodic the complex. Only distal Kounugu flows overlap the southern edge of the hedenbergite in a holocrystalline matrix of quartz, feldspar, sodic pyrox- Armadillo pile. The lower part of the Kounugu shield thus could be coeval ene, and opaques. Gravels of this unique rock are present beneath subse- with, or even older than, the Armadillo. The Nido and Kounugu basalts quent rhyolite flows of the upper Pyramid Formation. resemble the Raspberry in morphology and mineralogy. Each formation At the extreme northern and southern ends of the outcrop area, the contains as many as 20 flow units and is as much as 150 m thick. Aphyric upper Pyramid consists of thick rhyolite flows. These are characterized by to slightly feldspar- and pyroxene-phyric alkali olivine basalt predominates basal obsidian layers from 3 to 10 m thick, much thicker than beneath flows low in most sections and gives way upward to feldspar-phyric hawaiites. of similar size in the older salic formations. The crystalline rhyolite con-

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tains sparse phenocrysts of quartz, sanidine, and sodic hedenbergite in a includes a significant volume of intermediate rock types. Ice Peak rocks fine-grained, holocrystalline matrix of intergrown quartz and alkali feld- also contain an exceptional excess of 87Sr (Figs. 8 and 9), which suggests spars plus finely disseminated opaques, aenigmatite, and acmite. crustal contamination. The Pillow Ridge Formation, which is probably Between these two areas of flows, the upper Pyramid Formation is comagmatic with the upper Ice Peak, is also contaminated with crustal primarily clastic and at least partly water-laid. About 100 m of stratified, material, as discussed below. poorly sorted beds of lithic and vitric rhyolite clasts is interlayered with thicker and more uniform beds of pumice and fine ash. These are asso- Pillow Ridge Formation ciated with contorted and fragmented obsidian-rich flows and layers of subangular obsidian cobbles. Locally, the formation rests on polymict till, The Pillow Ridge Formation is confined to Pillow Ridge, a narrow suggesting that the chaotic, vitroclastic assemblage is an ice-contact spur on the northwest flank of Mount Edziza and to an elongate mound deposit. about 200 m high on the adjacent plateau. The lower one-half of the Two Pyramid dates (Table 1, nos. 17 and 18) are from nonhydrated succession on Pillow Ridge is crudely stratified sideromelane tuff-breccia comenditic obsidian and two (16 and 16F) are, respectively, a whole rock containing pillows, pillow fragments, and small globular masses of glassy and a feldspar from porphyritic, holocrystalline trachyte. The 4 dates have basalt in a yellowish-brown matrix of palagonitized, granular glass shards. a narrow range (0.94 to 1.2 Ma) and the weighted average, 1.1 Ma, is The ridge is capped by a mantle of pillow lava and tubular, glass-rimmed considered to be a fairly precise age for the Pyramid activity. pahoehoe toes that plunge steeply west, parallel to the slope. The adjacent mound consists entirely of pillow-like masses with quenched rims and Ice Peak Formation central voids. Both piles are cut by a stockwork of thin feeder dikes. Both pillows and dikes are highly vesicular, with small, spherical The Ice Peak is the oldest formation erupted during the third mag- open vesicles that occur in trains parallel with pillow rims and dike mar- matic cycle. It rests directly on the Pyramid rhyolite but is much more gins. The rock is moderately porphyritic alkali olivine basalt with small extensive and is commonly underlain by till or glacial-fluvial gravel. The phenocrysts of calcic plagioclase, titanaugite, and olivine in a glassy, oxide- period of glaciation during which these deposits were laid down is proba- rich matrix. bly coeval with ice-contact assemblages in the Pyramid Formation. Xenoliths of partly fused granitic and gneissic rock are abundant in Ice Peak rocks include picritic and alkali olivine basalt, hawaiite, the Pillow Ridge pile. They vary in size from single crystals or crystal benmoreite comenditic, and pantelleritic trachyte. Despite this composi- clusters to rounded masses more than 30 cm across and in degree of tional diversity, the formation represents a single composite cone that melting from lenses of frothy glass to subangular inclusions that have developed around a vent or cluster of vents near Ice Peak. The salic end undergone only minor discoloration. In most inclusions, the original members and some of the highly porphyritic hawaiites make up thick framework of white quartz and feldspar crystals is permeated by a network flows of the central edifice, whereas aphyric and moderately feldspar- of glass-lined voids. phyric basalts predominate in the distal parts of the surrounding shield. A single sample (Table 1, no. 8) from the Pillow Ridge Formation There is a transition from aphyric basaltic flows in the lower part of the Ice yielded a whole-rock K-Ar date of 5.9 ± 0.9 Ma. This is obviously too old, Peak Formation to more porphyritic and salic rocks in the upper part. probably due to contamination and introduction of excess argon. An Locally, trachyte and benmoreite flows are interstratified with basaltic independent check on the K-Ar date was made by means of fission-track sequences. Ice Peak activity culminated with an effusion of thick trachyte dating, using apatite from granitic clasts incorporated in the Pillow Ridge flows. basalt. The partly fused clasts obviously were raised far above the blocking Ice Peak basalt is similar to older basalts except that it commonly has temperature for apatite (-100 °C). The resulting dates of 0.9 ± 0.3 and 0.8 open vesicles, rather than calcite- or silica-filled amygdules. Ice Peak ± 0.25 Ma are consistent with stratigraphy. trachyte is similar to the overlying Edziza trachyte, from which it is separated by an erosion surface. The oldest dated sample from the Ice Peak (Table 1, no. 15) (3.7 ± 1.0 Ma) has a large error due to atmospheric argon and consequently is Figure 7. Plot of disregarded. The remaining dates fall into two groups: (13, 14) 2.8 Ma, initial ^Sr/^Sr ver- and (9, 10, 11, 12) between 1.6 and 1.2 Ma. The younger dates are from sus Sr abundance, samples with the highest potassium content and are least susceptible to showing the trend of fractionation or contamination errors. isotopic compositions The Ice Peak Formation is structurally and petrographically complex, of the Edziza suite (dot- and the range of dates may reflect real age differences within the Ice Peak ted line) compared to pile. The discrepancy between the Ice Peak dates, all greater than 1.5 Ma, the fields (shaded) for and concordant dates from the underlying Pyramid, all less than 1.2 Ma, volcanic-arc and ocean- suggests that the Ice Peak data are systematically biased toward older, floor samples. The anomalous results. The presence of excess argon is a likely explanation. feldspar sample (Table Correcting for a common excess of -0.05 * 10"6 cc/gm, which is the same 2, no. SE412166) plots magnitude as the solubility of Ar in magma at 1 atmosphere pressure, outside the diagram. would reduce all of the Ice Peak dates to about 1 Ma and bring them in The two anomalous line with dates from the underlying Pyramid and overlying Edziza samples, Raspberry Formations. 0.7U2 (no. 1) and Ice Peak 0 200 400 600 800 1000 The problem of excess argon may have affected individual dates for Sr, ppm (no. 2), are attributed several older formations, but a comprehensive and systematic bias appears ® Contaminated sample to crustal contamina- to be unique to the Ice Peak. The Ice Peak is the only formation that tion.

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Edziza Formation of the Spectrum Range. It rests mainly on pre-Tertiary basement rocks and, except for the much older Spectrum rhyolite, which it overlies locally, The Edziza Formation includes the composite trachyte cone of and a much younger Big Raven basalt, which overlies it, the Arctic Lake Mount Edziza and satellitic cones around its flank. The composite cone Formation is stratigraphically isolated. It is made up of partly subaqueous developed during the final stages of the third magmatic cycle and largely basalt flows, pyroclastic cones, and small composite cones. The pyroclastic buried the dissected north slope of older Ice Peak. Its circular summit cones include sideromelane tuff-breccia and glass-rimmed pahoehoe toes crater, 2 km in diameter and 2,787 m high, has been breached on its east from 0.5 to 1 m across, and portions of the composite cones are pillowed. side, exposing alternating sequences of explosion breccia and steeply The flows have spread over a large area and are only 2 to 3 m thick, outward-dipping flows from 30 to 90 m thick. Initial dips of as much as suggesting that they are subaerial. They are underlain by thick, basal 30° are present in the composite cone, and satellitic domes are bounded by quenched breccia of glassy blocks in a porous sideromelane ma trix. initial slopes of as much as 60°. The surface of the Arctic Lake flows is striated and covered with Rocks of the Edziza Formation are confined to a narrow range of erratics, kame fields, and glacial outwash deposits. Arctic Lake activity compositions near the pantelleritic trachyte/comenditic trachyte bound- appears to have taken place during a period of relatively thin, partial ice ary. Sparse phenocrysts of anorthoclase and less commonly of sodic cover followed by at least a local glacial advance. On the basis of its glacial ferrohedenbergite are surrounded by a pilotaxitic groundmass of sodic association and degree of dissection, its age is probably close to that of the plagioclase alkali feldspar, arfvedsonite, aenigmatite, kataphorite, and Pillow Ridge and Klastline Formations. A single whole-rock K-Ar date opaques. (Table 1, no. 5) of 0.71 ± 0.05 Ma places it intermediate in age between A single sample (Table 1, no. 6) of Edziza comenditic trachyte the subglacial, Pillow Ridge pile (0.8 to 0.9 Ma) and the subaerial, proba- yielded a whole-rock age of 0.9 ± 0.3 Ma. This is consistent with the dates bly interglacial, Klastline flows (0.62 ± 0.04 Ma). The 0.71-Ma age thus is from overlying and underlying formations. consistent with ice-contact features that are present locally in the largely subaerial Arctic Lake Formation. Arctic Lake Formation Klastline Formation The Arctic Lake basalt, which issued at the beginning of the fourth magmatic cycle, is restricted to the Arctic Lake Plateau on the west flank The principal source of the Klastline intravalley flows is an eroded pyroclastic cone on the northeastern edge of the Edziza trachy :e pile. Lava from this center flowed along Klastline River to its confluence with the Stikine River. Isolated remnants of these flows form smal' intravalley buttes and marginal buttresses underlain by pre-Klastline River gravel. Similar flows issued from a source on the plateau and flooded ':he lowlands south of Buckley Lake. The Klastline flows are highly porphyritic alkali olivine basalts with abundant phenocrysts of calcic plagioclase, titanaugite, and olivine in a groundmass of the same minerals plus disseminated opaques and minor glass. A sample from one of the large remnants in Klastline Valley (Table 1, no. 4) yielded a date of 0.62 ± 0.04 Ma, which is consistent with its stratigraphic position between the Edziza (0.9 Ma) and Kakiddi (0.3 Ma) formations.

Kakiddi Formation

Trachyte of the Kakiddi Formation was erupted at the end of the fourth magmatic cycle. It issued from vents satellitic to the central Edziza 1 /Sr ® Contaminated sample cone, after the cone itself had been deeply dissected. Massive flows of Kakiddi trachyte, as much as 18 m thick, occupy valleys on t rie east side of Figure 8. Plot of ^Sr/^Sr versus 1/Sr, showing the clustering of the Edziza oone. The largest of these originated from a vent near the high-Sr, low-initial-ratio basalts around 0.7028, representative of the 2,787-m summit and flowed eastward to the shores of Kakiddi and magma source. The high ^Sr/^Sr ratios of two samples, Raspberry Nuttlude Lakes at an elevation of 609 m. On the western side of Mount (no. 1) and Ice Peak (no. 2), are attributed to contamination with Edziza, lobes; of Kakiddi trachyte have spread from satellitic cones onto radiogenic crustal Sr. The high 87Sr/86Sr and low Sr of the more the adjacent shield. differentiated end members are probably due to extreme feldspar frac- Kakiddi trachyte is mineralogically similar to trachyt: and comen- tionation and growth of 87Sr during residence of the magma in crustal ditic trachyte of the Edziza Formation. However, the morphology of the reservoirs or to contamination with a low Sr and high ^Sr/^Sr Kakiddi flows, which have traveled as much as 10 km along gently sloping magma that might be produced by extreme fractionation of a hybrid valleys, suggests a more fluid lava. Patches of unconsolidated, steeply magma such as Ice Peak (no. 2). In this diagram, mixing of two dipping trachytic scoria and blocky explosion breccia on the east side of components produces a linear spread of points, with the pure compo- Mount Edziza are believed to be remnants of Kakiddi pyroclastic debris. nents lying at or beyond the ends of the line defined by observed A thick lobe of Kakiddi trachyte on the west side of the complex analyses. The nature of possible components involved in mixing proc- (Table 1, no. 1) yielded concordant whole-rock and feldspar ages of 0.30 esses that might produce the Mount Edziza Volcanic Complex ± .02 Ma and 0.29 ± 0.02 Ma, respectively. A thick intraval'.ey flow on the magmas is indicated in the diagram. east side (Table 1, no. 2) and the cap rock of an adjacent interfluve (Table

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TABLE 2. Sr-ISOTOPE DATA AND Rb/Sr RATIOS OF LAVAS OF THE MT. EDZIZA VOLCANIC COMPLEX

Initial Sample No. Formation Lithology ppm Sr ppm Rb "Rb/"Sr "Sr/"Sr Age "Sr/"Sr

ML270665 Big Raven Basalt 468 32 0.197 0.7029 -0.01 0.7029 SE442065 Kakiddi Trachyte 5.4 103 54.7 0.7036 0.30 ± 0.02 0.7034 ML2902a65 Edziza Trachyte 145 103 2.06 0.7026 0.8 0.7026

SE281967 Ice Peak Trachyte 467 75.5 0.466 0.7029 -1 0.7029 SE3815a67 Ice Peak Mugearite 13.5 82 17.6 0.7087 -1 0.7084 SE2902a66 Pyramid Comendite 0.8 119 408 0.7145 1.1 0.7081

SE 130172 Spectrum Comendite •1.64 185 327 0.720 3.0 ± 0.1 0.7060 SE140272 Spectrum Comendite glass •2.10 190 265 0.718 3.4 ± 0.1 0.7053 SE190265 Armadi1lo Comenditic glass 5.2 136 76.4 0.7111 6.4 0.2 0.7042

ML300765 Armadi1lo Basalt 431 18.5 0.124 0.7029 -6.5 0.7029 SE142166 Raspberry Hawai i te G65 23.7 0.103 0.7033 6.4 ± 0.3 0.7033 SE 142166 Raspberry (Feldspar) 1584 0.5 0.001 0.7027 11.4 * 0.3 0.7027

*Sr concentration determined by isotope dilution

Rb and Sr concentrations were determined by replicate analysis of pressed powder pellets using X-ray fluorescence. U.S. Geological Survey rock standards were used for calibration; mass absorption coefficients were obtained from Mo fox Compton scattering measurements. For concentrations above 20 ppm Rb/Sr ratios have a precision of 2% (1 o) and concentrations a precision of 5X (1 o). At low concentrations the precision is il ppm. Sr isotopic composition was measured on unspiked samples prepared using standard ion exchange techniques. Two samples with very low Sr were spiked with nearly pure ""Sr and the Sr concentrations thus determined by isotope dilution with a precision of 2X {1 o). The mass spectrometer (60° sector, 30 cm radius, solid source) is of U.S. National Bureau of Standards design, modified by H. Faul. Data aquisition is digitized and automated using a NOVA computer. Experimental data have been normalized to a B6Sr/"Sr ratio of 0.1194 and adjusted so that the NBS standard SrCO, (SRM987) gives a B7Sr/86Sr ratio of .71022 ± 2 and the Eimer and Amend Sr a ratio of 0.70800 ± 2. The precision of a single 87Sr/"Sr ratio is 0.00013 {1 o). Rb-Sr dates are based on a Rb decay constant of 1.42 x 10""y-1.

1, no. 3) yielded dates of 0.28 ± 0.02 and 0.31 ± .07 Ma, respectively. The genie Sr in other samples is generated by Rb decay in rocks with very high small error and close clustering of these four dates around 0.3 Ma suggest Rb/Sr ratios and, because the ages of these rocks are known from K-Ar that the Kakiddi activity was a short-lived event. dating, initial ratios can be calculated (Table 2). The initial ratios are clearly variable, suggesting that the volcanic suites have erupted without Big Raven Formation isotopic homogenization or rapid, closed-system fractionation. The extremely low Sr contents of more fractionated rocks are evident Postglacial eruption of Big Raven basalt from clusters of vents on the in Figure 7, which compares Sr abundance and initial isotopic composi- north and south flanks of Mount Edziza and from single vents elsewhere tion of the Edziza suite with volcanic arc and ocean-floor samples of a within the complex produced fields of blocky lava, tephra plumes, and global compilation (Faure and Powell, 1972). The Edziza trend lies out- 87 86 scores of small pyroclastic cones. These deposits are products of the fifth, side the arc field, either with lower Sr/ Sr ratio for samples rich in Sr or and youngest, cycle of activity. with much lower Sr content for samples enriched in radiogenic Sr. The The predominant lithology is fine-grained, vesicular hawaiite with observed pattern is like that of the Rainbow Range, another peralkaline 10% to 20% phenocrysts of calcic plagioclase and lesser pyroxene and central volcanic complex in British Columbia (Bevier, 1981). olivine phenocrysts in a groundmass of calcic plagioclase, clinopyroxene, Radiogenic Sr in low-Sr rocks at the time of eruption can be ex- glass, and opaques. Alkali olivine basalt and picrite occur locally as minor, plained by contamination or by radiogenic accumulation during residence olivine-rich phases. in a magma chamber prior to eruption. Both explanations may apply to Basaltic tephra on the northeast slope of Mount Edziza with a Mount Edziza. 14C date of 1,300 ± 130 yr B.P. probably dates one of the youngest Big Raven Two samples (Table 2, nos. SE142166, SE3815a67) show clear events (Souther, 1971). evidence of contamination with radiogenic, probably crustal, Sr. They are shown with distinctive symbols (Figs. 7, 8, 9) to distinguish them from Sheep Track Formation

Unconsolidated air-fall pumice of the Sheep Track Formation 0.710 blankets an area of about 40 km2 on the south flank of Mount Edziza. It is a completely vitreous, aphyric froth of trachytic composition. Maximum clast size, about 10 cm near the center of the area, decreases rapidly to fine ash around the margins. The pumice has been removed from the channels of intermittent streams on the plateau, but as much as 2 m remains on interfluves. It rests on all Big Raven lava flows and cinder cones on the south slope of Mount Edziza, but it may not postdate Big Raven basalt beyond its areas of deposition. 200 300 400

STRONTIUM ISOTOPIC COMPOSITION 87Rb/86Sr ® Contaminated sample Eleven rocks and one feldspar separate were analyzed for Rb, Sr, and Sr isotopic composition (Table 2).4 The present-day isotopic 87Sr/86Sr Figure 9. Rb-Sr isochron diagram for 11 whole rocks and 1 ratios vary from 0.7026 to 0.720. The basalts, relatively rich in Sr and low feldspar from the Mount Edziza Volcanic Complex. The anomalously 87 86 in Rb/Sr ratio, are for the most part between 0.7026 and 0.7029 and are high Sr/ Sr ratios of two samples, Raspberry (no. 1) and Ice Peak not subject to any correction for in situ decay. Much of the more radio- (no. 2), are attributed to crustal contamination. The increasing 87gr/86gr ra(jos ¡„ more highly differentiated end members may be due to the growth of 87Sr during residence of the magma in crustal 4To secure additional data for this tabular material, request Supplementary reservoirs for periods of 0.7 to 1 Ma or to contamination with Sr- Data 84-6 from the GSA Documents Secretary. depleted but moderately radiogenic material.

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samples for which contamination is less certain or unimportant. The DISCUSSION Raspberry Formation basalt-feldspar pair is discordant (Table 2, no. SE142166); the matrix rock is more radiogenic, yet lower in Sr. The Alkaline basalt, trachyte, and oversaturated peralkaline lavas in the difference cannot be explained by age or error. The rock has acquired Mount Edziza Complex have erupted episodically for the past 3 m.y. The radiogenic Sr between the time of feldspar crystallization and eruption. tectonic environment during this time was probably similar to that at Mugearite from the Ice Peak Formation (Table 2, no. SE3815a67) is present, with right-lateral movement of about 5.5 cm/yr between the likewise much too enriched in radiogenic Sr to be explained by any Pacific oceanic plate and the North American continent along north- process other than contamination with crustal Sr. In the Ice Peak Forma- westerly trending transcurrent faults of the Queen Charlotte system tion, excess radiogenic Sr (figs. 6, 7) is associated with excess Ar, a prob- (Riddihough, 1911), coupled with weak convergent motion of about 1 to lem that is most serious and virtually unique to the Ice Peak Formation. 2 cm/yr (Hyndman and Ellis, 1981). The Mount Edziza Volcinic Com- Whatever process supplied the radiogenic Ar may also have added radio- plex lies within the continental plate, about 300 km east of this transform- genic Sr and contributed to the distinctive chemical character of the Ice oblique convergent boundary. Its association with north-trending normal Peak rocks. Hybridization with fractionation, rather than closed-system faults and half-grabens suggests that it evolved in an environment of crustal fractionation, is indicated. extension, possibly in response to shear along the plate boundary (Souther, The high-Sr, low-initial-ratio basalts and the feldspar separate indi- 1977). Crustal reservoirs are commonly formed in such environments and cate a mafic magma source with >450 ppm Sr and 87Sr/86Sr approxi- the decreasing ascent velocity of magma passing into and through them mately 0.7028 ± 0.0001. These values are typical of mantle sources, in the favors fractionation by crystal settling. The same process may account for range observed for ocean-ridge basalts, and somewhat lower than the the virtual absence of megacrysts and lherzolite nodules in the Mount values reported for plume-type sources (Hart and Brooks, 1981). Low Sr Edziza Complex, in contrast to their abundance in small piles of unfrac- in more differentiated rocks is usually attributed to removal of large tionated basalt elsewhere in the Stikine Volcanic Belt. amounts of feldspar by fractional crystallization (Bevier, 1981). This is Prolonged crystal fractionation of mildly alkaline basalt in crustal required to achieve the 87Rb/86Sr ratios that exceed 100. reservoirs is the most widely accepted model for the generation of oversat- In rocks with 87Rb/86Sr ratios greater than 50, the growth of 87Sr is urated peralkaline rocks (Noble, 1968; Barberi and others, 1975; Bevier, relatively rapid. The isochron diagram (fig. 9) suggests that radiogenic 1981). The stratigraphy of the Mount Edziza Complex suggests that it is Sr-enrichment in all samples, except the two already discussed, could be the product of several cycles of magmatic activity, each of which began produced in time intervals of 0.7 to 1.0 Ma. If no contamination has with basalt and culminated with a saturated or oversaturated alkaline or occurred, these times would be effective pre-eruption residence times for peralkaline magma. The Sr data indicate a mantle source (87Sr/86Sr ap- the respective magmas. The actual residence times would be greater if the proximately 0.7028 ± 0.0001) and Sr concentrations support the idea that magma were undergoing progressive fractionation and increase in Rb/Sr the salic rocks evolved from alkali basalt by crystal fractionation in crustal ratio while awaiting eruption. Comparable initial isochrons of 0.6 to 0.9 reservoirs. Such a model, involving extensive feldspar fractionation, would Ma have been observed for other young peralkaline volcanoes in British account for the low Sr content and high Rb/Sr ratios of the salic end Columbia (Hoodoo Mountain, unpub. data; Rainbow Range, Bevier, members. The increase in 87Sr/86Sr initial ratios in these high Rb/Sr 1981) and elsewhere (Gibson, 1972). The similar results for volcanoes magmas may have developed during prolonged residence (~ 1 m.y.) of the sitting on different types of crust lends favor to the idea that a common fractionating magma in crustal reservoirs. process, residence of high Rb/Sr magmas in the crust prior to eruption, The first magmatic cycle, which produced the Raspberry basalt, the may be in effect. Little Iskut trachybasalt, and the Armadillo salic rocks, occurred between The alternate view, that the radiogenic additions are simply contami- 8 and 6 Ma. Part: of the initial magma batch rose rapidly from a mantle nation, is suggested in Figures 7 and 8. According to Faure (1977), simple source into crustal reservoirs; excess magma was erupted, forming the end-member mixing should produce a hyperbola on 87Sr/86Sr versus Sr lower, aphyric, alkali-olivine basalt. Fractionation of magma in the reser- and a straight line on 87Sr/86Sr versus 1/Sr. The low-Sr Edziza rocks voirs was controlled first by the separation of olivine, titanaugite, and follow the expected patterns but require an unusual contaminant, one that plagioclase, which drove the residual liquid toward hawaiite. Continued is fairly radiogenic (87Sr/86Sr > 0.7081) and very low in Sr (<1 ppm). No removal of large volumes of plagioclase, with Na/K ratios higher than that familiar crustal rock would qualify, but the crust-contaminated Ice Peak of the liquid, produced a trend of K enrichment (the "plagioclase effect" of magma, after further extreme fractionation to give the low Sr content, Bowen, 1945) toward trachybasalt composition. During this stage of pla- would provide a possible contaminant for the rock series. It seems rather gioclase fractionation, the reservoirs were periodically tapped, yielding the contrived to have such material repeatedly available or repeatedly created feldspar-phyric flows of upper Raspberry hawaiite and relatively K-rich during the long history of the volcanic complex, but that possibility cannot Little Iskut trachybasalt. Subsequent fractionation of the trachytic: residual be ruled out. liquid was dominated by separation of K-feldspar with an Na/K ratio The radiogenic initial Sr enrichments observed in Edziza rocks are lower than that of the melt, thus leading toward a residual liquid enriched probably of complex origin, in part direct contamination, in part contami- in Na and total alkalies relative to A1 (the "orthoclase effect" of Bailey and nation during fractionation, in part the result of magma mixing, and in part Schairer, 1964), as well as enrichment in silica. The resulting Na-rich salic the result of Rb decay in materials with high Rb/Sr ratio during residence melt produced the voluminous, oversaturated peralkaline flows of the in magma chambers for periods of as much as 1 Ma. Some of these Armadillo Formation, which concluded the first cycle of magmatic complexities were recognized by Stettler and Allegre (1979) in their study activity. of a slightly older alkaline volcano in France. The second magmatic cycle, which includes the Nido and Kounugu

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basalts and the oversaturated salic rocks of the Spectrum and Pyramid ACKNOWLEDGMENTS Formations, occurred between 3.5 and 1 Ma. The evolution from basalt through trachyte to oversaturated peralkaline rocks followed much the R. K. Wanless and R. D. Stevens provided the dates determined by same course as the first magmatic episode, except that no transitional the Geological Survey of Canada and provided helpful interpretation of rocks, equivalent to the K-rich, Little Iskut trachybasalt, were erupted. the results. K. Scott performed the Ar and Sr analyses. The third magmatic cycle differs from the first two in both duration We thank Dr. R. B. Campbell and Dr. G. J. Woodsworth for their and heterogeneity of eruption products. It includes the Ice Peak, Pillow comments. Critical reviews by Peter Lipman and Julie Donnelly-Nolan Ridge, and Mount Edziza Formations, which were erupted during a short are sincerely appreciated. Their constructive criticism greatly improved the interval between about 1 and 0.8 Ma. Unlike both earlier and later cycles, original manuscript. this activity produced a significant volume of transitional lavas (mugearite, benmoreite, trachyte). Basalts of the Ice Peak Formation are the only rocks in the Mount Edziza Complex in which lherzolite nodules were found, and REFERENCES CITED the Pillow Ridge basalt is the only lava that has brought a large volume of Armstrong, R. L., 1978, K-Ar dating: Late Cenozoic McMurdo Volcanic Group and dry valley glacial history, Victoria Land, Antarctica: New Zealand Journal of Geology and Geophysics, v. 21, p. 685-698. crustal inclusions to the surface. Strontium isotope data indicate crustal Aumento, F., and Souther, J. G., 1973, Fission-track dating of late Tertiary and Quaternary volcanic glass from the Mount Edziza Volcanic Complex, British Columbia: Geological Association of Canada, v. 10, no. 7, p. 1 156 1163. contamination of the Ice Peak rocks, and a bias toward inconsistently old Bailey, D. K., and Schairer, J. F., 1964, Feldspar-liquid equilibria in peralkaline liquids—The onhoclase effect: American K-Ar dates shows that excess radiogenic argon is also present. The lherzo- Journal of Science, v. 262, p, 1198-1206. Baksi, A. K., 1974, Isotopic fractionation of a loosely held atmospheric argon component in the Picture Gorge Basalts: lite nodules indicate that some Ice Peak basalts rose rapidly from the Earth and Planetary Science Letters, v. 21, p. 431-438. Barberi, F., Santacroce, R., Ferrara, G., Treuil, M., and Varet, J., 1975, A transitional basalt-pantellerite sequence of mantle without passing through a crustal reservoir. The hybrid nature of fractional crystallization, the Boina Centre (Afar Rift, Ethiopia): Journal of Petrology, v. 16, p. 22-56. the Ice Peak Formation suggests that it may be the mixing product of both Bevier, M. L., 1981, The Rainbow Range, British Columbia: A Miocene peralkaline shield volcano: Journal of Volcanol- ogy and Geothermal Research, v. 11, p. 225-251. new basaltic magma rising from a mantle source and older, partly frac- Bowen, N. L., 1945, Phase equilibria bearing on the origin and differentiation of the alkaline rocks: American Journal of Science, v. 243-A, p. 75-89. tionated magma that was still residing in crustal reservoirs. The chemically Dalrymple, G. B., 1969,4l,Ar/'lflAr analyses of historic lava flows: Earth and Planetary Science Letters, v. 6, p. 47-55. uniform and isotopically uncontaminated trachyte of the Mount Edziza Dalrymple, G. B., and Moore, J. G., 1968, Argon 40 excess in submarine pillow basalts from Kilanea Volcano, Hawaii: Science, v. 161, p. 1132-1135. Formation was probably derived by fractionation of basalt during the third Faure, G., 1977, Principles of isotope geology: New York, Wiley, 464 p. Faure, G., and Powell, J. L., 1972, Strontium isotope geology: Berlin, Heidelberg, New York, Springer, 188 p. magmatic cycle. Fisher, D. E., 1971, Excess rare gases in a subaerial basalt in Nigeria: Nature, Physical Science, v. 232, no. 29, p. 60-61. Gibson, [. L., 1972, The chemistry and pedogenesis of a suite of panteilerites from the Ethiopian Rift: Journal of The fourth magmatic cycle, which began with Arctic Lake and Petrology, v. 13, p. 31-44. Klastline basalt and culminated with the Kakiddi trachyte, spanned an in- Hamilton, T. S., 1981, Late Cenozoic alkaline volcanics of the Level Mountain Range, northwestern British Columbia: Geology, petrology and paleomagnetism [Ph.D. thesis]: Edmonton, Alberta, University of Alberta. terval of 0.7 to 0.3 Ma. The petrology and K-Ar data from these rocks Hart,S. R., and Brooks, C„ 1981, Sources of terrestrial basalts: Isotopic characteristics, in Basaltic volcanism study project: Basaltic volcanism on the terrestrial planets: New York, Pergamon Press, p. 987-1014. support the simple crystal fractionation model proposed for episodes 1 and Hyndman, R. D., and Ellis, R. M., 1981, Queen Charlotte fault zone: Microearthquakes from a temporary array of land 2, but the volume of magma is smaller and the period available for frac- stations and ocean bottom seismographs: Canadian Journal of Earth Sciences, v. 18, no. 4, p. 776-788. Irvine, T. M., and Baragar, W.R.A.. 1971, A guide to the chemical classification of the common volcanic rocks: Canadian tionation is shorter. Journal of Earth Sciences, v. 8, no. 7, p. 523-548. Krummenacher, Daniel, 1970, Isotopic composition of argon in modern surface volcanic rocks: Earth and Planetary The fifth and final cycle, which includes alkali olivine basalt and Science Letters, v. 8, p. 109-117. Macdonald, R., 1974, Nomenclature and petrochemistry of the peralkaline oversaturated extrusive rocks: Bulletin volca- hawaiite of the Big Raven Formation and trachytic pumice of the Sheep nologique, vol. 38, no. 3, p. 498- 547. Track Formation, involves a very small volume of magma, erupted within Noble, D. C., 1968, Systematic variation of major elements in comendite and pantellerite glasses: Earth and Planetary Science Leuets, v. 4, p. 167-172. the last few thousand years. If these magmas are genetically related, they Noble, D. C., and Parker, D. F., 1974, Peralkaline silicic, volcanic rocks of the western United States: Bulletin volcanolo- gique, v. 38, no. 3, p. 803-827. must have fractionated rapidly. It is possible that the fourth and fifth cycles Parrish, R. R., 1982, Cenozoic thermal and tectonic history of the Coast Mountains of British Columbia as revealed by may be part of a larger cycle that has not yet culminated. fission track and geological data and quantitative thermal models [Ph.D. thesis]: Vancouver, University of British Columbia, 166 p. In its eruption sequence and style, mineralogy, chemistry, and iso- Riddihough, R. P., 1977. A model for recent plate interactions off Canada's west coast: Canadian Journal of Earth Sciences, v. 14, p. 384-396. topic composition, the Mount Edziza Complex is similar to other areas of Rutford, R. H., Craddock, Campbell, Armstrong, R. L., and White, C. M., 1972, Tertiary glaciation in the Jones continental peralkaline volcanism, such as those of the Afar depression Mountains, in Adie, R. J,, ed., Antarctic geology and geophysics: Oslo, Universitetsforlaget, p. 239-243. Souther, J. G., 1971, Telegraph Creek map-area, British Columbia: Geological Survey of Canada Paper 71-44, 38 p. (Barberi and others, 1975), parts of the Great Basin of the western United 1977, Volcanism and tectonic environments in the Canadian Cordillera—A second look, in Baragar, W.R.A., Coleman, L. C, and Hall, J. M., eds.. Volcanic regimes in Canada: Geological Association of Canada Special States (Noble and Parker, 1974), and the Rainbow Range of central Paper, v. 16, p. 3-24. British Columbia (Bevier, 1981). A within-plate or extensional tectonic Souther, J. G., and Symons, D.T.A., 1974, Stratigraphy and paleomagnetism of Mount Edziza Volcanic Complex, northwestern British Columbia: Geological Survey of Canada Paper 73-22, 48 p. environment that favors the rise of mildly alkaline basalt from the mantle Stettler, A., and Allegre, C. J., 1979, Rr/ Sr constraints on the genesis and evolution of the Cantal continental volcanic system (France): Earth and Planetary Science Letteis, v. 44, p. 269-278. into crustal reservoirs is common to these peralkaline terranes as well as to Yagi, K., and Souther, J. G„ 1974, Aenigmatite from Mount Edziza, British Columbia, Canada: American Mineralogist, the Mount Edziza Complex. Large, long-lived reservoirs apparently are v. 59, p. 820-829.

required to sustain the prolonged fractionation needed to derive an over- MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 28, 1982 REVISED MANUSCRIPT RECEIVED APRIL 8, 1983

saturated peralkaline end member from parental basalt. MANUSCRIPT ACCEPTED APRIL 19, 1983

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