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Eruptive History and K-Ar Geochronology of the Late Cenozoic Garibaldi Volcanic Belt, Southwestern British Columbia

Eruptive history and K-Ar geochronology of the late Cenozoic Garibaldi , southwestern

NATHAN L. GREEN Department of Geology, University of Alabama, Tuscaloosa, Alabama 35487 cH u^ n h ^RMSTRONG 1 Department of Geological Sciences, University of British Columbia, , British Columbia, Canada V6T2B4 J. E. HARAKAL I J. G. SOUTHER Geological Survey of Canada, 100 W. Pender, Vancouver, British Columbia, Canada V6B1R8 PETER B. READ Geotex Consultants Limited, Suite 1200,100 W. Pender, Vancouver, British Columbia, Canada V6B1R8

ABSTRACT Fuca- configuration to about 40% by volume. The smaller volcanic along the . centers erupted mainly with only rare The Cenozoic Garibaldi belt comprises 6 . are generally restricted to the volcanic fields spaced at irregular intervals INTRODUCTION western margin of the volcanic belt, where they along an axis extending 240 km north- occur as valley-filling or as isolated flows northwest from the head of to The development of the Garibaldi volcanic that cap ridges. Geophysical and petrochemical the Bridge River area; 2 additional fields, the belt, a continuation of the High Cascades of the data suggest that the lavas of the Garibaldi vol- Franklin Glacier and Silverthrone fields, lie western United States, is closely linked to the canic belt were derived from fractionated liquids 140 and 190 km west of the north end of the late to Quaternary history of south- initially in equilibrium with mantle peridotite main volcanic belt. The volcanoes erupted western British Columbia (Souther, 1977). This above subducted oceanic lithosphère (Anderson, lavas ranging in composition from augite- belt of 6 volcanic fields trends subparallel to and 1975; Riddihough and Hyndman, 1976; Keen olivine , through hypersthene andesite, about 250 km inland from the convergent Juan and Hyndman, 1979; Lawrence, 1979; Green, hornblende andesite, and hornblende-biotite de Fuca-North American plate boundary. The 1981). andesite, to biotite . Many of the belt extends 240 km north-northwest from The volcanic fieldsar e underlain by the Coast volcanic complexes are characterized by Watts Point on Howe Sound to Salal Glacier. Crystalline Complex (Roddick and Hutchison, geomorphic features which indicate complex Two additional fields, which lie inland from the 1972). Isolated septa of metamorphic rocks in- interactions between and the Pleis- Explorer-North American plate boundary, are cluding metagraywacke, metaconglomerate, tocene ice sheets, but preglacial and postgla- 140 and 190 km west-northwest of the northern greenstone, crystalline limestone, quartz-mica cial phases are also present. end of the main volcanic belt (Fig. 1). schist, amphibolite gneiss, and migmatite are Whole-rock samples from 18 volcanic In general, the largest volcanic complexes are aligned along the persistent north-to-northwest complexes have been dated by the K-Ar stratovolcanoes built up of flows and pyro- structural trend. These septa are enclosed in method. Most of the results are internally clastic deposits, and the smaller volcanoes well-foliated to massive quartz , horn- consistent with stratigraphic relationships comprise single isolated flows to complex multi- blende-biotite granodiorite, and quartz monzo- and with limited I4C and paleomagnetic data. ple domes, clusters of pyroclastic cones, spines, nite that range in age from to They suggest that volcanic activity was epi- , and other subglacial forms. Most of the (Stephens, 1972; Mathews, 1958,1972; Woods- sodic; most of the analyzed andesitic and da- volcanoes occur in groups of three to nine and worth, 1975,1977; Roddick and Woodsworth, citic lavas were erupted in the intervals 2.3 to constitute (1) the Watts Point, , 1976). 1.7 Ma and 1.1 Ma to present in the northern and fields in the south, (2) the Garibaldi belt eruptive activity spanned sev- part and in the intervals 1.4 to 1.0 Ma and 0.7 , , and Salal Glacier eral Quaternary glacial periods. Armstrong and Ma to present in the southern part of the belt. volcanic fields in the center, and (3) the Franklin others (1965) recognized that to Basaltic volcanism occurred only during the Glacier and Silverthrone volcanic fields in the Holocene glacial deposits of southwestern Brit- past 0.15 m.y., except in the Salal Glacier area north (Fig. 1). ish Columbia and northwestern Washington where hawaiite and alkali-olivine basalt, The volcanoes erupted lavas ranging in com- represent two major glacial periods: Salmon which are perhaps an "edge effect" related to position from augite-olivine basalt, alkali-olivine Springs Glaciation (>50,000 yr B.P.) and Fraser of the , were basalt, and hawaiite, through hypersthene ande- Glaciation (10,000-26,000 yr B.P.), separated erupted as early as 0.97 Ma. The timing of site, hornblende andesite, and hornblende-bio- by a nonglacial interval (Olympia Interstade). Garibaldi belt volcanism provides informa- tite andesite, to biotite rhyodacite. The majority Little is known concerning the extent and timing tion bearing on the distribution of pre- of stratovolcanoes consists of weakly porphyritic of earlier Pleistocene glaciations in southwestern Wisconsin glaciers in southwestern British hornblende-biotite dacite and rhyodacite, but in- British Columbia. Columbia and constrains interpretations of tercalated or underlying andesite is commonly This paper describes the stratigraphic rela- late Cenozoic changes in Explorer-Juan de present in amounts ranging from a few percent tions of Garibaldi belt volcanoes, evidence for

Geological Society of America Bulletin, v. 100, p. 563-579,7 figs., 6 tables, April 1988.

563

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BRITISH COLUMBIA

48°

Figure 1. Tectonic setting of Garibaldi volcanic belt in southwestern British Columbia. Major Pliocene-Quaternary volcanic fields indicated by solid dots. Stars represent the locus of the Alert Bay volcanic belt (Bevier and others, 1979; Armstrong and others, 1985). Stippled area indicates the general trend of the Miocene Pemberton volcanic belt at 12-22 Ma; ruled area indicates possible trend of the Pemberton volcanic belt at 6-12 Ma (Souther, 1977; Berman and Armstrong, 1980). Dotted line is the subcrusta! projection of the Nootka trend under southwestern British Columbia. Arrows indicate late Cenozoic directions of plate motion (Riddihough, 1977).

lava-glacial ice interaction, and the results of an hornblende or biotite) that could be separated graphic relationships, the samples were reana- ongoing K-Ar study of the volcanic products. for dating. Where present, modal hornblende lyzed with any visible, coarse crystalline material The timing of Garibaldi belt volcanism provides and biotite are commonly mantled by reaction (primarily plagioclase crystals) deliberately re- information bearing on the distribution of pre- rims or partly replaced by anhydrous break- moved to reduce possible contamination. The Wisconsin glaciers in southwestern British Co- down products. Methods used in the K-Ar de- presence of excess radiogenic argon in the crys- lumbia and constrains interpretations of late terminations have been described previously talline fraction of some lavas is suggested by Cenozoic changes in Explorer-Juan de Fuca- (White and others, 1967; Armstrong and others, K-Ar dates obtained for two Meager Creek North American plate configuration along the 1985). Results are given in Table 1, where errors (Table 1, samples MM293 and continental margin. quoted are one standard deviation. MM353). The groundmasses of these lavas yield The K-Ar dates on samples from individual K-Ar dates which are 3 to 5 times lower than K-Ar DATING volcanoes agree well with one another. The those obtained from plagioclase separates and concordancy of ages from each volcano with which are consistent with dates for overlying 40K-40Ar analyses of 43 unaltered, hyaloha- observed stratigraphic relationships and limited and underlying stratigraphic units (see below). line to hypocrystalline rocks representative of 14C and paleomagnetic data, together with vari- distinct stratigraphic units from 18 complexes of ations in the K content of dated rocks, is taken RANGE OF K-Ar DATES the Garibaldi volcanic belt have been com- to indicate that the problems of incorporation of pleted. A majority of the Garibaldi belt volcanic excess radiogenic argon are minimal. A few ex- K-Ar dates of samples from the Garibaldi rocks are only weakly porphyritic (generally ceptions are noted at appropriate places in the volcanic belt generally range from 2.3 to less <15% phenocrysts), and none contains sufficient text. In cases in which such problems were sug- than 0.1 Ma (Fig. 2). These dates, coupled with amounts of K-rich mineral phases (<2% modal gested by a K-Ar date discordant with strati- limited 14C and paleomagnetic data, suggest that

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SILVERTHRONE VOLCANIC FIELD Figure 2. Diagram showing the dis- FRANKLIN GLACIER VOLCANIC FIELD tribution of isotopic dates along the Garibaldi volcanic belt from north to NM i -RM SALAL GLACIER south and the subducted oceanic plates VOLCANIC FIELD *t currently beneath the volcanic fields of MEAGER CREEK the belt. Available paleomagnetic data VOLCANIC FIELD •fr ri. (Mount Garibaldi field, Thompson, ELAHO VALLEY 1968; Salai Glacier field,Lawrenc e and VOLCANIC FIELD others, 1984) are shown as follows: MOUNT CAYLEY 7* RM, reversed magnetic polarity; NM, VOLCANIC FIELD normal magnetic polarity. 14C data from ¡a, Read (1979), Green (1981), and Low- den and Blake (1975, 1978). Units ex- hibiting either ice-contact (i) features GARIBALDI LAKE or overlying till (t) are also indicated; VOLCANIC FIELD where possible, age of ice-contact units has also been estimated on basis of field relations if K-Ar dates are un- available. Samples which have yielded anomalous K-Ar dates that have been MOUNT GARIBALDI VOLCANIC FIELD shown to reflect analytical problems (1.2 Ma, MG465, Mount Garibaldi WATTS POINT -CD- field; 2.7 and 3.8 Ma, SE1516A79, VOLCANIC FIELD t I I 1 I I I I I I I I I ) I I I 1 I I I I I I I Mount Cayley field; 0.7 Ma, RdA74- 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 23-3, Meager Creek field) or inconsis- MILLION YEARS tencies with observed field relations

14 (1.0 and 1.1 Ma, 77-1-7B, Silverthrone K-Ar DATE POSTGLACIAL C DATE field) have not been plotted. # basalt basalt A basalt O A dacite/rhyodacite Ä basaltic andesite • andesite il andesite • dacite/rhyodacite @ dacite/rhyodacite

there has been episodic volcanic activity in the volcanic belt coincides with the subcrustal pro- in order of occurrence from the southern to Garibaldi volcanic belt for the past 2.5 m.y. In jection of the Nootka fault system (Fig. 1; northern limits of the belt. Volcanic complexes particular, most of the analyzed andesitic and Green, 1981). Lawrence and others (1984) sug- within each of these fieldsar e described individ- dacitic rocks were erupted in the intervals 2.3 to gested that basaltic volcanism in the Salal Gla- ually in terms of eruptive history and geomor- 1.7 Ma and 1.1 Ma to present in the northern cier area may be related to processes within the phic development. Where possible, the devel- part and in the intervals 1.4 to 1.0 Ma and 0.7 mantle causing generation of alkalic melts near opment of individual complexes has been Ma to present in the southern part of the belt the edge of the subducted Juan de Fuca plate. separated into stages, defined by the presence of (Table 1). The length of individual eruptive epi- The late appearance of basaltic lavas along the erosional discontinuities and(or) glacial deposits sodes at each volcanic center varied from single remainder of the Garibaldi volcanic belt may within the volcanic succession and(or) by vol- eruptive events to time intervals as long as 0.5 reflect changes in the conditions of gene- canic products that originated from different m.y. Products of the earliest eruptive episode sis adjacent to the subducted oceanic slab be- vents or groups of vents. Stratigraphic relations appear restricted to the northern end of the vol- neath southwestern British Columbia concurrent and morphologic features of the volcanic com- canic chain (Meager Creek and Franklin Glacier with decreasing Juan de Fuca-North American plexes are summarized in Tables 2-6, and their fields). The younger period of volcanic activity plate convergence rates during the past 0.5 m.y. location and extent within the different fieldsar e occurred along almost the entire belt, although (Riddihough and Hyndman, 1976). indicated in Figures 3-7. there may be a trend of decreasing age when volcanism was initiated at different volcanic cen- ERUPTIVE HISTORY AND Watts Point Volcanic Field ters from north to south (Fig. 2). GEOMORPHIC DEVELOPMENT Extrusion of andesitic and dacitic OF VOLCANOES Hornblende dacite flows occupy a small was accompanied by contemporaneous basaltic semicircular depression on Watts Point, about volcanism within the past 0.15 m.y. along most Each volcano of the Garibaldi belt has a 50 km north of Vancouver (Fig. 1). The flat- of the Garibaldi volcanic belt. Older (1.0 to 0.5 unique eruptive history and geomorphic devel- lying, columnar-jointed lavas were ponded Ma) basaltic lavas occur only in the Salal Gla- opment. Eruptions were not strictly synchronous against a thick deposit of glaciofluvial sediments cier area, where the N30°W trend of the along the belt, and the volcanic products are (Table 2). These poorly stratified sediments, ex- volcanic axis shifts gradually to a north-south spatially distinct. In the following sections, the posed in a quarry on the western shore of the orientation and the northern limit of the main eight Garibaldi belt volcanic fields are discussed point, mark the southern boundary of the lava-

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filled basin and possibly represent part of a TABLE 1. K-Ar DATA FOR ROCKS OF GARIBALDI VOLCANIC BELT raised delta or kame terrace developed during No. retreat of a continental ice sheet from Howe Sample Rock type Location Analysis %K «Ar« %"»Ar' Age (Ma) Sound. A feeder dike for thick pyroxene da- Watts Point volcanic field (Fig. 1)

cite flows that cap the lava succession intruded 1 WP-1 Dact. 49°39.

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TABLE 1. (Continued) structed the Glacier Pike dome on the northern flank of the Mount Garibaldi edifice. While the No. Sample Rock type Location Analysis %K «Ar* Ar» Age (Ma) still stood above the 1,158

MeagtT Creek volcanic field (Fig. 6) m level, subglacial extrusion of basaltic andesite 26 MM160 Dact. stnsff WR 1.49 0.1118 11.4 1.9 ±0.2 formed a drumlin-shaped mass within Eeno- 123°33.5' stuck Meadow, approximately 7 km southeast 27 MM136B Andt. 50°35.r WR 1.12 0.0452 8.5 1.0 ±0.1 123°32.r of Atwell Peak. Laterally restricted basaltic an- 28 RdA74-23-3 Andt. 50°35.1' WR 1.05 0.0307 2.1 0.7 ±0.4 123°32.r desite and pyroxene andesite flows extend down 29 MM81 Andt. 5C35.2' WR 2.13 0.0438 4.4 0.5 ±0.1 the south flank of Paul Ridge and terminate 123°31.5' 30 MM236 Andt 50°37.r WR 1.38 0.0483 3.2 0.9 ±0.2 abruptly at the 762 m level, suggesting that the 123°34,3' 31 64110-V Andt. 50°37.3' WR 1.00 0.0846 10.1 2.2 ±0.1 lavas flowed southward onto ice that occupied 123°35.6' Ring Creek and Mamquam River valleys. WR 1.00 0.0773 12.6 2.0 ±0.1 32 MM293 Rydc. 50°37.8' WRM 1.46 0.0055 0.46 0.10 ± 0.02 Opal Cone Complex. The final episode of 123°30.9" PL 0.51 0.0099 2.0 0.5 ±0.3 volcanism in the Mount Garibaldi volcanic field 33 MM435 Rydc. 50°38.4' WR 2.50 0.0148 1.3 0.15 ± 0.10 123°31.8' occurred at Opal Cone, near the southern mar- 34 MM353 Rydc. 50°39.2' WRM 2.28 0.0097 5.6 0.11 ±0.02 gin of Garibaldi Neve, about 3 km southeast of 123»31.r PL 0.85 0.0093 0.6 0.3 ±0.3 Atwell Peak (Fig. 3). The Ring Creek lava, 35 RdR74-4B Bslt 50°40.2' WR 1.03 0.0037 1.3 0.09 ± 0.06 123°34.7' which issued from the cone, flowed southward Salai Glacier pluton (Fïg. Ij within Ring Creek valley and then extended

36 Rd74-44210M Msgt. 50°45.5' MS 7.04 2.287 16.0 8.3 ±0.5 westward within the glaciated Mamquam River 123°24.6' valley (Fig. 3). As the lower half of the Ring Salal Glacier volcanic field (Fig. 1) Creek lava shows no evidence of glacial erosion 37 C5-5 Aob. 50°47.0' WR 0.92 0.0210 15.6 0.59 ± 0.05 or glacial ponding, the Opal Cone eruptions 123°22.6' 38 M5-14 Hwtt 50°47.9' WR 1.21 0.0456 24.3 0.97 ± 0.05 probably occurred after the Late Wisconsin 123°23.5' (Fraser Glaciation) ice sheet had disappeared Franklin Glacier volcanic field (Fig. 1) from the vicinity of the flow terminus (Mathews, 39 Rd7M61S5 Qzmt. 51°17,4' BT 6.81 1.852 50.7 7.0 ±0.2 125°24.0' 1958). The presence of Mazama ash within a 40 Rd7646156 Dact 51°17.4' BT 5.77 0.909 26.2 4.0 ±0.1 succession of limaic peat resting on glacial drift 125°24.0' 41 JR10-61A Daa 51°21.5' BT 6.99 0.5874 19.2 2.2 ±0.1 near the westernmost extension of Brohm Ridge 12S°35.5' indicates that the ice sheet had evacuated Siiverthrone volcanic field (Fig. 7) the Mount Garibaldi area before 6,670 yr ago 42 77-1-1 Andt 51°24.5' WR 1.42 0.0211 4.2 0.4 ±0.1 126° 13.3' (Mathews, 1972). 43 77-1-4C Rhyt. sra.o- WR 3.74 0.1125 26.9 0.77 ± 0.08 126°18.0- 44 77-1-7B Band. 51-33.3' WR 0.85 0.0322 5.0 1.0 ±0.2 126°21.5' Garibaldi Lake Volcanic Field WRL 1.28 0.1911 17.2 1.1 ±0.1 Quaternary activity occurred at nine eruptive Note: sample numbers keyed to Figures 3-7, except where indicated: 1, Watts Point field; 25, Elaho Valley flow: 36, Salal pluton; 37-38, Salal Glacier field; 10 10 4 centers in the Garibaldi Lake area (Fig. 4). 39-41, Franklin Glacier field. Decay constants used in calculation: \t =• 4.96 x 10" yr'. ^ - 0.581 x 10~ yr , and ^fc/K = 1.167 x 10" ; errors are one standard deviation. Estimated precision of K contents is 2%. Analyst: K. L. Scott for K, J. E. Harakal for Ar. Analyzed samples: WR, whole rock; PL, plagioclase; BT, Eruptions generally produced small andesite biotite; MS, muscovite; WRM, whole-rock matrix after removal of visible plagioclase; WRL, acid-leached whole rock. Rock types: Bslt., hy-normative basalt; Aob., Alkati-olivine basalt; Hwtt., hawaiite; Band., basaltic andesite; Andt., andesite; Dact, dacite; Rydc., rhyodacite; Rhyt, rhyolite; Qzmt., quartz monzonite; (Mount Brew, The Table) and dacite (Tricouni, Msgt, muscovite , Ar* refers to radiogenic An all values of are x 10"6 cm3g-1 STP- Cheakamus River) complexes, but two andesitic cones ( and Mount Price) were constructed during multiple stages of activity. which the Cheekye River incised a deep valley opened north of the Atwell Peak plug. This Basaltic andesite lavas were erupted at The into the western flank of the Cheekye cone. third, or Dalton Dome, stage produced a , about 2 km east of The Black Volcanism began at Atwell Peak during the hornblende-biotite dacite lava that flowed Tusk, and at the Sphinx Moraine complex on waning stage of the , while westward down the landslide scar which trun- the eastern shore of Garibaldi Lake. Basaltic the Cordilleran ice sheet still filled the Cheekye cated more gently dipping tuff breccias of the eruptions produced a sequence of columnar River basin. This middle, or Atwell Peak, stage Atwell Peak cone (Fig. 3). Partial destruction of flows within the Cheakamus River and Cal- of Mount Garibaldi activity was characterized this flow by additional collapse of the tuff brec- laghan Creek valleys and a pyroclastic cone with by Pelean eruptions that produced a supraglacial cias on which it lies suggests that the lava was associated lava flow at The Cinder Cone (Table tuff breccia cone surrounding the funnel-shaped extruded not long after withdrawal of the ice 3). No stratigraphic continuity exists between plug dome that forms Atwell Peak (Fig. 3; sheet (Mathews, 1952a). volcanic products of the Garibaldi Lake centers. Mathews, 1952a). Melting of the ice tongue that Glacier Pikes, Paul Ridge, and Eenostuck Tricouni Complex. The Tricouni complex filled the Cheekye River basin led to the collapse Complexes. Three complexes formed possibly (Fig. 4) apparently was erupted from a vent lo- of the western flank of the Atwell Peak cone. contemporaneously with Atwell Peak and Dal- cated about 6 km west of Daisy Lake. Pervasive After most of the ice sheet within the Cheekye ton Dome eruptive activity at Mount Garibaldi alteration of these dacitic flows and lithic tuffs River valley had disappeared, a new vent (Fig. 3). Eruptions of hornblende dacite con- makes them unsuited for dating.

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TABLE 2. STRATIGRAPHIC, PETROLOGIC, AND GEOMORPHIC FEATURES OF WAITS POINT AND The Black Tusk Complex. The Black Tusk MOUNT GARIBALDI VOLCANIC FIELDS (Fig. 4) is a glacially dissected complex built

during two stages of volcanism. Products of the Field/complex Latitude Age (Ma) Stratigraphie relations Geomorphic/contact relations older eruptions consist of platy which have whole-rock K-Ar dates of 1.3 ± 0.1 Ma Watts Point N49°39.0' 0.09-0.13 Gray, flat-lying,hypocrystallin e Hy-Aug-Hb Ponded against gtaciofluvial sand and dacite lavas; >150 m thick; overlain gravel; >20 m thickness; sediment (MG612), 1.1 ± 0.06 Ma (MG610), and 1.3 ± by black, vitric Hy-Aug dacite flows. cut by Hy-Aug-Hb dacite dike. 0.07 Ma (MG622). After a period of dormancy, Round Mountain MSPA1.V <0.75 Hy-Aug andesite pyroclastic and Fill Mashiter paleovalley; cut into laharic breccia, ioterlayered in and across older north-trending eruptions of vitric hypersthene andesite buried upper part with andesites and minor breccia-filled paleovalley. the deeply eroded older cone. The culminating 0.46-0.51 Hb andesite and Hb-Hy dacite flows; Fill Btohm and Cheekye event of the hypersthene andesite eruptions ap- >150 m thick. paleovalleys. pears to have been the extrusion of a mushroom- Mount Garibaldi N49°48.3' 0.22-0.26 Cheekye stage: Composite cone; Hb Erosional remnants. dacite lava and tuff breccia; minor shaped plug dome and related lava near the Hb andesite lava; coalescing Hb summit of the second-stage cone. The absence of dacite domes. >0.02* Atwell Peak stage: Hb-Hy dacite tuff Supraglacial composite cone; basal glacial erratics and striations on exposed surfaces breccia cone surrounding Pelean plug cross-bedded and graded sandstones dome. rest on basement tonalite 300 m (2,311m level) of the summit lava suggests that above present Cheekye River it stood above the upper limit of late Pleistocene 0.01-0.02* Dalton Dome stage: Hb-Bt dacite Nonglaciated surface; eruption ice sheets. Three andesites representative of the summit lava and dome. followed glacial retreat and plug dome stage yield K-Ar dates of 0.09 ± 0.08 collapse of supraglacial cone. Glacier Pikes N49°52.5' >0.015* Hb dacite dome on northern flask Precipitous 50-m-high ice-contact Ma (MG634-1), 0.21 ± 0.04 Ma (MG634-2), of Mount Garibaldi edifice. margins with subhorizontal and 0.17 ± 0.04 Ma (MG635-2). columns. Paul Ridge N49°44.y >0.015* 01 andesite and Hy-Aug andesite flows Subglacial/intraglacial lavas; on upper slopes of Paul Ridge. locally basal breccia; Sphinx Moraine Complex. The Sphinx Mo- extended into and above U-shaped raine complex (Fig. 4) comprises several gla- valley. cially rounded basaltic andesite knobs on the Eenostuck N49°47.(r >0.015* Hb-bearing basaltic andesite; Subglacial/intraglacial lava; eastern shore of Garibaldi Lake. The basaltic 1 km long; >150 m high. quenched ice-contact margins. Opal Cone N49°49.5' <0.015* Agglutinate cone and Ring Creek lava; Nonglaciated lava surface; occupies andesite locally contains small partially fused changes along extent from Hb-Hy to U-shaped valley xenoliths of basement quartz diorite, suggesting Hb-Bt dacite (Sivertz, 1976)

that it may be unsuited for dating owing to con- Note: Hy, hypersthene; Hb, hornblende; Aug, augile; Bt, biotite; 01, olivine. Ages indicated are based on K-Ar and paleomagnetic data except where estimated tamination. Hemlock wood buried within over- based on geomorphic relationships and Holocene glacial relations. lying moraines of the Sphinx Glacier yields a radiocarbon date of 7640 ± 80 yr B.P. (GSC- 1993; Lowden and Blake, 1975). The extreme "older" cone. An andesitic flow, which origi- ring; basaltic lava, which was emitted from the dissection of the basaltic andesite complex sug- nated at the summit crater and flowed down the base of the cone, yields a whole-rock K-Ar date gests a considerably older age. northern flank of the mountain, has been dated of 0.04 ± 0.04 Ma (MG458). Cheakamus River Complex. Hornblende- as 0.3 ± 0.2 Ma (MG567). Posteruptive en- The Table Complex. Mathews (1951) de- biotite dacite flows of the Cheakamus River croachment of a continental ice sheet is indi- scribed the stratigraphy of the hornblende complex were extruded onto the glacially cated by the presence of numerous glacial andesite of The Table complex (Fig. 4) and scoured floor of the northern Cheakamus River erratics on the mountain slopes. postulated that this pillar of flat-lying flows filled valley (Fig. 4). The lavas locally contain The third eruptive stage constructed an ande- a pit within the continental ice sheet. The ab- rounded, striated pebbles and cobbles of tonalité sitic scoria cone on the northern flank of Mount sence of glacial erratics on the summit and lack and are crossed by at least two directions of Price. Possibly contemporaneous activity oc- of erosional features attributable to glaciation glacial striations. The extreme dissection of the curred at , a breached lava ring on suggest that The Table eruptions occurred dur- dacite complex suggests that eruptions predated the western shoulder of Mount Price (Fig. 3). ing the late stages of Fraser Glaciation. advance of the Fraser Glaciation ice sheet. Two lavas (Barrier and Culliton Creek flows) Mount Brew Complex. Sparsely porphyritic Mount Price Complex. The eruptive history emanated from the Clinker Peak summit and black, glassy hypersthene andesite forms steep- of Mount Price (Fig. 4) can be divided into three were ponded against ice within the lower Rubble sided bulbous knobs on the eastern slope of distinct stages. During the oldest stage, a small Creek and Culliton Creek valleys. The age of Mount Brew, about 5 km west of Brandywine was constructed at the eastern these unglaciated lavas cannot be less than 6000 Falls (Fig. 4). It seems likely that the Mount edge of a cirque-like basin. A hornblende ande- yr B.P.; they postdate the disappearance of the Brew complex formed as a subglacial mass site intercalated with palagonitic lithic tuff at the Cordilleran ice sheet from higher altitudes in late when the late Wisconsin (Fraser Glaciation) ice base of the volcanic succession yields a K-Ar Fraser Glaciation time but predate the final dis- sheet stood above the 1,219 m level within the date of 1.2 ± 0.1 Ma (MG522). The flow and appearance of the ice sheet in the Cheakamus Cheakamus River valley. pyroclastic material overlie a drift-covered sur- River valley (Mathews, 1952b). Cheakamus Valley Basalts. Episodic erup- face; the underlying sediment may be related to The Cinder Cone Complex. Initial activity at tions produced a thick sequence of flat-lying either a local alpine glacier or a continental ice The Cinder Cone (Fig. 4) produced a broad tuff sparsely porphyritic olivine basalts (Fig. 4) that sheet of early Pleistocene age. Mathews and ring and a basaltic andesite flow within a north- overlie an ice-scoured valley floor and locally Rouse (1986) have documented a contempo- trending glacial valley carved into the eastern bury thin accumulations of fluvioglacial sand raneous (>1.1 Ma) regional glaciation in south- flank of The Black Tusk. The basaltic andesite, and gravel (Mathews, 1958). A whole-rock central British Columbia. now exposed only at the northern end of the K-Ar analysis of the stratigraphically youngest The second eruptive stage occurred subse- valley and as tuffs in the pyroclastic cone, yields flow of the early episode of volcanism gives a quent to a quiescent period of unknown dura- a whole-rock K-Ar date of 0.11 ± 0.03 Ma date of 0.05 ± 0.05 Ma (MG36-4). tion. Eruption of andesitic lava and pyroclastic (MG208). Following a period of quiescence, re- Advance and subsequent retreat of a conti- material formed the nearly symmetrical central newed eruptions constructed a Strombolian nental (Salmon Springs Glaciation) ice sheet is mass of Mount Price and partly buried the cinder cone on the eastern rim of the older tuff indicated by thin layers of lacustrine silt and clay

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/4/563/3380343/i0016-7606-100-4-563.pdf by guest on 27 September 2021 OPAL CONE COMPLEX ¡¡¡¡¡I Cheekye Stage: composite volcano comprising Hornblende-biotite dacite flow and minor Sa hornblende andesite, hornblende-biotite breccia. dacite and minor rhyodacite flows, domes and tuff breccias. ENOSTUCK COMPLEX Subglacial pile of hornblende-bearing basaltic COLUMNAR PEAK COMPLEX andesite. fcyy'gff*' Hornblende dacite dome and minor breccias.

PAUL RIDGE COMPLEX ROUND MONTAIN COMPLEX Subglacial piles of basaltic andesite and BS&ras&g Valley-filling andesitic breccias, lahars and pyroxene andesite. 188^888981 polymict conglomerates; andesite, dacite and minor basalt flows. GLACIER PIKES COMPLEX Hornblende dacite dome and lavas. BASEMENT ROCKS | Undivided plutonic and metamorphic rocks of 1 Coast Crystalline Complex. MOUNT GARIBALDI COMPLEX }Jiiiiii{9 Dalton Dome Stage: Hornblende-biotite KH»hta dacite flow. Glacier At well Peak Stage: composite volcano reSSMgaa comprising hornblende-biotite rhyodacite intrusive core and Pelean tuff breccias.

Figure 3. Map showing Quaternary volcanic suites in the Mount Garibaldi volcanic field.Lava s and pyroclastic rocks of the Round Mountain complex filled the Brohm (BV), Cheekye (CV), Mashiter (MV), and Paul Ridge (PV) paleovalleys; dashed lines indicate approximate strike of the paleovalleys. Contour interval is 1,000 ft.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/4/563/3380343/i0016-7606-100-4-563.pdf by guest on 27 September 2021 Figure 4. Map showing Quaternary volcanic suites in the Garibaldi Lake volcanic field. Contour interval is 1,000 ft.

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FIGURE 4 EXPLANATION TABLE 3. STRATIGRAPHIC, PETROLOGIC, AND GEOMORPHIC FEATURES OF GARIBALDI LAKE VOLCANIC FIELD

Complex Latitude Age (Ma) Stratigraphic relations Geomorphic/contact relations CHEAKAMUS VALLEY BASALTS 1 L"" '•'•. 1Olivine basalt flows; locally interbedded Tricoum N49°58.8' Unk. Yellowish-gray Hb andesite, and Erosional remnants; glacially I •- '. ..-im/ith glacial tills and fluvioglacial sediments. Hb-Bt dacite flows and tufts. striated. MOUNT BREW COMPLEX The Black Tusk N49°58.8' 1.1-1.3 Stage I: Platy Hy-Aug andesite, Isolated erosional remnants; vTjT] Subglacial piles of glassy hypersthene Hb dacite, and minor lithic tuff. glacially striated. •¿¿•I andesite. 0.17-0.2J Stage II: Hy andesite lava cone with Quenched, >50-m-high ice-contact endogenous summit dome and feeder margins; nonglaciated surface. dike. THE TABLE COMPLEX bbgçwççg Flat-topped, subglacial Tuya; mainly flows Sphinx Moraine N49°57.5' Unk, Hb-bearing basaltic andesite. Steep-sided subglacial knobs; re88S8o88l of hornblende andesite. fanned columnar joints; glacially striated; overlain by till.

THE CINDER CONE COMPLEX Cheakamus River N50°0.45' Unk. Flow-banded, 10- to 30-m-thick Contain glacial erratics and surface K'i'K':! Olivine basalt, mugearite and basaltic columnar-jointed Hb-Bt dacite cut by two directions of glacial andesite flows and . lavas. striations; occupy glacial valley. Mount Price N49°55.0' 1.2 Stage I: Hb andesite stratovolcano Overlies drift-covered surface; MOUNT PRICE COMPLEX within cirque-like basin; basal modified by erosion and covered by Composite volcano comprising hornblende palagonitic lithic tuffs. glacial erratics. §§¡§§¡¡111 andesite, hornblende-biotite andesite and 0.3 Stage II: Hb-Bt andesite/dacite Extensive cover of glacial erratics. minor dacite flows and tuff breccias. composite cone. <0.012' Stage III: Hy andesite (Price Bay) No glacial erratics on surfaces; CHEAKAMUS RIVER COMPLEX scoria cone on north flank; Clinker Clinker Peak lavas have {ÏÎÎÎBÎJM Subglacial flows of hornblende-biotite Peak lava ring and Hb-Bt andesite >300-m-thick ice-dammed flow Hows (Barrier and CuJJiton Creek) snouts (Mathews, 1952b). on west flank.

SPHINX MORAINE COMPLEX The Cinder Cone N49°58.9' 0.11 Stage I: Palagonitic tuff ring and Ol >100-m-thick, ice-contact flow Subglacial piles of hornblende-bearing andesite flow. snout; occupies U-shaped valley. &ïfl&V:&yl| basaltic andesite. 0.04 Stage II: Strombolian cinder cone; Ol- Glacially striated lava surface; Aug basalt/Hb-Aug mugearite lava. glacial erratics on cone surface. THE BLACK TUSK COMPLEX The Table N49°53.7' <0.012* Flat-lying 20- to 50-m-thick Hb Tuya; youngest lava flowed down Hypersthene andesite dome and lavas with andesite flows. southwest margin onto glacial till; WSMghzi peripheral ice-contact features; minor elongate parallel to glacial striae. hornblende andesite and dacite flows and Mount Brew N49°58.8> Unk. Sparsely porphyritic black, glassy Hy Quenched ice-contact margins; tuffs. andesite. steep-sided bulbous knobs. TRICOUNI COMPLEX Cheakamus Valley N50°05.0' 0.05 Stage I. >50-m-thick, flat-lying Ol-Aug Episodic subaerial eruptions; I Hornblende andesite lavas and ash-flow basalts; stream channels at top basal flows overlie fluvioglacial tuffs. of some flow units. sediment; occupy U-shaped valley. <0.032 Stage II: Anastomosing Ol-Pl basalt Subglacial/intraglacial extrusion; flows; 50-100 m wide; 10-20 m overlie hyaloclastite breccia thick; fanned entablature columns; and/or glacial till and lacustrine Glacier blocky flow tops sediments

Note: Hy, hypersthene; Hb, hornblende; Aug, augite; Bt, biotite; Ol, olivine. Ages indicated are based on K-Ar and 14C data except where estimated based o geomorphic relationships and Holocene glacial relations; Unk., age unknown.

and underlying till sandwiched between younger ice-contact features, but subaerial products are Mount Cayley Complex. The Mount Cayley plagiophyric basalts and the earlier olivine ba- also present (Table 4). complex (Fig. 5) formed during at least three salts north of Brandywine Falls (Fig. 3). Wood Complex. Five small patches distinct eruptive periods: the Mount Cayley, contained in the lacustrine sediment yields a ra- of hornblende-bearing basaltic andesite, which Vulcan's Thumb, and Shovelnose stages. The diocarbon date of 34,200 ± 800 yr B.P. (GSC- crop out on the ridge south of (Fig. earliest, or Mount Cayley, stage produced a 2169; Green, 1981). This date is correlative with 5), are similar in lithology and structure. They composite pile of dacite flows, tuffs, and breccia. the Olympia Interstade, the nonglacial interval are approximately coeval but are believed to On the northwestern side of Turbid Creek, the preceding the Fraser Glaciation (Fulton and have issued from separate vents. The activity basal flow of this succession is separated from others, 1976). The K-Ar and radiocarbon chro- almost certainly occurred during a period of ex- unaltered basement granodiorite by well-indu- nologies are not in conflict. tensive ice cover, causing the rapidly quenched rated, unstratified clastic sediment. The sparse The overlying plagiophyric basalts occur as lava to pile up in the form of steep-sided exog- matrix of this deposit, which consists of sand- anastomosing flow units that stand as isolated enous domes directly over the vents. sized fragments rather than clay or silt, and the ridges above the glaciated surface of the older Mount Fee Complex. Mount Fee (Fig. 5) is angular character of the clasts suggest that it is flows. The absence of erosional features attribu- a narrow elliptical spine of intrusive biotite colluvium that mantles the west-sloping, prevol- table to glaciation suggests that these flows rhyodacite. The mantle of pyroclastic rocks that canic surface. The narrow serrated ridge of erupted during the waning stages of Fraser Gla- must once have enclosed it has been completely Mount Cayley itself is an intrusive spine, intru- ciation, possibly by passage of lava along stripped away except for a small remnant on the sion of which is believed to be the culminating trenches through the ice sheet formed by heated western side and along the northern end where event of the Mount Cayley stage. melt water (Mathews, 1958). the lip of the conduit is exposed in cross section. During the subsequent Vulcan's Thumb stage At its northern end, the intrusive spine wedges of activity, an extensive tephra cone was super- Mount Cayley Volcanic Field out and appears to cut a coarse, blocky tuff- imposed on the southwestern flank of the Mount Seven volcanic piles form a north-south belt breccia which, in turn, is overlain by thick Cayley edifice (Fig. 4). Vulcan's Thumb, the along the ice-covered height of land between rhyodacite flows. Intrusion of the central spine largest in a cluster of slender pinnacles, repre- Cheakamus and valleys in the of viscous rhyodacite, which cuts the early- sents a remnant of vent breccia deposited in the central part of the Garibaldi belt (Fig. 5). These formed breccia and was probably initially com- upper part of this volcano. The base of the Vul- volcanoes include the Ember Ridge, Mount Fee, pletely enclosed by it, appears to have been the can's Thumb succession rests on a steep westerly Mount Cayley, Pali Dome, Cauldron Dome, culminating event in the Mount Fee activity. An dipping surface that truncates older deposits of Slag Hill, and Crucible Dome complexes. Many old age is consistent with the nearly complete the Mount Cayley stage and laps onto the base- of the centers are characterized by subglacial, denudation of the central spine. ment surface. A majority of the Vulcan's Thumb

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rocks are extensively weathered. An unaltered porphyritic dacite clast from tuff breccia within the saddle between Mount Cayley and Vulcan's Thumb yields a K-Ar date of 3.8 ± 0.7 Ma (SE1516A79), which seems too old. This old age is possibly due to analytical problems (com- pare with Souther and others, 1984) or to excess radiogenic At in xenocrystic plagioclase derived from basement granodiorite. Such contamina- tion is suggested by a somewhat younger date (2.7 ± 0.7 Ma) obtained for the sample after separation of feldspar crystals from the matrix, but the uncertainties are too large to draw a firm conclusion. The third, or Shovelnose, stage of activity produced two domes and related flows of hypersthene-biotite dacite in the valley of Shovelnose Creek (Fig. 5). The more northerly exogenous dome rests on tephra which is sepa- rated from basement diorite by bouldery till containing clasts of exclusively basement lithol- ogies. The second dome, an endogenous mass, was intruded into and quenched against Vul- can's Thumb tephra. The biotite dacite has a whole-rock K-Ar date of 0.31 ± 0.05 Ma (SE0502A79). The age relationship of this dome to flows in Shovelnose Creek is not known, but it seems probable that the eruptive event began with effusion of the flows and cul- minated with emplacement of the dome. Pali Dome Complex. Hornblende-hyper- sthene andesites exposed as isolated nunataks on Pali Dome (Fig. 5) form a composite . Peripheral lobes of lava on both the east and west sides rest directly on basement, except for one lobe on the southwest corner which over- laps Mount Cayley tephra and flows (Fig. 5). The distal flows are clearly quenched and termi- nate in nearly vertical, ice-contact cliffs. The boundary cliffs on the southeastern side of the pile are flanked by more than 70 m of granular glass which must have spalled off the advancing flow front and accumulated in the moat between lava and ice. Cauldron Dome Complex. The Cauldron Dome edifice (Fig. 5) is a nearly flat-topped elliptical pile of thick hypersthene andesite flows. It has the classical form of a tuya, but subsequent erosion has removed all direct evi- dence of quenching. It seems likely that Caul- dron Dome itself formed as a subglacial mass and that the flows were directed into a melt- water channel that breached the enclosing bar- rier of ice during the later stages of activity. The hypersthene andesite yields a whole-rock date of 0.49 ± 0.08 Ma (SE150779). Slag HiU Complex. The Slag Hill complex Figure 5. Map showing Quaternary volcanic suites in the Mount Cayley volcanic field. (Fig. 5) is obviously a subglacial pile but lacks Dashed lines indicate approximate subglacial limits of volcanic complexes. Contour interval is the classical tuya form. The volcanic complex 1,000 ft. rests directly on a steep northwesterly sloping

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TABLE 4. STRATIGRAPHIC, PETROLOGIC, AND GEOMORPHIC FEATURES OF MOUNT CAYLEY VOLCANIC FIELD Although the unit is unsuited for dating, its time of formation falls in the interval of 1.9 ± 0.2 Ma Complex Latitude Age (Ma) Stratigraphic relations Geomorphic/contact relations for the youngest underlying unit and 1.0 ± 0.1

Ember Ridge N50°04,0' Unk. Hb-bearing basaltic andesite flows; Subglacial extrusion; some flows, Ma for the oldest overlying unit. coltiform structure due to as much as 60 m thick; separated by overlapping and interfingering, deep furrows which expand into Pylon Complex. Aphanitic andesite flows of steeply inclined bulbous cooling small elliptical glass-lined the Pylon complex (Fig. 6) locally overlie the units. caverns. Pliocene basal breccia and The Devastator Mount Fee Nsmr link. Elliptical intrusive Bt rhyodacite Composite cone; greatly modified spine; contacts nearly vertical; by erosion. complex. The base of the andesite flows marks shattered, granulated granitic wall rock; at north end, spine cuts coarse, the end of a period of volcanic quiescence and blocky tuff-breccia overlain by deep dissection. A K-Ar date of 1.0 ± 0.1 Ma Bt rhyodacite flows. (MM136B) for the andesite supersedes an earlier Mount Caytey N50°06.4' <2.7 Mount Cayley stage: Lower southwest- Greatly modified by erosion; dipping flows and pyroclastics cut by northwest of Turbid Creek, basal date of 4.3 ± 0.2 Ma (RdA74-23-3; Anderson, numerous dikes and sills; older flows flow separated from unaltered overlain by Hy-Hb dacite flows of basement granodiorite by about 1975; Woodsworth, 1977) for a sample from Wizard Peak (500 m thick); 150-m-thick 25 m of colluvium. the same stratigraphic level. The latter date cooling units dip southeast from Mount Cayley intrusive spine. probably reflects either dating of contaminated Unk. Vulcan's Thumb stage: Massive Hb-Bt Base rests on a steep westerly dacite lava and agglutinate breccia; dipping surface that truncates material or sample bakeout problems (Arm- central mass overlain and flanked by older Mount Cayley stage deposits, strong and others, 1985). When sample remnants of tephra cone. and laps onto basement surface. 0.31 Shovelnose stage: Two domes and Underlying tephra separated from RdA74-23-3 was rerun after removal of visible related flows of Hy-Bt dacite in basement diorite by as much as 12 m of Shovelnose Creek valley; northern bouldery till. feldspar, the resulting age was 0.7 ± 0.4 Ma. exogenous dome; southern endogenous dome intruded into, quenched against Plagiophyric andesite, the most extensive unit Vulcan's Thumb stage tephra. of the Pylon complex, covers the southeastern, Pali Dome N50°08.5' Unk. Hb-Hy andesite flows exposed as Proximal portion of flows appear central, and northwestern parts of the Meager isolated nunataks; southeastern subaerial; distal flows terminate boundary cliffs flanked by >70 in nearly vertical, 100- to 200-m-high, Creek field. The topographically highest lava m of granular glass. ice-contact cliffs. gives a K-Ar date of 0.9 ± 0.2 Ma (MM236), Cauldron Dome N50°09.7' 0.49 Hy andesite flows; two 100- to 130-m-thick, Tuya; nearly flat-topped elliptical complexly jointed lavas pile. whereas a flow within 100 m of the base of the extend 2 km southwest from main pile. succession yields a younger date of 0.5 ±0.1 Ma Slag Hill N50°10.8' 0.25-0.73 Black, glassy basaltic andesite and Steep-sided, bulbous, subglacial (MM81). andesite domes. masses; complex, small diameter, curved and radiating columns. Devastation Glacier, Capricorn, and Plinth Crucible Dome N5O°13.0' Unk. Nearly circular Hy andesite mass Tuya; greatly modified by erosion Complexes. Rocks of the late Pleistocene epi- sode of rhyodacite volcanism underlie the Note. Hy, hypersthene; Hb, hornblende; Aug, augite; Bt, biotite; 01, olivine. Ages indicated are based on K-Ar data; Unk., age unknown. northeast corner of the Meager Creek field and form the summits of , Mount Capri- corn, Plinth Mountain, and Meager Mountain basement surface. Only a small amount of lava Pleistocene episodes (Table 5). The earliest (Fig. 6). Several centers erupted areally re- was initially ice ponded to form the flat-topped Pleistocene episode of volcanism is represented stricted complexes of steeply dipping tuff, bluff in the source area. Most lava was chan- by The Devastator complex of rhyodacite lavas, breccia, and flows. The earliest of these, best neled, along with melt water, into subglacial tuff breccias, and hypabyssal intrusions. The in- exposed on the east side of Affliction Glacier, is caverns where it was quenched and solidified termediate episode produced the Pylon complex the Devastation Glacier complex of rhyodacite into its present bulbous forms. Two andesites of andesitic lavas and pyroclastic rocks; products flows, of which the basal flow yields a K-Ar date from the main Slag Hill mass have whole-rock of the latest eruptive episode, which extends into of 0.15 ± 0.1 Ma (MM435). Overlying the K-Ar dates of 0.73 ± 0.07 Ma (SE1501A79) the Recent, include several rhyodacitic com- Devastation Glacier flows are rhyodacite flows and 0.60 ± 0.03 Ma (14098-V1), but a basaltic plexes (Devastation Glacier, Capricorn, Plinth, and breccia lenses of the Capricorn complex. A andesite from a nunatak on the southern margin and Bridge River) and minor basaltic lavas. few metres above the basal flow of the Capri- of the complex has a K-Ar date of 0.25 ± 0.03 Pliocene Activity. Pliocene flows, intrusions, corn complex, a sample gives a K-Ar date of 0.1 Ma (64088-V1), suggesting that products of two and basal breccia overlie plutonic and meta- ± 0.02 Ma (MM293). Rhyodacite flows, brec- eruptive episodes may be present. morphic basement along the southwest edge of cia, and tuff of the Plinth complex form the top Crucible Dome Complex. Crucible Dome the field (Fig. 6). A sample from the sequence of of Mount Capricorn, the uppermost 600 m of (Fig. 5) is a nearly circular flat-topped mass of dacite flows and breccia overlying basal breccia Meager Mountain, and the bulk of Plinth Moun- hypersthene andesite. Its lower contact is com- near the head of Meager Creek yields a K-Ar tain. East of Affliction Creek and north of Lil- pletely obscured by talus, and its original outer date of 1.9 ± 0.2 Ma (MM160). Surrounded by looet Valley, basal flows of the Plinth complex margins have been greatly modified by erosion. glaciers and isolated from the section southwest overlie till. A whole-rock age of 0.11 ± 0.02 Ma The lava pile is probably a tuya on which the of Pylon Peak, possibly related andesite flows (MM353) from the margin of the lava dome upper flows, at least, were subaerial. and breccia, which yield whole-rock K-Ar dates underlying Plinth Mountain suggests approxi- of 2.0 ± 0.1 and 2.2 ± 0.1 Ma (64110-V2), lie mate contemporaneity of the Plinth and Capri- corn complexes. Meager Creek Volcanic Field directly on the basement along the western edge of the field. Olivine Basalt. Olivine basalt underlies the The volcanic field, located west of the junc- The Devastator Complex. Rocks of the uppermost 22 km of the Elaho River valley (Fig. tion of Meager Creek and Lillooet River, con- complex (Fig. 6) cut through the Pliocene basal 6) located immediately south of the Meager sists of the products of four episodes of breccia and 1.9-Ma-old dacite. The source ap- Creek volcanic field (Fig. 1), and extends volcanism: a Pliocene episode and three major pears to have been located south of Pylon Peak. another 3 km to its probable source at the 1,375

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m level in the south fork of Meager Creek. The TABLE 5. STRATIGRAPHIC, PETROLOGIC, AND GEOMORPHIC FEATURES OF MEAGER CREEK VOLCANIC FIELD basalt yields a K-Ar date of 0.14 ± 0.10 Ma (44084-VI). Scattered remnants of locally de- Complex Latitude Age (Ma) Stratigrapbic relations Geomorphic/contact relations

rived basaltic andesite and basalt flows also lo- Pliocene Nstmy 1.9-2.2 Andesite flows, intrusions, and basal Greatly modified by erosion; breccia; as much as 300 m thick; south of overlie plutonic and metamorphic cally overlie till and rocks of the Plinth complex Pylon Peak, where breccia thickest basement on southwestern and (Fig. 5). An age of 0.09 ± 0.06 Ma (RdR74-4B) and basement lowest, partly exhumed western margin of field- vent exposed; 200-m-thick Hb-Bt is consistent with its observed field relations to dacite flows and breccia sequence other units. overlie basal breccia. The Devastator N50°35.2' <1,9 Succession of hydrothermally altered Cuts through sequence of Pliocene Bridge River Tephra. Bridge River tephra rhyodacite tuff, breccia, flows, and basal breccia and dacite; greatly (Fig. 6) is a continuation of late Pleistocene hypabyssal intrusions; as much as 500 in modified by erosion. thickness. rhyodacite volcanism into the Holocene. It in- Pylon N50°36.0' 0.5-1.0 Aphanitic andesite flows locally overlie Greatly modified by erosion. completely blankets the area between the north Pliocene basal breccia and The Devastator complex. Flow-banded and and east ridges of Plinth Mountain. Within this platy-jointed plagiophyric andesites, area, crudely stratified tephra deposits are over- reddened breccia, and tuff; underlain by as much as several hundred metres of lain by welded tuff and pyroclastic breccia monomict andesite breccia. flows. The tephra and welded pyroclastic flows Devastation N50°37.5' 0.15 Hb-Bt rhyodacite flows. probably erupted from the same vent at 1,525 m Glacier Capricorn N50°37.2' 0.1 Hb-Bt rhyodacite flows and breccia; Overlies Devastation Glacier flows. near the headwall of Fall Creek. They overran a form uppermost 600 m of Mount Job forest 4 km away on the banks of Lillooet River, and main mass of Mount Capricorn. from which the core of a charred tree in living Plinth N50°38.3' 0.1 Columnar- and platy-jointed East of Affliction Creek and north 14 rhyodacite flows, breccia, and tuffs; of Lillooet Valley, basal flows position yielded a C date of 2500 ± 50 yr B.P. lava domes on Meager Mountain and at overlie till. three areas of north ridge and flat-topped (Lowden and Blake, 1978). After application of summit of Plinth Mountain. a correction for the approximate age of the tree, Olivine basalt N50°27.0' 0.14 Olivine basalt underlies uppermost Probable source at 1,375 m level the Bridge River tephra is dated at 2350 ± 50 yr 22 km of Elaho River valley. in south fork of Meager Creek. B.P. (Read, 1979). N50°40.1' 0.09 Locally derived basaltic andesite and Erosional remnants; locally overlie basalt flows and tuff; veneer valley till and rocks of Plinth complex. walls, cap few ridges, and cover some valley floors. Salal Glacier Volcanic Field Bridge River N50°38.8' 0.0025 Crudely stratified tephra deposits, Overran forest on banks of as much as 20 m thick on some ridges, Lillooet River overlain by welded tuff and The Salal Glacier area, at the north end of the pyroclastic breccia flows as much as 6 main group of vents in the Garibaldi volcanic km long; 145 m thickness belt, was the site of several small alkali-olivine Note: Hy, hypersthene; Hb, hornblende; Aug, augite; Bt, biotite; 01, olivine. Ages indicated are based on K-Ar and ,4C data. basalt and hawaiite eruptions (Table 6). Law- rence and others (1984) described two overlap- ping flow units of the Salal Glacier volcanic field, located at the 2,300 m level on the eastern margin of the glacier. The more northerly flow, FIGURE 6 EXPLANATION a hawaiite having reversed magnetic polarity, has a K-Ar date of 0.97 ± 0.05 Ma (M5-14), BRIDGE RIVER TEPHRA Ui whereas normally polarized alkali-olivine basalt z Hornblende-biotite rhyodacite air-fall and welded tephra of the southern flow is dated at 0.59 ± 0.05 Ma LU (C5-5). Steep flow margins, pillow-palagonite u 1 | Olivine basalt O accumulations, and waterlaid tuffs indicate that 0 DEVASTATION GLACIER, CAPRICORN AND PLINTH COMPLEXES both of these flows were ponded against glacial X ice. In contrast, basalt flows at the head of Float 1 Hornblende-biotite rhyodacite flows, tuff breccia and intrusions Ui Creek are underlain by poorly consolidated gla- z PYLON COMPLEX UJ cial till (Stephens, 1972) and appear to postdate o glaciation. o : 1 Plagioclase andesite flows, tuff breccia and intrusions H- ÇO THE DEVASTATOR COMPLEX LU Franklin Glacier Volcanic Field Rhyodacite tuff, breccia« flows and hypabyssal intrusicns

The Franklin Glacier volcanic field (Mc- Knight, 1965; Ney, 1968) occupies an approxi- Dacite porphry flows and tuff breccia mately elliptical, northwesterly trending area I* «"« „ « Andesite flows and tuff breccia about 20 km long and 6 km wide in which the J Basal breccia with basement clasts local topographic relief exceeds 2,000 m. Within o. it, granitic and metamorphic rocks of the Meso- BASEMENT ROCKS zoic to early Tertiary Coast Crystalline Complex | | Undifferentiated granitic and metamorphic rocks of the are fractured,hydrothermall y altered, and cut by Coast Crystalline Complex

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Figure 6. Map showing Quaternary volcanic suites in the Meager Creek volcanic field. Contour interval is 1,000 ft.

dike swarms and high-level plutons of biotite- Medium-grained quartz monzonite forms the Pleistocene-to-Holocene volcanic rocks (Read, quartz porphyry, biotite-quartz-feldspar por- largest of the subvolcanic plutons in the Franklin 1979; Lawrence, 1979) and are considered to be phyry, and quartz monzonite. At least some of Glacier field. Biotite from this body yields a K- high-level, possibly subvolcanic, plutons belong- these younger intrusive phases appear to be Ar age of 7.0 ± 0.2 Ma (Rd76-46155), which is ing to the pre-Garibaldi, Pemberton volcanic feeders for an overlying succession of eruptive only slightly younger than the Salal Creek belt (Souther, 1975). A porphyritic dacite dike rocks comprising mainly dacite breccia, minor pluton of the Salal Glacier area (8.3 ± 0.5 Ma, that cuts the Franklin Glacier quartz monzonite dacite flows, and a few remnants of hornblende Rd74-44210M, Table 1) and the Fall Creek yields a K-Ar (biotite) date of 4.0 ±0.1 Ma andesite lavas. The breccias may be of epiclastic stock of the Meager Creek area (10.1 Ma; (Rd76-46156), and a fine-grained dacite dome origin, possibly formed during collapse and infill- Stevens and others, 1982). The later two granitic or flow yields a K-Ar (biotite) date of 2.2 ± 0.1 ing of a cauldron subsidence structure (Table 6). bodies are overlain unconformably by the late Ma (JR10-61A). The later date is only slightly

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TABLE 6. STRATIGRAPHIC, PETROLOG1C, AND GEOMORPHIC FEATURES OF SALAL GLACIER, FRANKLIN GLACIER, 8 major climatic cycles in southern British Co- AND SILVERTHRONE FIELDS lumbia during the past 800,000 yr (Shackleton

Age (Ma) Stratigraphie relations Geomorphic/contact relations and Opdyke, 1973). Many of the climatic cycles may have been accompanied by widespread Salai Glacier NM^-O" Erosional remnants of alkali-olivine Ice-contact margins, some basalts glaciation (Clague and others, 1982). These gla- basalt and hawaiite lavas; pillow- underlain by poorly consolidated palagonite accumulations; waterlaid glacial till (Stephens, 1972). ciations may not have progressed to the conti- tuffs. nental ice-sheet stage but rather peaked with ice Franklin Glacier N51°17.4' Granitic and metamorpbic rocks of Roughly elliptical, northwesterly Coast Plutonic Complex fractured, trending area about 20 km long and cover consisting of a series of separate glacier bydrothermally altered, and cut by 6 km wide in which local complexes confined to or spreading beyond younger dike swarms and high-level topographic relief exceeds 2,000 m. plutons. mountain ranges (Clague, 1986). The geomor- Dacite breccia with angular to sub- Collapse and infilling of cauldron angular dasts of porphyritic dacite subsidence structure. phology, stratigraphy, and geochronology of and basement rocks; absence of Garibaldi belt volcanic complexes provide im- or vitric blocks suggests epiclastic origin; minor dacite/Hb portant clues to the distribution of ice sheets at andesite lavas at top of succession. elevations that presently do not support glaciers Silverthrone NSl'Xiff Basal breccia; 1.200 m thick; contains Steep oontacts with older angular to subangular granitic, metamorphic crystalline rocks suggest that breccia and to the timing of Quaternary glaciations in and volcanic dasts; locally may occupy fault-bounded southwestern British Columbia. welded shards define eutaxitic texture. subvolcanic structure- Thick, lenticular flows of rhyolite, Overlies breccia; greatly modified Armstrong and others (1965) recognized two dacite, and andesite; 900 m composite by erosion. thickness. late Pleistocene to Holocene glacial periods: Basaltic andesite flows; remnants of Fill U-shaped valleys; rest on source pyrodastic cones project unconsolidated, cross-bedded tuff Salmon Springs Glaciation (>50,000 yr B.P.) through glacial ice on eastern side of breccia and fluvial gravel and Fraser Glaciation (10,000-26,000 yr B.P.). complex Huesser (1972), Hansen and Easterbrook Note. Hb, hornblende; ages indicated are based on K-Ar, 14C, and paleomagnetic data. *K-Ar age inconsistent with geomorphology of volcanic products. (1974), and Easterbrook (1976) have suggested that the Salmon Springs Glaciation in north- western Washington consisted of an early , followed by an interstade between about older than that of the basal Meager Creek rocks raphy was established before eruption of basaltic 40,000 and 50,000 yr B.P., and a later glacial (-2 Ma). andesite from numerous centers around the episode which lasted from about 34,000 to periphery of the field (Fig. 7). The flows rest on 40,000 yr B.P. Fulton and others (1976), how- Silverthrone Volcanic Field several metres of unconsolidated, cross-bedded ever, argued that British Columbia was undergo- tuff breccia and fluvial gravel. The basaltic an- ing nonglacial conditions between 26,000 and The Silverthrone volcanic field lies about 50 desite flow, which occupies the Pashleth Creek 50,000 yr B.P. The presence of lacustrine silts km west-northwest of the Franklin Glacier field. and Machmel River valleys, yields K-Ar dates of and clays beneath younger Cheakamus Valley Between the two, several small remnants of an- 1.0 ± 0.2 Ma and 1.1 ± 0.1 Ma (77-1-7B). It basalts of the Garibaldi Lake volcanic field is desitic and rhyolitic lava cap ridge crests and seems likely that the lava is much younger than compatible with ice-free conditions in south- occupy valleys. Although deeply dissected and indicated by the K-Ar dates because these high- western British Columbia about 34,200 yr B.P. as rugged as the Franklin Glacier field, the ap- energy, glacier-fed streams have only begun to The widespread interaction between Gari- proximately circular Silverthrone field is com- etch a channel along the edge of the flow. The baldi belt volcanism and late Pleistocene ice posed mostly of eruptive rocks (Table 6). Many whole-rock dates may be influenced by assimila- sheets documents the extent and timing of pre- of the volcanic products postdate a glaciated to- tion of material derived from the underlying Salmon Springs (>50,000 yr B.P.) glaciations in pography and are clearly younger than those Mesozoic granitic basement. The analyzed sam- southwestern British Columbia. Although it is associated with the Franklin Glacier center. ple did not contain recognizable contaminants not possible to discriminate between alpine, val- The lowest unit is a breccia exposed from that could be removed to test this hypothesis. ley, or continental glacier, precipitous ice- valley bottoms up through a vertical distance of The sample was reanalyzed after it was ultrason- contact margins of andesites associated with the at least 1,200 m (Fig. 7). Contacts with the older ically washed in deionized water containing a plug-dome stage of The Black Tusk complex crystalline rocks of adjacent peaks are steep, few percent hydrochloric acid to remove dust suggest that an ice sheet was present above the suggesting that the breccia may occupy a fault- and possibly unseen carbonate and to minimize 1,800 m level at 0.17-0.21 Ma in the Garibaldi bounded subvolcanic structure. The breccia is trapped atmospheric Ar. The percentage of ra- Lake field. Geomorphic relations indicate that a overlain by thick, lenticular flows of rhyolite, diogenic Ar, 17.2%, in this second analysis deep U-shaped valley was incised into the east- dacite, and andesite. Rhyolite glass about 100 m (Table 1) is sufficiently large to preclude analyt- ern flank of The Black Tusk complex after ex- stratigraphically above the top of the basal brec- ical problems as an explanation of the anoma- trusion of the 0.17-0.21 Ma plug dome but cia and a thick andesite flow overlying rhyolite lously old date. prior to eruption of 0.11 Ma basaltic andesite at in the central part of the complex yield whole- The Cinder Cone. The subglacial form of Caul- rock K-Ar ages of 0.75 ± 0.08 Ma (77-l-4c) and DISCUSSION dron Dome and Slag Hill complexes in the 0.4 ± 0.1 Ma (77-1-1), respectively. Both of Mount Cayley field requires that ice covered the these ages appear to be consistent with the ob- Volcanism and Glaciation 1,829 m level at 0.49 and >0.6 Ma, respectively. served state of dissection. The presence of pillow lavas and palagonitic wa- terlaid tuffs in the Salal Glacier area indicates The early flows and underlying breccia were Isotopic and magnetic studies of deep-sea sed- that ice that filled major valleys stood at 2,340 deeply eroded, and much of the present topog- iments have demonstrated that there have been

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Post —topography ÜOTJTJTTU >0000000000001 ooooooooooooo Basaltic andesite ooooooooooooo QOOOOOOOOODOC - ' ooooooooooo flows and pyroclastic lOOOOO oooc 200000000000 100000000000C ooooonoootpoo cones

Pre —topography Rhyolite, dacite and andesite flows and domes

Polymict breccia, in pari pyroclastic

Sample No. (Table 1)/ Age in million years rf^ CJ i .

Inferred boundary of subsidence \ structure

0 4 • I • KM.

Contour interval 1000 feet

Figure 7. Map showing Quaternary volcanic suites in the Silverthrone volcanic field.

m 0.59 m.y. ago and that before 0.7 Ma, there by a 1.2 Ma volcanic succession in the Garibaldi Volcanism and Tectonic Implications was ice at 2,280 m levels (Lawrence and others, Lake field. Collectively, these relations record 1984). Till and(or) glaciofluvial sediments are the earliest episodes of glacial advance and re- During late to Holocene time, a overlain by 0.09-0.13 Ma lavas in the Watts treat (prior to 0.09, 0.3, and 1.2 Ma) so far triple point between a sediment-filled trench, the Point and Meager Creek fields, by a 0.31 Ma documented in the of south- Juan de Fuca ridge system, and the Queen Char- volcanic dome in the Mount Cayley field, and western British Columbia. lotte fault was situated off the northwest coast of

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Vancouver Island (Riddihough and Hyndman, Nootka fault zone separating the Juan de Fuca along the continental margin. The volcanoes 1976). Subduction of the Juan de Fuca plate plate from the slower-moving Explorer plate erupted lavas ranging in composition from beneath southwestern British Columbia gave (Fig-1). augite-olivine basalt, through hypersthene an- rise to the Miocene Pefflblerton volcanic belt Late Pliocene to early Pleistocene (1.7-2.3 desite, hornblende andesite, and hornblende- (Fig. 1; Souther, 1977; Berman and Armstrong, Ma) lavas and pyroclastic rocks of the Franklin biotite andesite, to biotite rhyodacite. Many of 1980). Extinction of Pemberton belt volcanoes Glacier and Meager Creek fields, which define a the centers are characterized by geomorphic fea- occurred at about 7 Ma and followed a seaward northern segment of the Garibaldi belt roughly tures which indicate complex interactions be- jump in the locus of Juan de Fuca plate subduc- parallel to the 6-12 Ma locus of the Pemberton tween volcanism and the Cordilleran ice sheets, tion along the continental margin (Dickinson, belt (Fig. 1), constitute the first episode of wide- but preglacial and postglacial phases are also 1973; MacLeod and others, 1977). The cessa- spread volcanic activity. This volcanism was present. These relations record several episodes tion of volcanism coincided closely with the probably related only to subduction of Explorer of late Pleistocene glacial advance and retreat initiation of the Sovanco fault zone between plate because (1) at 2.6 Ma, the Nootka fault (prior to 0.09, 0.3, and 1.2 Ma) in the Coast the Juan de Fuca and Explorer segments of the zone was located beneath the continental margin Mountains of southwestern British Columbia. spreading ridge system, with breakup of the at a position south of the Meager Creek field Early Pliocene igneous activity of the Gari- Miocene Juan de Fuca plate into the present (Hyndman and others, 1979) and (2) the onset baldi belt, represented by 4 Ma dacite dikes that Juan de Fuca and Explorer plates, and with of volcanic activity coincided with a rapid cut high-level plutons within the Franklin Gla- initiation of the brief episode of Alert Bay vol- change in the direction of Explorer-North cier field, may be related to disruption of canism on (Fig. 1; Riddi- American plate motion to an almost northerly steady-state plate consumption patterns along hough, 1977; Bevier and others, 1979; Keen and direction and with subsequent migration of the the northern edge of the subducted Explorer Hyndman, 1979; Armstrong and others, 1985). triple point northwestward toward its present plate under Vancouver Island. Late Pliocene to Garibaldi belt eruptions, which began after a position off the northern end of Vancouver Is- Holocene lavas and pyroclastic rocks constitute long hiatus in volcanic activity along the conti- land (Riddihough, 1977). The general absence the products of the several episodes of wide- nental margin (=3 m.y. in the north and as of volcanism south of the Franklin Glacier and spread volcanic activity. Andesitic and dacitic much as 20 m.y. in the south; Table 1; Berman Meager Creek fields may indicate that south- lavas were erupted in the intervals 2.3 to 1.7 Ma and Armstrong, 1980), may be related to sub- ward along the convergent margin, magmatic and 1.1 Ma to present in the northern part and duction along the Explorer-North American systems required increasingly longer periods of in the intervals 1.4 to 1.0 Ma and 0.7 Ma to and Juan de Fuca-North American plate post-Miocene readjustment to changes in con- present in the southern part of the belt, but ba- boundaries and to "edge effects" along the vergence rate and dip of the subducting Juan de saltic volcanism occurred only during the past Explorer-Juan de Fuca and northern Explorer Fuca plate and(or) to westward rotation in its 0.15 m.y. The 1.7-2.3 Ma volcanic episode was plate boundaries. subcrustal position. probably related only to subduction of the Ex- Earliest Pliocene igneous activity along the Hyndman and others (1979) concluded that plorer plate, whereas subsequent activity ac- continental margin is represented by 4 Ma dacite the position of the northeast-trending Nootka companied subduction along both Explorer- dikes that cut 7-8 Ma high-level quartz monzo- fault has migrated both northward and south- North American and Juan de Fuca-North nite plutons within the Franklin Glacier field ward along the coast of Vancouver Island over American plate boundaries. The establishment (Table 1). These subvolcanic intrusions may the past 2.6 m.y. Because 0.97 Ma alkaline ba- of widespread volcanism along the entire Gari- represent either the final eruptions within the salts of the Salal Glacier area appear related to baldi volcanic belt at 0.7-1.1 Ma suggests that Pemberton volcanic belt or a transition phase magma genesis along the edge of the subducted the Nootka fault, which separates the Juan de between Pemberton and Garibaldi belt volcan- Juan de Fuca plate (Lawrence and others, Fuca and Explorer plates, stabilized in its pres- ism. It seems more likely, however, that the 1984), it seems likely that the Nootka fault sta- ent position off southwestern British Columbia Franklin Glacier activity represents a distal seg- bilized in its present position off southwestern about the same time. ment of the 2.5-8 Ma Alert Bay volcanic belt British Columbia about the same time (1.0-1.3 (Fig. 1) and resulted from disruption of steady- Ma) that widespread volcanism, associated with state plate consumption patterns along the de- subduction along the Juan de Fuca-North scending plate edge as the Explorer-North American plate boundary, occurred along ACKNOWLEDGMENTS American plate convergence rate decreased and the N30°W-trending Pleistocene to Holocene changed in direction between 5 and 2.5 Ma volcanic axis south of the Salal Glacier area This work was supported by the Geothermal (Riddihough, 1977; Armstrong and others, (Fig. 1). Project of the Geological Survey of Canada and 1985). As the locus of the Alert Bay belt is a National Science and Engineering Research almost coincident with the projected trace of the SUMMARY Council of Canada operating grant to R. L. northern edge of the subducted plate under Van- Armstrong. Krista Scott performed the K anal- couver Island at 6.5 Ma (Riddihough, 1977), The development of Garibaldi belt volcanic yses. Critical reviews by William E. Scott, Paul displacement of Franklin Glacier activity from complexes was intimately related to the timing E. Hammond, and Anita L. Grander are sin- the main Alert Bay trend might reflect a slight and extent of late Pleistocene to Holocene gla- cerely appreciated; their constructive criticism clockwise rotation of the plate edge between 6.5 ciations in southwestern British Columbia and greatly improved the original manuscript. This and 4 Ma. Alternatively, volcanism may have to late Cenozoic changes in Juan de Fuca-Ex- work owes much to the insight and observations been related to perturbations along the incipient plorer-North American plate configuration of W. H. Mathews.

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