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Geology and Petrology of Quaternary Volcanic Rocks, Garibaldi Lake Area, Southwestern British Columbia: Summary

Geology and Petrology of Quaternary Volcanic Rocks, Garibaldi Lake Area, Southwestern British Columbia: Summary

Geology and petrology of Quaternary volcanic rocks, Garibaldi area, southwestern : Summary

NATHAN L. GREEN" Department of Geological Sciences, University of British Columbia, , British Columbia,

INTRODUCTION GEOLOGY

Subduction of the beneath the North Ameri- The volcanic suites record at least two periods of can plate has been accompanied by Pleistocene-Holocene eruptions eruptive activity. Hornblende- flows and pyroclastic mate- of , andesite, and along the Garibaldi in rials were erupted during the earliest phase of activity at The Black southwestern British Columbia (Souther, 1977; Keen and Tusk and Mount Price complexes (Fig. 1); the latter (Red Hyndman, 1979). The 25-km-wide belt, comprising at least eigh- Mountain) were extruded onto glacial sediments that teen volcanic complexes, lies approximately 250 km inland from suggest the existence of an earlier continental ice sheet or a local the convergent margin. It extends from , at the alpine glacier. After quiescent periods of unknown length, activity head of , northwesterly for 140 km to Bridge River. was renewed at both centers. Eruption of hypersthene-andesite in the Garibaldi Lake area, about 6 km north of Mount , culminating in the extrusion of an endogenous dome, con- Garibaldi, produced a succession of flat-lying olivine in the structed the second-stage cone of . Hornblende- Cheakamus River valley, basaltic-andesite lavas at The Cinder biotite andesite flows and tuff breccias formed the nearly symmetri- Cone and Sphinx Moraine centers, and a line of three andesitic cal Mount Price cone on the western fllank of the Red Mountain complexes: The Table, Mount Price, and The Black Tusk. volcano. Phreatic eruptions at The produced a broad

TABLE 1. PHENOCRYST EQUILIBRATION TEMPERATURES AND ESTIMATED PRE-ERUPTIVE WATER CONTENTS f melt C B Suite Lithology" Tl" T2 T? PHs0 (bars)" Ptotai (bars)' X» 20 Wt % h2o

Cheakamus Valley BSLT 1027 500 1000 0.21 1.0 Desolation Valley BSAT 1025 1024 1090 3900 0.27 2.8 Sphinx Moraine HBST 992 973 2240 5750 0.38 3.6 Table Meadow HADT 939 936 2230 3800 0.43 3.8 Table HADT 909-937 909- 949 2530-2900 3400-3900 0.42-0.47 3.7-5.5 The Black Tusk (Stage II) HYAN 965-982 960-1010 924 1510-1880 2900-3300 0.32-0.39 3.1-3.5 Mount Price/ HBAN 805-875 2980-3930 3500-4000 0.49-0.57 5.5-7.7

Notes: a. BSLT, basalt; BSAT, amphibole-free ; HBST, hornblende-bearing basaltic andesite; HADT, hornblende andesite; HYAN, hypersthene andesite; HBAN, hornblende-biotite andesite. b. Tl: temperatures (°C ± 60°C) calculated using average pyroxene phenocryst compositions and the Wood and Banno (1973) two-pyroxene geothermometer. c. T2: temperatures (°C ± 70°C) calculated using average pyroxene phenocryst compositions and the Wells (1977) two-pyroxene geother- mometer. d. T3: temperatures (°C ± 30°C) calculated using co-existing Fe-Ti oxides (Buddington and Lindsley, 1964). e. Water pressures estimated from the plagioclase geothermometer of Kudo and Weill (1970) and phenocryst equilibration temperatures using the method of Stormer and Carmichael (1970). Plagioclase temperatures, calculated with average plagioclase and bulk rock compositions, approximate low-pressure liquidus temperatures. Calculated water pressures represent maximum values, with estimated uncertainty of ±750 bars. f- Protai and XflJ'o estimated from Fig. 18 and 19 of Burnham and Davis (1974) using temperatures calculated with pyroxene and Fe-Ti oxide

geothermometers and PHl0 derived from Kudo-Weill plagioclase thermometer; PH,0 assumed to equal equilibrium water pressure (P™"). Esti- 1 mated uncertainties are ±1000 bars in PXotal and ±0.05 in Xfi'o -

g. Maximum pre-eruptive water contents estimated using XS'o' and activity-composition relations in the NaAlSi3Qj —H20 system (Burnham and Davis, 1974). Estimated uncertainties are ±1.0% water.

The complete article, of which this is a summary, appears in Part II of the Bulletin, v. 92, no. 10, p. 1359-1470.

* Present address: Department of Geology, University of Alabama, University, Alabama 35486.

Geological Society of America Bulletin, Part I, v. 92, p. 697-702, 2 figs., 2 tables, October 1981.

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CLINKER PEAK CHEAKAMUS, QC Hornblende-biotite andesite flows and scoria: a - VALLEY Barrier andesite; b - Culliton Creek andesite. PRICE BAY CONE QPP Hornblende-biotite andesite aggultinate breccia; minor dykes. CHEAKAMUS VALLEY Qv Olivine basalt. THE CINDER CONE

QH Helm Creek alkali basalt and mugearite flows; minor pyroclastic material. THE BLACK THE TABLE ,QB* 0.04O /THE c|NDER QTV Table hornblende andesite flows and tuff breccia. o> QoW/jcohJE ÛTm Table Meadow hornblende andesite flow. 13 QBA MOUNT PRICE *Easf Bluff QMb Hornblende-biotite andesite flows. Helm J Glacier QMa Hornblende andesite flows and tuff breccias; minor hornblende-biotite daclte flows and welded tuff. THE CINDER CONE

QD Desolation Valley basaltic andesite flow and related pyroclastic material. SPHINX SPHINX MORAINE MORAINE Qs Hornblende-bearing basaltic andesite flow. THE BLACK TUSK A NL MOU Stage II: hvpersthene andesite flows and plug dome. RED MOUNTAIN CONE ÛMr Hornblende andesite flows and tuff breccias. QMg Basal agglomerate; glacial outwash and minor tuffs» THE BLACK TUSK Qßa Stage I: platy pyroxene andesite and hornblende dacite flows, dykes and lithic tuffs.

Figure 1. Distribution of Pliestocene-Holocene volcanic rocks in the Garibaldi Lake area. Rock units shown in legend are listed in sequence o 1.2 K-Ar date in millions of years. of eruptive activity inferred from field relations and K-Ar dates Levee. (J. Harakal, 1976, personal commun.). I 2 3 »«""' Pressure ridges. KILOMETERS © Vent.

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TABLE 2. COMPARATIVE CHEMISTRY OF PLEISTOCENE-RECENT VOLCANIC SUITES FROM GARIBALDI LAKE AREA

Cheakamus Valley (25) Desolation Valley (4) Sphinx Moraine (3) Range Mean Range Mean gdms§ Range*'1 Mean gdms§

Si02 48.57-52.13 50.08 55.38-56.45 55.99 60.35 54.58-54.96 55.06 64.60 Ti02 1.44- 1.59 1.52 0.84- 0.98 0.94 1.28 0.87- 0.89 0.88 0.81 AI2O3 15.08-16.31 15.73 16.85-18.04 17.33 18.01 17.29-17.40 17.42 18.76 Fe203* 10.02-12.32 11.28 6.95- 7.75 7.44 5.71 6.71- 6.76 6.76 2.38 MnO 0.13- 0.16 0.13 0.12- 0.14 0.13 0.11 0.10 0.10 0.03 MgO 6.42- 8.81 8.43 4.84- 5.46 5.21 2.12 5.80- 6.73 6.31 1.48 CaO 8.47- 9.00 8.64 6.78- 7.11 6.97 6.01 8.05- 8.25 8.14 5.29

Na20 2.77- 4.14 3.47 4.18- 4.77 4.51 4.97 3.70- 3.98 3.84 4.69 K2O 0.38- 0.63 0.47 0.99- 1.14 1.06 1.43 1.09- 1.18 1.14 1.95 P2O5 0.25- 0.33 0.28 0.40- 0.41 0.41 0.31 0.31

Nb 9- 12 11 3- 12 9 5 5 Y 18- 23 21 20- 27 23 22 22 Zr 90-109 102 88-141 118 124 124 Rb 4- 9 7 9- 12 10 13 13 Sr 448-559 467 752-964 774 1300-1309 1306 Ba 80-163 132 305-462 356 462 462 Cu 47- 60 57 38- 51 43 56 56 Zn 93-115 107 79- 84 82 55 55 Ni 69-147 118 72- 95 83 89- 111 100 Cr 141-192 172 105-104 118 104- 135 120 V 176-228 196 120-163 146 141- 174 158

K/Rb 470-987 774 809-934 872 688-754 716

Note: Total iron reported as Fe^j. ""t Single values indicated two analyses. § Determined by wide-scan microprobe analysis.

tuff ring. During the late stages of The Cinder Cone activity, the spinellid, magnesian olivine (Fo87-84), aluminous (4.24% A1203) associated Desolation Valley basaltic andesite spread northward bronzite, diopsidic augite, pargasitic hornblende, plagioclase, and within the glacial valley. The Sphinx Moraine basaltic andesite is- magnetite. The amphibole formed through reaction of earlier- sued from a now-concealed vent near the eastern shore of Garibaldi crystallized olivine with the enclosing andesitic melt. In the Desola- Lake, and olivine basalts were extruded into the glacially scoured tion Valley basaltic andesite, the order of crystallization inferred floor of the Cheakamus River valley. from crystal clots was: spinellid, then olivine, followed closely by The latest phase of volcanism commenced during the last major plagioclase, clinopyroxene, and orthopyroxene. These pétro- (Fraser) continental glaciation. The Table, a hornblende-andesite graphie observations argue strongly against an interpretation that , formed when repeatedly flooded a deep, cylindrical pipe the basaltic andesites represent primary . thawed through the ice sheet (Mathews, 1951). After evacuation of (3) The Desolation Valley basaltic andesite contains titano- the continental glacier from higher elevations, the small Price Bay magnetite as inclusions within olivine xenocrysts and as isolated adventive cone was constructed on the northern flank of Mount microphenocrysts. The included grains are enriched in Al, Mg, and Price, and two hornblende-biotite andesite flows were emitted from Cr, and depleted in Ti, Fe, and Mn, compared with the mic- Clinker Peak, a breached lava ring in the western shoulder of rophenocrysts. As magnetites coexisting with silicate liquids at 10

Mount Price cone. At lower elevations, "esker like" basaltic lavas kbar are notably richer in Cr203 plus Al20:1 and poorer in Ti02 were extruded within tunnels or trenches thawed in glacial ice that and total iron, than those precipitated at 1 kbar (Osborn and Wat- still occupied the Cheakamus River valley (Mathews, 1958). son, 1977), the lack of continuous compositional variation between the oxide inclusions and microphenocrysts supports microscopic PETROLOGY evidence that a /3 phase was precipitated during two distinct stages of the lava's crystallization history. Lavas of each Garibaldi Lake eruptive suite exhibit distinctive (4) Mount Price and Clinker Peak andesites contain xenocrysts phenocryst assemblages. The extended report in Part II describes in of almost completely resorbed pargasitic hornblende and micro- detail the petrography and chemical mineralogy of the lavas, which phenocrysts of magnesiohornblende. Compositional differences be- form the basis for geochemical interpretation of magmatic differ- tween the xenocrysts and microphenocrysts suggest that the latter entiation. The following observations are of particular significance: amphiboles crystallized under lower temperatures and higher water (1) The basaltic andesites and andesites contain more magnesian pressures than the coexisting xenocrysts. These differences possibly olivines and clinopyroxenes than those in the Cheakamus Valley reflect progressive changes in physiochemical conditions within dif- basalts. This feature strongly argues against fractionation of the ferentiating bodies. basalts of produce the andesitic rocks. (5) All andesites contain cognate xenoliths which are composed (2) The basaltic andesites contain microxenoliths that indicate an largely of minerals present as phenocrysts or xenocrysts in the host extensive crystallization history. The Sphinx Moraine lava, for lava, although in different proportions. Gabbroic microxenoliths, example, shows the following crystallization sequence, determined interpreted to indicate segregation of crystals and liquid in deep- from the textural relationships of the various xenolith phases: seated magma chambers, are dominated by plagioclase or am-

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TABLE 2. fContinued)

Table Meadows (4) Table (7) Mount Price (4) Range4'1 Mean Range Mean Range Mean

Si02 58.22-60.24 59.81 57.25-58.19 57.83 58.37-63.11 61.88 Ti02 0.64- 0.69 0.68 0.63- 0.70 0.66 0.54- 0.72 0.58 AI2O3 18.30-18.75 18.47 18.15-18.75 18.50 17.77-18.55 18.48 Fe,Q, 5.60- 6.53 5.63 5.81- 6.50 6.21 4.33- 6.02 5.08 MnO 0.10 0.10 0.08- 0.10 0.10 0.08- 0.09 0.09 MgO 3.10- 3.83 3.16 3.59- 3.88 3.76 2.35- 3.57 2.84 CaO 5.74- 6.72 5.79. 6.56- 6.80 6.72 4.88- 6.67 5.54

Na20 4.22- 4.83 4.68 4.20- 4.96 4.63 3.13- 4.54 3.97 K2O 1.23- 1.39 1.37 1.13- 1.33 1.23 1.12- 1.36 1.28 P2O5 0.24- 0.30 0.29 0.30- 0.39 0.36 0.25- 0.31 0.27

Nb 2 2 4- 7 5 Y 13 13 12- 18 15 Zr 99 99 75- 92 84 Rb 15- 18 17 11- 13 12 12- 15 14 Sr 926-118 6 969 1260-1397 1316 846-1304 970 Ba 410- 444 414 411- 492 461 416- 540 484 Cu 27- 32 30 12- 58 39 14- 50 25 Zn 62- 102 76 67- 77 72 52- 77 60 Ni 26- 39 30 29- 37 33 21- 49 31 Cr 21- 36 22 26- 39 33 20- 40 26 V 91- 118 100 117- 128 122 97- 140 107

K/Rb 715- 791 750 771- 953 873 715- 957 800

phibole. The medium-grained xenolith assemblages include: ampb were derived from a common mantle source. The uniformly low + plag + mgt in hornblende-bearing andesites, or plag + cpx + Rb/Sr (0.008 to 0.029) and Sr87/Sr8li ratios imply that the source opx + mgt in hypersthene andesites. Plagioclase is the predominant region was depleted in Rb and related lithophile elements as a result phase in porphyritic, hyalo-ophitic, and diktytaxitic inclusions that of previous melt extraction. also occur in most siliceous andesites. These cognate xenoliths, The Cheakamus Valley basalts are characterized by low Rb, K,

which are regarded as crystal accumulates torn from the walls of Ni, Cr, V, and Mg/(Mg + iFe), moderately high A1203 and Ti02, shallow reservoirs, show less disaggregation than the grabbroic in- and high total-iron contents (Table 2). The limited compositional clusions and illustrate several possible high-level crystallization variation in the basalts can be explained by low-pressure fractiona- paths: plag + ampb + opx + mgt in The Table, and The Black tion of the observed phenocryst phases: olivine, plagioclase, and Tusk Stage I hornblende andesites; plag + ambp + opx + biot + clinopyroxene.

mgt ± ilm ± qtz in Mount Price, Price Bay, and Clinker Peak The calc-alkaline lavas are characterized by low Rb and Ti02,

hornblende-biotite andesites; and plag + opx + mgt ± ilm in The low to moderately high Ni and Cr, and high A120:), Sr, and K/Rb Black Tusk Stage II hypersthene andesites. The presence of cognate contents (Table 2). There is a complete gradation from basaltic xenoliths in the andesitic rocks provides direct evidence that these andesite to dacite in the region as a whole, but this compositional lavas have undergone a comparatively complex evolution. range does not characterize products of any single volcanic center. (6) Coexisting Fe-Ti oxides and pyroxenes define systematic dif- Lava sequences produced during different eruptive stages of The

ferences in temperature (805 to 1,025°C) and -log f02 (10 to 13) Table, Mount Price, and The Black Tusk complexes exhibit re- between individually coherent andesitic eruptive groups. Estimated stricted chemical variation. Moreover, the earliest lavas of each se- pre-eruptive magmatic-water contents are less than 1 wt. % in quence have more primitive compositions than the final products of basalts, 3.0 to 3.5 wt. % in hypersthene andesites, 3.6 to 5.5 wt. % the preceding stage, suggesting that each eruptive suite involved a in hornblende andesites, and 5.5 to /.7 wt. % in hornblende-biotite separate batch of magma tapped at different stages of differentia- andesites (Table 1). tion. When considered individually, the andesitic suites exhibit CHEMISTRY major-and trace element features that can be related qualitatively to differences in their phenocryst mineralogy. For example, the Deso- The Garibaldi Lake lavas represent two distinct magmatic as- lation Valley basaltic andesite and The Black Tusk hypersthene sociations, one comprising the "mildly" alkaline Cheakamus Val- andesites have lower large-ion-lithophile element (LLLE) contents

ley basalts, with Si02 between 48.57 and 51.09%, and the other than other members of the calc-alkaline series (Table 2). The lower

comprising calc-alkaline rocks with Si02 ranging from 54.58 to L1LE concentrations cannot be explained by simple accumulation 64.29%. The two groups display separate but subparallel K-Rb or fractionation of mineral phases observed in the andesites, but trends (Fig. 2), consistent with mineralogical evidence that they are they correlate with an absence of amphibole in The Black Tusk and not related by fractionation processes. However, Sr87/Sr811 ratios, Desolation Valley lavas. It is suggested that these different levels of which range from 0.7030 to 0.7036 for basalts and from 0.7028 to L1LE enrichment may have been inherited, in part, at an early crys- 0.7036 for andesitic rocks, suggest that the basalts and andesites tallization stage, during which more-hydrous conditions resulted in

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TABLE 2. (Continued)

Clinker Peak (9) The Black Tusk I (4) The Black Tusk II (9) Range" Mean Range* * Mean Range Mean

SiOz 59.03-64.31 60.98 58.92-65.29 62.66 58.25-62.20 60.30 TiOa 0.52- 0.79 0.69 0.49- 0.80 0.61 0.58- 0.80 0.70 AI2O3 17.31-18.24 17.93 17.78-18.30 18.05 17.34-18.93 18.36 FesO;, 4.54- 5.94 5.36 2.38- 6.41 4.40 5.07- 6.61 5.90 MnO 0.08- 0.10 0.09 0.08- 0.11 0.09 0.09- 0.11 0.10 MgO 2.23- 3.51 2.97 1.68- 2.63 2.14 2.33- 3.01 2.81 CaO 4.57- 6.49 5.82 5.05- 6.06 5.47 5.36- 6.06 5.95

NazO 3.77- 4.74 4.41 4.32- 4.73 4.60 3.63- 4.95 4.31 K2O 1.22- 1.59 1.42 1.16- 1.58 1.44 1.18- 1.38 1.27 P2O5 0.20- 0.37 0.31 0.18- 0.37 0.25 0.22- 0.34 0.29

Nb 8 8 1- 5 3 Y 17- 18 17 16- 22 19 Zr 94- 128 111 96-156 123 Rb 13- 18 16 11- 26 19 11- 18 13 Sr 751-1056 944 698-910 809 848-942 895 Ba 459- 668 532 533-555 544 363-483 409 Cu 9- 28 19 22 22 26- 46 36 Zn 44- 66 55 50 50 63- 80 72 Ni 23- 33 27 12 12 18- 39 37 Gr 23- 32 28 8- 27 16 12- 37 28 V 75- 122 103 85-106 92 74-129 114

K/Rb 675-1046 788 506-891 677 744-958 858

a lesser amount of plagioclase, and a greater proportion of olivine relative to pyroxene, fractionated from the parental magmas of the amphibole-bearing, high-LILE rocks than from those of the amphibole-free, low-LILE rocks.

ORIGIN OF THE CALC-ALKALINE SERIES

Observed chemical and mineralogical variations in the Garibaldi Lake calc-alkaline rocks can be explained in terms of multistage fractionation processes. However, the individually coherent erup- tive groups have evolved from separate and chemically distinct pa- rental magmas. The lavas and their cognate xenoliths therefore il- lustrate several different evolutionary paths. These crystallization trends probably reflect varied physiochemical conditions within Garibaldi Lake subvolcanic systems. Petrographic evidence indicates that ampb + plag + mgt and plag + cpx + opx + mgt, respectively, precipitated during the late crystallization stage of Sphinx Moraine and Desolation Valley basaltic andesites. Similar mineral assemblages constitute gabbroic microxenoliths in The Table, Mount Price, and The Black Tusk 5 10 50 100 andesites. Using simple fractionation models, it is possible to derive the andesitic compositions from precursor basaltic-andesite liquids by subtracting the appropriate xenolith minerals in amounts simi- Rb(p.p.m.) lar to those observed in the modes of gabbroic xenoliths in selected Figure 2. K versus Rb diagram. Garibaldi Lake volcanic rocks. daughter lavas. It appears, therefore, that fractional crystallization Open circles, Cheakamus Valley basalts; filled circles, calc-alkaline of basaltic-andesite magmas can produce the andesites of the rocks. Inset shows variation of K and Rb in individual eruptive Garibaldi Lake volcanoes. By analogy with melting relations in groups of the calc-alkaline series. Inset symbols represent: open

andesite-H20 systems (Eggler, 1972; Eggler and Burnham, 1973; stars, Desolation Valley lava; solid stars, Sphinx Moraine lava; Maksimov and others, 1978), it is proposed that the parent basaltic solid triangles, Table lavas; open triangles, Table Meadow lava; andesites fractionated (1) amphibole, plagioclase, and magnetite open squares, Mount Price lavas; solid squares, Clinker Peak and assemblages, to produce hornblende-bearing andesites under nearly Price Bay lavas; solid diamonds, first-stage lavas of The Black

water-saturated conditions (>3.5% H20) at 900 to 975°C and Tusk; open diamonds, second-stage lavas of The Black Tusk. Note pressures greater than 3.5 kbar, and (2) plagioclase, two-pyroxene, the similar range of K/Rb shown by the basaltic and calc-alkaline and magnetite assemblages to produce hypersthene andesites under groups.

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water-undersaturated conditions (<3% H20) at 960 to 1,100°C REFERENCES CITED and pressures above 3 kbar. Buddington, A. F., and Lindsley, D. H., 1964, Iron-titanium oxide minerals The hyalo-ophitic, porphyritic, and diktytaxitic xenoliths appear and their synthetic equivalents: Journal of Petrology, v. 12, p. 310— to represent accumulations of phases which occur as phenocrysts 357. Burnham, C. W., and Davis, N. F., 1974, The role of water in silicate melts and microphenocrysts in the host siliceous-andesite and dacite — II. Thermodynamic and phase relations in the system lavas. Fractionation of this material probably produced limited NaAlSiaOg-HsO to 10 kilobars, 700° to 1100°C: American Journal of silica enrichment in andesite magmas of The Table, Mount Price, Science, v. 274,p. 902-940. and The Black Tusk complexes. Major- and trace-element frac- Eggler, D.H., 1972, Water-saturated and undersaturated melting relations tionation models demonstrate that elemental variations within in- in a Paricutin andesite and an estimate of water content in a natural magma: Contributions to Mineralogy and Petrology, v. 34, p. 261- dividually coherent eruptive groups of these volcanoes can be ex- 271. plained adequately by removal of phenocryst phases in amounts Eggler, D.H., and Burnham, C. W., 1973 Crystallization and fractionation

similar to observed xenolith modes from the most-basic andesite in trends in the system aridesite-H20-C02 -02 at pressures to 10 kb: each group. The different fractionation paths of the andesitic erup- Geological Society of America Bulletin, v. 84, p. 2517-2532. tive groups, represented by the distinctive mineral assemblages of Keen, C.E., and Hyndman, B..D., 1979, Geophysical review of the continen- tal margins of eastern and : Canadian Journal of Earth their hyalo-ophitic, porphyritic, and diktytaxitic xenoliths, may be Sciences, v. 16, p. 712 -747. related to cessation of magma crystallization at different tempera- Kudo, A. M., and Weill, D. F., 1970, An igneous plagioclase thermometer: tures within upper-crustal magma reservoirs, as suggested by tem- Contributions to Mineralogy and Petrology, v. 25, p. 52—65. perature differences inferred from various geothermometers (Table Maksimov, A. P., Kadik, A. A., Korovushkina, E. Ye., and Ivanov, B. E., 1). 1978, Crystallization of an andesitic melt with a fixed water content at pressures to 12 kbar: Geochemistry International, v. 15, p. 20—29. Experimental studies (Nicholls and Ringwood, 1973; Mysen and Mathews, W.H., 1951, The Table, a flat-topped volcano in southern British others, 1974; Wyllie, 1979) indicate that, if basaltic-andesite mag- Columbia: American Journal of Science, v. 250, p. 830-841. mas are partial melting products of mantle peridotite, they can be 1958, Geology of the Mount Garibaldi map-area, southwestern British generated only under water-saturated conditions. However, the Columbia-II. Geomorphology and Quaternary volcanic rocks: Geological Society of America Bulletin, v. 69, p. 179-198. late appearance or non-appearance of amphibole in the crystalliza- Mysen, B. O., Kushiro, I., Nicholls, I. A., and Ringwood, A. E., 1974, A tion sequence of Garibaldi Lake basaltic andesites indicates that possible mantle origin for andesitic magmas-Discussion of a paper by these magmas were water-undersaturated or, at most, water- Nicholls and Ringwood: Earth and Planetary Science Letters, v. 21, p. saturated only at shallow levels. The presence of magnesian 221-229. olivines, aluminous bronzite, diopsidic augite, and chromian Nicholls, I. A., and Ringwood, A. E., 1973, Effect of water on olivine sta- bility in tholeiites and the production of silica-saturated magmas in the titanomagnetite in the basaltic andesites suggests the existence of island-arc environment: Journal of Geology, v. 81, p. 285—300. basaltic precursors, but these parental magmas apparently have not Osborn, E. F., and Watson, E. B., 1977 Studies of phase relations in subal- erupted at the surface. The Cheakamus Valley basalts cannot repre- kaline volcanic-rock series: Carnegie Institute of Washington Year sent the parental magmas from which the andesitic suites were de- Book 76, p. 472-478. rived. Based on experimental data for water-undersaturated olivine Souther, J. G., 1977, Volcanism and tectonic elements in the Canadian Cordillera-A second look,/« Baragar, W.R.A., Coleman, L. C., and and Si02 —saturated tholeiite compositions (Nicholls and Ade-Hall, J. M., eds., Volcanic regimes in Canada: Geological As- Ringwood, 1973), basaltic magmas have high-pressure liquidus sociation of Canada Special Paper 16, p. 3-24. temperatures lower than their 1-atm liquidus temperatures and Stormer, J. C., and Carmichael, I.S.E., 1970, The Kudo-Weill plagioclase must therefore fractionate during ascent, when their water contents geothermometer and porphyritic acid glasses: Contributions to Mineralogy and Petrology, v. 28, p. 306-309. exceed 2% at 5 kbar, 5% at 10 kbar, or 10% at 15 kbar. As these Wells, P.R.A., 1977, Pyroxene thermometry in simple and complex sys- water contents agree reasonably well with values for Garibaldi tems: Contributions to Mineralogy and Petrology, v. 62, p. 129—139. Lake andesitic rocks estimated independently from geother- Wood, B. J., and Banno, S., 1973, Garnet-orthopyroxene and mometry (Table 1) and from relevant melting data on andesitic sys- orthopyroxene-clinopyroxene relationships in simple and complex tems (Eggler, 1972; Eggler and Burnham, 1973; Maksimov and systems: Contributions to Mineralogy and Petrology, v. 42, p. 109 — 124. others, 1978), it is suggested that calc-alkaline progenitors have not Wyllie, P. J., 1979, Magmas and volatile components: American erupted in the Garibaldi Lake area because they were more hydrous Mineralogist, v. 64, p. 469-500. than the Cheakamus Valley basalts and, thus, were forced to un- dergo a greater amount of fractionation before they reached the MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 20, 1980 REVISED MANUSCRIPT RECEIVED MAY 7, 1981 surface. MANUSCRIPT ACCEPTED MAY 19, 1981

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