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 volcanic belt, southwestern British Columbia 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, Vancouver, 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-North American plate configuration to about 40% by volume. The smaller volcanic along the continental margin. centers erupted mainly andesite with only rare The Cenozoic Garibaldi belt comprises 6 dacite. Basalts 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 lavas or as isolated flows northwest from the head of Howe Sound 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 Tertiary 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 basalt, 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 rhyodacite. Many of the belt extends 240 km north-northwest from The volcanic fields are 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 volcanism 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 lava 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 diorite, 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 Jurassic to Miocene They suggest that volcanic activity was epi- tuyas, 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, Mount Garibaldi, 1976). 1.7 Ma and 1.1 Ma to present in the northern and Garibaldi Lake 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 Mount Cayley, Meager Creek, 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 late Pleistocene 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 subduction of the Juan de Fuca plate, 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 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 564 GREEN AND OTHERS 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 fault 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 rhyodacites (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 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 CENOZOIC GARIBALDI VOLCANIC BELT, BRITISH COLUMBIA 565 SILVERTHRONE VOLCANIC FIELD Figure 2. Diagram showing the dis- FRANKLIN GLACIER VOLCANIC FIELD tribution of isotopic dates along the Garibaldi

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