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40Ar/39Ar Ages for Plutons of the Monteregian Hills, Quebec: Evidence for a Single Episode of Cretaceous Magmatism

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40Ar/39Ar Ages for Plutons of the Monteregian Hills, Quebec: Evidence for a Single Episode of Cretaceous Magmatism 40Ar/39Ar ages for plutons of the Monteregian Hills, Quebec: Evidence for a single episode of Cretaceous magmatism K. A. FOLAND LISA A. GILBERT* Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210 CHERYL A. S1ÏBRING CHEN JIANG-FENG* ABSTRACT graphic expression as monadnocks rising hun- hypothesis is the mantle hotspot or plume model dreds of metres above the St. Lawrence Low- (Wilson, 1963; Morgan, 1971); in this model, a ^Ar/^Ar data for biotite and amphibole lands; distribution in a quasi-linear, nearly hotspot "trace" of intrusives might be expected from six plutonk complexes of the Montere- east-west belt extending for more than 200 km; to show a geographic age progression in an ap- gian petrographic province in Quebec, Can- and pronounced alkaline affinity with attendant proximately linear belt. The ages could rnle out ada, are presented. Generally uncomplicated peculiar rock types and uncommon minerals a hotspot model in favor of another, such as a and concordant incremental heating spectra have stimulated interest over the years. Isotopic fracture model (for example, Uchupi and others, are observed, although there is evidence for age data for these intrusions have been reported, 1970). minor excess 40.\r at several localities. Bio- although most reports are of limited scope and In this report, 40Ar/39Ar mineral ages for tites and amphiboles are concordant, consist- of 1960s vintage. seven Monteregian intrusions are discussed. This ent with rapid cooling of the high-level The significance of the ages in the Montere- work is part of an ongoing project to investigate complexes. Ages for different phases of indi- gian province and analogs elsewhere in a mod- the pedogenesis and to identify the mantle vidual complexes are analytically indistin- ern framework of igneous petrology and plate sources of such complexes. A recent paper guishable and suggest that the various princi- tectonics transcends a simple mission of geo- (Eby, 1984a) reported Rb-Sr and fissioi-track 40 39 pal lithologies are cogenetic. The Ar/ Ar chronology. The ages are key parameters in dates suggesting very different ages not only for ages of six Monteregian complexes (St. Hil- understanding not only the genesis of specific various Monteregian complexes but also for dif- aire, Rougemont, Johnson, Shefford, Brome, rock compositions but also the formation of the ferent phases of individual ones. The new Megantic) all fall within the very restricted primary magma or magmas. Alkaline, epizonal 40Ar/39Ar results are in significant conflict with range of 124 ± 1 in.y. It is suggested that all of intrusive complexes, such as the Monteregians, these dates and indicate not only a short intru- the Monteregian plutons, with the exception usually consist of a series of intrusions or rock sive history at individual centers but also essen- of Oka, were foimed within a short interval types of substantially different chemistries. The tially one major episode of magmatism :br the (1-2 m.y.) during the Cretaceous, consistent ages of various intrusions are important in estab- province. with paleomagnctic results. The new ages, lishing whether the associated units are coge- along with geoctiemical characteristics, sup- netic or whether a petrogenetic link between GEOLOGIC SETTING AND port derivation of the alkaline magmas by a different compositions is likely or needed. Large PREVIOUS WORK fixed mantle hoi spot which also produced age differences (for example, 10 to 20 m.y.) be- broadly similar complexes in New England tween intrusives in a shallow-level complex The Monteregian province of Quebec consists and the chain of New England Seamounts. would suggest that the units are not derived of 10 major plutons and numerous small plugs, from a single-parent magma because it seems dikes, and sills; the intrusions crop ou ; in a INTRODUCTION unlikely that a magma chamber could exist for roughly east-west belt from the Oka complex, The intrusions of the Monteregian petro- such extended periods. Such intrusions are -30 km west of Montreal, to the Mount Megan- graphic province in southern Quebec, Canada, probably produced by different episodes of par- tic complex, ~ 190 km east of Montreal (see Fig. have been the subject of numerous studies since tial melting of the mantle or crust. Alternately, 1). The general geologic and petrologic features first described by Adams (1903). Their topo- intrusives that have essentially identical ages or of the intrusives have been reviewed by Gold small age differences are likely to be cogenetic, (1967) and Philpotts (1974). The location of being related by magmatic processes or, con- intrusions appears to be controlled by crustal *Present addresses: (Gilbert) Earth Technology ceivably, by partial melting events. structures. The province is situated at the junc- Corporation, 3777 Long Beach Boulevard, Long The ages of different localities are important tion of the east-west Ottawa graben and the Beach, California 90807; (Chen) Department of Earth in evaluating the causes of intraplate alkaline north-northeast-trending St. Lawrence graben; and Space Sciences, University of Science and Tech- nology of China, Hel'ei, Anhui, People's Republic of magmatism such as that represented by the several intrusions are located at intersections of China. Monteregian province. A frequently advocated individual faults of these graben systems [Phil- Additional material for this article (Table A) may be obtained free of charge by requesting Supplementary Data 86-16 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 97, p. 966-974, 12 figs., 2 tables, August 1986. 966 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/8/966/3445159/i0016-7606-97-8-966.pdf by guest on 26 September 2021 ^Ar/^Ar AGES FOR PLUTONS OF MONTEREGIAN HILLS, QUEBEC 967 74° model, providing that the ages are consistent. 46* Extrapolation of Duncan's (1984) predicted hotspot tracks would require Monteregian igne- ous activity within a restricted interval -125 m.y. ago. It is important to recognize that the direction of North American plate translation over a fixed mantle hotspot is to the northwest, not east to west as might be inferred from the Monteregian outcrop pattern. On the basis of early isotopic work (Lowdon, 1960, 1961; Fairbairn and others, 1963; Shafi- qullah and others, 1970), the timing of Mon- teregian activity was thought to be ~ 110 ± 20 m.y. ago. Conventional K-Ar biotite determina- 45* tions by the Geological Survey of Canada (Lowdon, 1960, 1961) gave dates (with quoted 1 Oka 6 Johnson (124) uncertainties of ±8 m.y.) of 125 m.y. for the 2 Royal 7 Yamaska nordmarkite at Mount Brome and 129 and 118 m.y., respectively, for the gabbro and granite at 3 Bruno (127) 8 Shefford (124) Mount Megantic.1 Fairbairn and others (1963) 4 St. Hilaire (124) 9 Brome (123) reported K-Ar biotite dates of 113 ± 10 m.y. for 5 Rougemont (125) 10 Megantlc (124) the essexite of Mount Johnson and 98 ± 5 m.y. for the carbonatite at Oka. Shafiqullah and oth- Figure 1. The distribution of Monteregian intrusions (after Gold, 1967). The numbers fol- ers (1970) reported K-Ar dates for minerals lowing the intrusion names are ^Ar/^Ar ages in millions of years that are discussed in the text. from the Oka complex; the biotites gave a mean K-Ar date of 116 ± 4 m.y. The only other argon potts, 1974). The intrusions themselves are New England-Quebec province of Cretaceous isotopic data are for the Mont St. Hilaire com- generally pluglike and have steep, even vertical, age, extending over southeastern Quebec and plex (Gilbert and Foland, in press) that indicate contacts that extend to considerable depth including Vermont, New Hampshire, and east- a short intrusion history for the complex at (Philpotts, 1974). era Maine in New England. 124.4 ±1.2 m.y. Small, yet significant, amounts 40 Petrologically, the rock types of the major Morgan (1972) proposed that the southeast- of excess Ar have been noted at Oka and intrusions are broadly similar and have alkaline trending belt of eastern North American intru- Mont St. Hilaire (Shafiqullah and others, 1970; affinities; at present erosional levels, the compo- sions together with the southeast-trending chain Gilbert and Foland, in press). sitions range from ultrabasic peridotites-pyrox- of New England Seamounts, which lie roughly Early, reconnaissance-level investigation by enites of cumulate origin to syenites and on strike, represent the trace of a mantle hotspot. Fairbairn and others (1963) was sufficient to granites, although varieties of alkali gabbro Subsequently, Foland and Faul (1977) reported indicate low initial 87Sr/86Sr ratios and suggest dominate. Individual complexes are generally the ages of New England intrusions to be spread an age (or ages) from 120 to 130 m.y. Some of composite, consisting of several distinct intrusive over some 100 m.y., making this model difficult the 6 biotite analyses of Fairbairn and others phases. The Oka complex is carbonatitic and to explain. Concluding that more than one hot- (1963) were discordant, and the apparent Rb-Sr petrographically distinct from the others. The spot would be required, they preferred a model mineral ages ranged from 87±2toll8±7 mafic and carbonatitic rock types clearly suggest in which the complexes were generated along m.y., a feature attributed by the authors to 87Sr a mantle origin for the primary magmas; this is the extension of a transform fault during periods loss. It seems probable that these dates are the supported by low initial 87Sr/86Sr ratios ob- of major changes in sea-floor spreading. Re- result of unrecognized analytical problems. served by Fairbairn and others (1963), Bell and newed interest in the hotspot hypothesis was Recently, Eby (1984a), on the basis of Rb-Sr others (1982), and Eby (1984b, 1985) and in stimulated by Crough (1981a), who proposed whole-rock and fission-track dating, found that our unpublished work at several locations.
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