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

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74° model, providing that the ages are consistent. 46* Extrapolation of Duncan's (1984) predicted hotspot tracks would require Monteregian igneous 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. The that only the Cretaceous complexes were the the dates clustered at —118 and 136 m.y.; intru- Sr-Nd isotopic relations of the Monteregian and result of a hotspot which produced the seamount sions of the older group were generally slightly similar New England complexes suggest time- chain. The various aspects of this hypothesis undersaturated to saturated, whereas those of integrated, large-ion-lithophile-element de- have been discussed extensively (Crough, 1981a, the younger one generally were moderately to pleted sources similar to those for recent 1981b; McHone, 1981; Morgan, 1983; McHone strongly undersaturated. Because some com- ocean-island basalts (Bell and others, 1982; and Butler, 1984; and others) and need not be plexes contain both types and the age differences Chen and others, 1984). repeated here. The New England Seamounts are are large, the various associated intrusions are The general tectonic framework of the Mon- the key to evaluation of the hotspot model be- apparently not strictly cogenetic or comagmatic. teregian intrusions in the context of broader cause the chain extends > 1,200 km, whereas the These dates have important implications not Mesozoic igneous activity in the eastern Appala- continental intrusions extend only a few hun- only for the petrogenesis of the various intru- chian region of North America has recently dred kilometres. Duncan (1984) showed that been reviewed by McHone and Butler (1984). seamount volcanism was indeed time progressive and consistent with a hotspot trace. The They included the Monteregian intrusions, as 1 Monteregian and younger White Mountain All ages reported or discussed herein incorporate well as New England counterparts of the White the currently accepted decay constants recommended Mountain magma series (Billings, 1956), in their magma-series intrusions would fit a hotspot in Steiger and Jäger (1977).

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sions but also for tectonic hypotheses, as appar- TABLE 1. SUMMARY OF wAr/3,Ar DETERMINATIONS BASED ON THE DATA IN TABLE A AND DISCUSSED IN THE TEXT ent ages spanned. -20 m.y. COMPLEX Rock type Mineral Total-gas 40Ar/39Ar plateau ^Ar/^Arage it of total Agi METHODS AND RESULTS (m.y.)' 39Art (m.).)

BRUNO gabbro- biotite 132.0 85.6 127.5 - 0.4 Biotite and amphibole separates (>95% pure pyroxenite biotite 131.5 53.0 127.2 :: 0.5 biotite 131.0 except in cases rioted) were analyzed using the 40 39 SHEFFORD diorite biotite 124.0 99.5 123.6 :: 0.9 Ar/ Ar method. This variant of Ar dating is biotite 123.0 necessary because of the occurrence of excess JOHNSON pulaskite biotite 123.6 99.9 123.6 :: 0.8 '"'Ar at some Monteregian localities. Addition- biotite 123.8 ally, good precision is obtainable because only biotite 124.3 Ar isotopic ratios are measured, only a single ROUGEMONT gabbro biotite 125.2 84.4 124.6 :: 0.7 biotite 124.6 calibration (determination of the J parameter) is biotite 124.3

involved, and aliquoting problems are avoided. BROME nepheline amphibole 125.0 96.9 123.3 J 0.9 Furthermore, a high relative precision is possible diorite amphibole 125.5 91.9 123.2 i 0.7 gabbro biotite 123.6 84.4 123.1 i 0.7 if several aliquot; are analyzed and if the sam- gabbro biotite 123.3 84.4 123.2 J 1.1

ples are relative to the same monitor and irra- MEGANTIC granite biotite 123.6 94.1 123.4 i 1.0 diated together (see Randall and Foland, in biotite 123.9 biotite 123.7 press). granite biotite 123.5 93.5 123.9 i 0.6 biotite 123.3 Analytical details of the procedures have been biotite 123.8 diorite biotite 123.5 99.2 123.3 i 0.7 described elsewhere (Foland and others, 1984). biotite 123.3 Except for those from Mount Megantic, all of gabbro amphibole 128.5 98.3 127.4 l 2.2 amphibole 128.3 92.4 125.6 i 0.3 the samples, and also those of Gilbert and Fo- amphibole 132.4 93.0 127.4 ± 1.9 syenite amphibole 130.0 58.8 124.8 ± 2.5 land (in press), were irradiated simultaneously amphibole 127.7 60.0 124.1 t 1.4 using a biotite from Brome as the monitor. The syenite amphibole 123.6 76.4 123.0 ± 1.5 amphibole 124.0 87.5 122.9 ± 0.5 40 39 Ar/ Ar total-gas age of this biotite is 121.7 amphibole 125.3 89.3 123.0 ± 2.3 (±1%) m.y., determined by cross calibration 40 10 * K \tQta| = 5.543 x 10" yr"'. Uncertainties for the plateau ages are standard deviations of the weighted gas fractions that compose the plateat and do not with other standards. The Mount Megantic include a systematic uncertainty in the irradiation parameter J. samples were irradiated separately using an in- ^The sum of the 39Ar of plateau fractions as a percentage of total 39Ar. tralaboratory biotite standard, (K-Ar age of

124.4 (±1.5%) m.y., as the monitor. Relative to TABLE 2. SUMMARY OF 40Ar/3,Ar AGES OF MONTEREGIAN INTRUSIONS these calibrations, the "MMhb-1" interlabora- tory amphibole standard (Alexander and others, Complex Age (m.y.)' Comment 1978) has a 40Ar/39Ar age of 513.5 m.y. The St. Bruno 127.4 ± 1.5 Average plateau age of biotite from gabbro-pyroxenite; age is a maximum and may be irradiations were performed in the H-5 position too old due to excess ^Ar of the Ford Nuclear Reactor at the University of St. Hilaire 124.4 ± 1.2 Average total-gas age of four biotites from nepheline diorite and nepheline syenite Michigan for whi ch the correction factors for (Gilbert and Foland, in press) 2 interfering Ar reactions are given in Table A. Rougemont 124.7 ± 1.5 Average total-gas age of biotite from gabbro All ages incorporate the decay constants from Johnson 123.9 t 1.4 Average total-gas age of biotite from pulaskite Steiger and Jäger (1977), and all uncertainties quoted are at the la level. Shefford 123.5 ± 1.5 Average total-gas age of biotite with amphibole from diorite Brome 123.1 ± 1.2 Average plateau age for amphibole from nepheline diorite and for biotite from gabbro The detailed analytical results of the >150 gas analyses of 31 samples are given in Table A, Megantic 123.6 ± 1.9 Average total-gas age of three biotites from two granites and a diorite

whereas individuell total-gas and plateau ages 'Constants are given in Table 1. Uncertainties are total analytical uncertainties quoted at the 68% confidence level. are given in Table 1. The ages of the complexes are summarized in Table 2. Incremental release spectra are shown in Figures 2-7 and 9-12. Age plateaus are defined to include as large a percen- The various replicate analyses (Tables A and -124 m.y., and have a total spread in average tage of Ar as possible and to be composed of 1) generally show very close agreement. Most, 40Ar/39Ar age of -1.5 m.y. successive increments such that the first and last but not all, samples yield highly concordant apparent ages are indistinguishable at the 2 a spectra. Generally, the total-gas ages are consist- DISCUSSION OF RESULTS level and all intervening fractions overlap with ent with the plateau ages. If there are exceptions the plateau at the la level. Fractions that have to this, the total-gas ages exceed the plateaus and Mount St. Bruno relatively large uncertainties are omitted. All the incremental release spectra suggest the pres- 39 plateaus contain >50% of the Ar, and most ence of excess 40Ar. There are two striking fea- The biotite from a gabbro-pyroxenite of exceed 80% (see Table 1). tures of the Âr/Âr data. First, for any given Mount St. Bruno, the westernmost complex complex, all of the ages (both amphibole and studied, contained -5% impurities, mostly am- biotite) are indistinguishable. Second, with the phibole and pyroxene. It was analyzed three 2Table A may be obtained free of charge by requesting Supplementary Data 86-16 from the GSA Docu- exception of a slightly older date for Mount St. times, once by simple fusion and twice by in- ments Secretary. Bruno, all of the ages are remarkably similar, at cremental heating. The total-gas Âr/Âr

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dates show good agreement at -131.5 m.y., and Figure 2. 40Ar/39Ar incre- 150 the spectra agree well with one another. The mental release spectrum for a incremental heating spectra (Fig. 2) show signif- biotite from a gabbro-pyrox- " | MT ST BRUN0

icant discordance with some high apparent ages enite of Mount St. Bruno. In _ 140 - ! pyroxenitinsi Kifimafie - biotitt e particularly for low-temperature fractions. The this and other age spectra, 'j- apparent ages decrease from -400 m.y. to ~ 127 the sample number and anal- ~ m.y. at >60% gas released. Omitting the discor- ysis identification number are ^130

dant fractions, which have higher apparent ages, given in parentheses; tp is the ~ yields plateau ages of 127.2 and 127.5 m.y. apparent plateau age in mil- £ These apparent dates are significantly lower lions of years; t^ is the total- g than are the total-gas dates. gas date in millions of years < The incremental heating relationships suggest determined from the weight- the presence of significant quantities of excess ed apparent ages of all frac- 40Ar. The shape of the Âr/Âr spectra is sim- tions; and the stippled areas q 20 60 80 100 ilar to those frequently cited to show evidence cover ±1 a analytical uncer- for excess 40Ar (for example, Lanphere and Dal- tainties in the dates, exclud- rymple, 1976), and such plateaus are generally ing uncertainties in J. As regarded as maximum dates. The average pla- discussed in the text, this spectrum suggests excess 40Ar and that the 127.5-m.y. plateau date is teau date of 127.4 m.y. is thus interpreted as a a maximum age. maximum age of Mount St. Bruno, a date significantly older than any other Monteregian younger eastern portion of nepheline syenites three times; the total-gas ages (125.2 to 124.3 40 39 Ar/ Ar age. Two apatite fission-track dates and syenite breccias. An extensive set of m.y.) show close agreement. An incremental of 135 ± 11 m.y. for Mount St. Bruno (Eby, 40Ar/39Ar and K-Ar data for this complex has heating experiment (Fig. 3) shows general con- 1984a) overlap with this date. been reported elsewhere (Gilbert and Foland, in cordance; the apparent plateau age (124.6 m.y.) The 40Ar/39Ar spectra indicating excess 40Ar press). The biotites yielded concordant spectra is not appreciably different from the total-gas for the Bruno biotite are significant in two re- and analytically indistinguishable ages; the average (125.2 m.y.). The apparent ages show a spects. First, the results suggest that excess 40Ar age 40Ar/39Ar ages for 4 biotite samples ranged slight decrease with increased heating tempera- could be a problem even in high-K minerals in only from 124.1 to 124.6 m.y. The results sug- ture which may be real. The spectrum is broadly some high-level, unmetamorphosed plutons gest a short intrusion or cooling history on the similar to those for the Mount St. Bruno biotite, where it is generally not regarded as posing diffi- order of 0.5 m.y. or less. Excess 40Ar was ob- but in this case, there are no highly discordant culties. The apparent concentrations of excess served in some amphiboles, pyroxenes, and fractions and the differences are only marginally 40Ar (>6 x 10"11 mol/g) are not large com- feldspars, but the biotites do not carry quantities significant. The possibility of small amounts of pared to those observed in some metamorphic sufficient to affect their ages significantly. The excess 40Ar cannot be excluded, although the biotites (see, for example, Foland, 1983) but are amphiboles have incremental heating spectra quantities, if present, must be very small in com- sufficient to cause significantly anomalous dates. with progressively decreasing apparent ages, in- parison to St. Bruno. The total-gas ages are be- Second, the 40Ar/39Ar incremental heating dicating heterogeneous distributions of excess lieved to reflect closely an age of 124.8 ±1.5 method, in this case, is clearly capable of 40Ar. All of the Mont St. Hilaire data are con- m.y. for Rougemont. 40 indicating the presence of excess Ar, although sistent with the 124.4 ± 1.2 m.y. age. Eby (1984a) reported apatite fission-track it may not resolve the age. This is an important dates of 136 ± 10 and 138 ± 11 m.y. for a observation pertinent to the interpretation of Mount Rougemont gabbro and pyroxenite from Rougemont. These 40 39 Ar/ Ar dates in general and those presented dates are significantly older than is the below in particular. The shapes of the Biotite from a gabbroic sample at the 40Ar/39Ar age, although only marginally so, 40 39 Ar/ Ar showing decreasing apparent age Rougemont small mafic intrusion was analyzed considering the uncertainties. with increasing gas release indicate heterogeneities in the biotite Âr/^K and are similar to those of amphibole spectra (for example, Harri- 138 - 40 son and McDougall, 1981) with excess Ar. MT. ROUGEMONT Foland (1983) suggested that heterogeneities in 134 gabbro - biotite 40 39 Ar/ K distributions in biotite might not be (Q82-10;# 31104) revealed because of mica decomposition dur- jp30 ing vacuum heating (see also Harrison, 1983), <§.12 6 which is clearly not always the case. Unfortu- Figure 3. 40Ai/39At age nately, the possible effect of the minor impurities spectrum for biotite from a g 122 is unknown. gabbro of Mount Rouge- <0 -y124.6- mont. <â 118 V125.2 Mont St. Hilaire 114

Mont St. Hilaire (Currie, 1983; Greenwood 110110 and Edgar, 1984) has two major lithologic divi- o 20 40 60 80 100 sions, an older western gabbroic portion and a % ^Ar released

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Biotite data for two gabbro samples are 138 MT. JOHNSON shown in Figure 6. Both spectra are concordant and show close agreement of both plateau and 134 puiaskite - biotite (Q82-24;«31093> total-gas ages at -123 m.y. Biotite from gabbro g 130 Figure 4. Age spectrum for Q83-59 shows minor discordance in low- SMT biotite from a puiaskite at temperature fractions in which the higher appar- 40 »126 Mount Johnson. The spec- ent ages suggest minor excess Ar in the mica ® 122 trum is internally concor- or in impurities. Release spectra (Fig. 7) 1'or am- w CO dant. phibole from a nepheline diorite are highly con- -tp=123.6- 1118 cordant with some discordant high apparent ttg=123.6 ages for small low-temperature fractions. The 114 plateau ages, which include >90% of ihe Ar, agree well at 123.3 and 123.2 m.y.; the total-gas 40 60 80 100 ages are slightly, yet significantly, higher at % released 125.5 and 125.0 m.y. and suggest minor quantities of excess 40Ar. In preliminary experiments, a Mount Johnson ages of 120.3 ± 1.0 and 128.5 ± 3.0 m.y. are, in less pure separate of this amphibole w.is ana- our opinion, consistent with 123.5 ±1.5 m.y. lyzed and yielded discordant spectra with, higher A biotite separate having -4% amphibole The analyzed diorite from Shefford is from total-gas ages (-135 m.y.). Further purification from the puiaskite phase of the small Johnson the first intrusive rock type, according to Vali- of the amphibole by hand picking to remove intrusion was analyzed 3 times. The total-gas quette and Pouliot (1977) as mentioned above. compound grains yielded the results in Figure 7. ages (123.6 to 124.3) show good agreement, and Obviously, the ages of successive intrusive The contrast between the preliminary measure- the age spectrum (Fig. 4) is highly concordant. events cannot be evaluated from the new data. ments and those given here demonstrate the oc- 40 The average total-gas age of 123.9 ± 1.4 m.y. is Whether the 4-point Rb/Sr whole-rock date of currence of excess Ar in impurities or, taken as the age of the puiaskite. 120 m.y. for the nordmarkites is valid cannot as conceivably, along the grain borders. The highly discordant fractions (Fig. 7) can be attributed to The only other age information for Mount yet be tested. By analogy with other complexes, remaining minor impurities. The amphibole is Johnson is fission-track dates for apatite (Eby, it seems preferable to accept a short intrusion highly zoned, and this may be significant. Never- 1984a) from essexite (117 ± 9) and puiaskite history and to ascribe the somewhat young theless, the agreement of the amphibole plateau (120 ± 8) and a K-Ar date for biotite (Fairbairn Rb/Sr date to a limited number of samples that 87 86 ages with those of biotites provides strong sup- and others, 1963) from essexite (113 ± 10 m.y.). may not have had a common initial Sr/ Sr port for the validity of the 123.2-m.y. average These dates are younger but consistent, within ratio. 40 39 plateau date. There is evidence for mino:: excess uncertainties, with the Ar/ Ar age. 40Ar at Brome, but it is resolved in the step Mount Brome heating analysis. Mount Shefford The magmatic complex at Brome, one of the The results for two rock types, the g.ibbroic A diorite sample from the large intrusive largest Monteregian intrusions, includes units first intrusive phase and a later, undersaturated mafic mass, supposedly the oldest lithology of both under- and over-saturated in silica (Vali- nepheline-bearing phase, yield indistinguishable the Shefford complex (Valiquette and Pouliot, quette and Pouliot, 1977). Our detailed work on ages that give 123.2 ± 1.2 m.y. The monitor 1977), was studied. The biotite separate con- all of the major units is currently in progress and used in the 40Ar/39Ar experiments is a biotite tained -10% amphibole and was analyzed by the details will be presented at a later time. Here, from a puiaskite at Brome which gives a total- total fusion and incremental heating yielding four incremental heating analyses are described; gas age of 121.7 m.y.; although the apparent total-gas ages of 123.0 and 124.0 m.y. The in- these serve to define within narrow limits the difference of 1.4 m.y. is analytically significant, cremental heating spectrum (Fig. 5) is highly age of Brome. this sample has not yet been analyzed by incre- concordant with a plateau age (123.6 m.y.) not significantly different from the total-gas date. The decreasing K./Ca ratios during step heating 138 support progressive release of Ar from amphi- MT. SHEFFORD bole at higher temperatures; the agreement of ~134 diorite - biotite the apparent ages with decreasing K/Ca sug- (Q82-26;#31094) ¿130 gests that the amphibole and biotite are concor- Figure 5. Age spectrum for œ dant. The two determinations of the Shefford a biotite separate from OB 40 39 <126 separate indicate a Ar/ Ar age of 123.5 ± Mount Shefford. This sepa- 1.5 m.y. £122 rate contains significant am- (0 a. Eby (1984a) published fission-track and Rb- phibole yet still yields a con- a. < 118 Sr whole-rock results for Shefford samples. cordant 40Ar/39Ar incre- Apatite dates of 119 ± 8 and 131 ± 8 m.y. were mental release spectrum. 114 reported for a nepheline diorite and diorite, respectively; within uncertainties, these dates are 110 w 39 20 40 60 80 100 consistent with the kr/ kr age. The Rb-Sr O data for nordmarkites and pulaskites that yield % released

134 ^ 136 I 132 •j MT. BROME 130 Figure 6. Age spectra for biotites from two 13 2 'I neph. diorite - amphibole H > (*3170:Q62-3) 128 gabbros of Mount Brome. The spectra are 128 '126 concordant with minor discordance for Q83- 124 124 59 as discussed in the text. JI E= fld=n 2 122 120 1 g 120 E •u tp:123.3 £ 118 116 t,g = 125.0 <9 7-1 1 1 1 1—I 1 1 h | 134 Figure 7. Two age spectra for amphibole | 138 ;Q MT. BROME neph. diorite- amphibole 1130 from a nepheline diorite from Mount Brome. 2.134 < 1*3168:082.3) The two experiments show good agreement < 130 126 with discordance of low-temperature frac- tionsn . 126 122 122

118 123 2 118 V - V125.5-

114 0 20 40 60 80 100 The 40Ar/39Ar age of the complex is most 0 20 40 60 80 100 % 39Ar released precisely defined by the biotite analyses. The % released incremental heating spectra for the biotites (Fig. 9) are highly concordant with plateau ages placement of the first and last units. The amphi- mental heating. At present, it is concluded that within uncertainty of the total-gas ages. The 6 bole data support this conclusion. the complex has an age of 123 m.y. and that all analyses of biotite from the granite yield an av- The amphibole analyses are less definitive be- of the later units were emplaced within a short erage total-gas age of 123.6 m.y., whereas the cause the relative uncertainties are much larger interval not exceeding 1-2 m.y. two for the diorite give 123.5 m.y. The 8 anal- (due to small amounts of Ar) and the separates This interpretation conflicts with that of Eby yses give 123.6 with a standard error of 0.1 contained significant impurities. Incremental re- (1984a), who concluded that the rocks near sil- m.y.; this result, together with a 1.5% (system- lease spectra for the three samples (Figs. 10, 11, ica saturation are -20 m.y. older than are the atic) uncertainty in the monitor, leads to 123.6 and 12) show significant discordance, suggesting appreciably undersaturated ones. Apatite fis- ±1.9 m.y. for the complex. minor excess40 Ar, and the plateau ages are not sion-track dates of 139 ± 13 and 117 ± 9 m.y., The granite and diorite analyses indicate ages especially well defined. Amphibole from syenite reported for the gabbro and nepheline diorite, that are analytically indistinguishable, suggesting Q82-40, containing -5% impurities, mostly respectively, are consistent within uncertainties a geologically very short time span between em- pyroxene, gives total-gas ages of 123.6, 124.0, with the 40Ar/39Ar date. Rb-Sr data for nepheline-bearing diorites, foyaites, and tingua- T 1 1 1 1 r ites suggested an age of 118.4 ± 2.2 m.y. (Eby, MT. MEGANTIC, QUEBEC 1984a). In view of the Rb-Sr scatter and uncertainty, these data do not conflict with the 40Ar/39Ar age. Eby (1984a) also reported Rb- Sr data for the pulaskites that gave 136.2 ± 1.7 m.y., significantly older than observed using 40 39 Ar/ Ar. There is no obvious reason for this Figure 8. Geologic old date except that all of the pulaskites may not sketch map of the Mount 87 86 have had a common initial Sr/ Sr ratio. Sr- Megantic complex, based isotope data (Eby, 1985, and our unpub. results) on the map of Reid indicate significant heterogeneities and add sup- (1976). The patterned port for this alternate interpretation. The 125- areas represent bed rock, m.y. K-Ar date for biotite from the nordmarkite and the open areas within reported by Lowdon (1960) is consistent with the complex represent 40 39 the Ar/ Ar data. those covered by glacial debris, according to Reid Mount Megantic (1976). Solid circles and numbers indicate locali- Megantic, the easternmost intrusion, is com- ties of the samples ana- posed of three principal intrusive phases, which lyzed. are, in order of emplacement, gabbro-diorite, syenite, and granite (Reid, 1976). Geological relations and sample locations are shown in Figure 8. Two samples from each phase were studied: biotite from two granites and a diorite, amphibole from two syenites and a gabbro. [~] granite ^ syenite gabbro

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T—I—i—i—r 150 III I I I I T" MT. MEGANTIC MT. MEGANTIC granite-biotite syenite - amphibole 140 8 «3C 97:092 32) t#30*6:Q92-40> Figure 9. Age spectra for biotite samples from Mount Megantic. Biotite from two gran- 130 - ites yields concordant ages with essentially identical plateau and total-gas ages. Biotite ¿120 E3 <#3C94;Q82-31) from the diorite yields an internally concor- 123 0 -»=123.4- a -y « 123.0- show only minor discordance and are con- t, = 123.6 «,g=123.5 112 8 • 1 cordant with the biotite ages. 20 40 60 80 100 20 40 60 80 100 39 Ar released % released

and 125.3 m.y. (average, 124.3) with apparent cantly exceed those of intermediate fractions, diate between the 118 and 129 m.y. K-Ar dates plateaus near 123 m.y. (Fig. 10). These data are which approach -125 m.y. The apparent pla- of Lowdon (1961), which are consistent within in good agreement with those for biotite. Am- teaus (Table 1, Fig. 12) of-125-127 m.y. are their quoted uncertainties. phibole (87% pure) from syenite Q82-46 gives interpreted as maximum dates due to the pres- total-gas ages of 128 and 130 m.y. with discor- ence of excess 40Ar. All Complexes dant spectra; the spectra (Fig. 11) are somewhat The 40Ar/39Ar data suggest rapid emplace- saddle-shaped, suggesting excess 40Ar. The pla- ment and cooling of all Mount Megantic units, The 40Ar/39Ar ages discussed abcve are teaus (-60% of Ar) at -124 m.y. and the low- 123.6 ± 1.9 m.y. ago. This interpretation differs summarized in Table 2; the ages for 6 complexes est apparent ages (~ 123 m.y.) are interpreted as from that of Eby (1984a), who reported a 134 ± show a total spread of -1.5 m.y. Thosi; of St. maximum ages due to excess40Ar. 8 m.y. fission-track date for apatite from the Hilaire, Rougemont, Johnson, Shefford, and The amphibole from a gabbro (Fig. 12) gabbro and Rb-Sr whole-rock dates of 127.6 ± Megantic are analytically indistinguishable. Be- shows significant discordance with high appar- 1.7 and 132.5 ±1.1 m.y. for the granite and cause a number of replicates were analysed and ent ages for the low-temperature fractions. This nordmarkite (syenite), respectively. The granite the same monitor was used, St. Hilaire appears fine-grained amphibole separate contained Rb-Sr, fission-track, and 40Ar/39Ar dates al- to be -1 m.y. older than Brome. The ages are -13% clinopyroxene, mostly as cores. The spec- most overlap at the la level. The syenite Rb-Sr thus all very similar, although some small differ- tra, which suggest excess 40Ar, are inadequately date, based upon five samples, is distinctly older. ences probably exist. defined because of the small number of frac- This probably results from variable initial The observation of nearly identical dates 87 86 40 39 tions. The total-gas ages (-129 m.y.) signifi- Sr/ Sr ratios. The Ar/ Ar age is interme- within and among complexes is extremely im-

i i i i i 146 MT.MEGANTIC 142 MT. MEGANTIC syenite - amphibole gabbro - amphibole 138 («3090; 092-40) Figure 11. Age spectra for an amphibole 1*3091:092-37) from a Mount Megantic syenite. Relatively 134 c P large analytical uncertainties limit the resolu- 130 tion of the spectra. Some discordance due to ~ 126 40 £ 122 •iTtFi excess Ar is suggested, whereas the inter- ' tps125.6 • i V124.8-j-^1300 128 mediate temperature fractions are consistent

portant and merits some discussion. Of the var- three of these plutons have ages near 124 m.y. recent ocean-island basalts (Chen and others, ious geological (for example, excess 40Ar, 40Ar On the basis of K-Ar (Shafiqullah and others, 1984) whose origin has been ascribed to hotspot loss, weathering) and analytical (for example, 1970) and biotite Rb-Sr (Wen and others, 1985) activity. Both the age-geographic and geochemi- 39Ar recoil loss or redistribution, uncertainties) data, the carbonititic Oka complex appears to be cal aspects thus support a hotspot origin. factors that could pose problems for ^Ar/^Ar -10 m.y. or more younger than the rest of the The lack of age progression from west to east dates, all would be expected to increase the dis- Monteregian plutons. does not pose a problem with the hotspot model persion rather than reduce it. The tight cluster of An extensive set of paleomagnetic data (La- if the possible effects of crustal structures and the consistent and concordant ages suggests that rochelle, 1969; Foster and Symons, 1979) pro- scales of the ocean-basin relationships are con- these factors are not important. There is no evi- vides significant support for both the intra- and sidered. The nearly west-to-east belt of-200 km dence for a regional event which might have intercomplex temporal relationships. Polarity from Mount Royal to Mount Megantic does not completely reset all the ages. reversals in this part of the Cretaceous are on the correspond to the predicted motion of North Furthermore, biotite and amphibole have scale of -0.5 to 1.0 m.y. (Larson and Hilde, America over a fixed hotspot because the trace concordant ages that substantiate rapid cooling 1975), and various samples from the separate should be northwest to southeast. The positions and essentially simultaneous closure for these plutons indicate the same polarity without ap- of the plutons are probably controlled by crustal minerals, which have Ar blocking temperatures parent mixed polarities (Larochelle, 1969; Fos- structures (Philpotts, 1974; Seguin, 1982). Typi- differing by -200 °C. Because of the rapid cool- ter and Symons, 1979). The paleomagnetic data cal oceanic mid-plate hotspot swells (Crough, ing of these small, high-level bodies, the thus support the suggestion of short intrusion 1978) have diameters of several hundred ki- 40Ar/39Ar dates are taken to be emplacement histories and rapid cooling of the individual lometres, and judging from bathymetry, regions ages. Resetting of earlier phases by later ones complexes. of active hotspot magmatic activity in ocean ba- might also be postulated; however, this is un- Of the ten major complexes, all show re- sins (for example, present-day Hawaii) extend likely for several reasons. There is no geographic versed remanence polarity except Oka and well beyond a hundred kilometres. The length of pattern; concordant ages are obtained from Mount Johnson. The paleopole positions are the entire Monteregian belt is very roughly the older units far removed from the younger ones. statistically indistinguishable and are consistent diameter of regions of oceanic hotspot magma- Additionally, the amphibole and biotite are not with a remanence acquired 124 m.y. ago (Foster tism for short periods. Magmatism synchronous, only concordant but also yield internally con- and Symons, 1979). The directions and polari- or nearly so, for belts on the order of 200 km cordant spectra. None of the spectra show evi- ties of magnetizations led Foster and Symons long is a plausible, perhaps even probable, prod- dence for partial resetting. (1979, p. 1724) to propose that "intrusion of the uct of hotspot activity on the continents. The considerable time span suggested by Eby Monteregian Hills likely happened over a rela- The general distribution of the Cretaceous (1984a) is not supported by the 40Ar/39Ar re- tively short time interval of-2 Ma and certainly New England-Quebec province plutons de- sults. The fission-track dates have large uncer- of <~5 Ma." The paleomagnetic data thus agree scribed by McHone and Butler (1984) is consist- tainties and basically overlap with the more well with the short interval of Monteregian ent with the hotspot trace proposed by Duncan precise 40Ar/39Ar results. Close inspection of plutonism. (1984). Furthermore, the ages of the Cretaceous the Rb-Sr whole-rock data indicates significant plutons to the south in New England are basic- scatter beyond analytical uncertainties. With the PETROLOGIC AND TECTONIC ally consistent. These ages appear to cluster at exception of the Shefford nordmarkite which SIGNIFICANCE -120 m.y. (Foland and Faul, 1977), although has only 4 points, our regression analysis of the the precision is less than that which now exists Eby (1984a) data, with 1 a uncertainties quoted Both the 40Ar/39Ar and paleomagnetic re- for some Monteregian counterparts. in Eby (1984b) using the standard routine of sults suggest that the Monteregian complexes, In summary, the apparent temporal distribu- Brooks and others (1972), indicates excessive with the probably exception of Oka, were tion of Cretaceous alkali magmatism recorded in scatter about the regression lines. The mean formed within a single, geologically short epi- southern Quebec, northern New England, and square of weighted deviates (MSWD) parame- sode, -124 m.y. ago and apparently not more the chain of New England Seamounts supports ter for the Mclntyre I model are generally unac- than -1 to 2 m.y. in duration. Furthermore, the the hypothesis of production of the magmas due ceptably large, indicating significant geological various intrusive phases of individual complexes to mantle hotspot activity. Clearly, however, a error (see Brooks and others, 1972). The Shef- were emplaced within a limited period of time, fracture model or a nonstationary hotspot can- ford pulaskite and Megantic granite data, for less than can be resolved analytically. not be ruled out on the basis of age data. which the indicated Rb-Sr ages do not differ It appears that the various units of individual 40 39 markedly from the Ar/ Ar ones, have complexes are cogenetic and are related by ACKNOWLEDGMENTS MSWD's near 2; the data for Brome and the magmatic and/or partial melting processes. The Megantic nordmarkite, for which the ages differ age data do not support the sequence of mantle The authors wish to thank Jeff Linder for markedly, have large MSWD's of -6 to 7. It is processes postulated by Eby (1984a). performing most of the argon analyses while thus apparent that these systems do not strictly The Early Cretaceous age and the geographic maintaining the laboratory and a cheerful atti- satisfy the requirements of the Rb-Sr whole-rock position of the Monteregian province fit very tude; Kim Randall and Michael Bugenstein for method (see Faure, 1977). It is likely that the well with the predicted path (Duncan, 1984) of field and laboratory assistance; Gary Cook for various oogenetic rocks had different initial North America over a fixed mantle hotspot coordinating the neutron irradiations at the Ford 87 86 Sr/ Sr ratios, probably due to the effects of which produced the chain of New England Nuclear Reactor; Nelson Eby for supplying a list crustal contamination. Seamounts. The geochemical characteristics of of his Rb-Sr data; and Gunter Faure, Keith Bell, The ages of Royal, Bruno, and Yamaska are, the primary Monteregian magmas, high 87Sr/ and Daniel Lux for particularly helpful reviews. as yet, not well defined. The 40Ar/39Ar data for 86Sr and low I43Nd/144Nd ratios and elevated This work was supported in part by National Bruno suggest excess 40Ar and indicate a maxi- incompatible-element concentrations (relative to Science Foundation Grants EAR-8018345 and mum age of -127 m.y. It is suggested that all mid-ocean ridge basalts), are similar to those of EAR-8408988.

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REFERENCES CITED Fairbairn, H. W., Faure, G., Pinson, W. H., Hurley, P. M., and Powell, J. L., McHone, J. G., and Butler, J. R., 1984, Mesozoic igneous provimes of New 1963, Whole-rock age and discordant biotite in the Monteregian igne- England and the opening of the North Atlantic Ocean: Geological ous province, Quebec: Journal of Geophysical Research, v. 68, Society of America Bulletin, v. 95, p. 757-765. Adams, F. D., 1903, The Monteregian Hills: A Canadian petrographical prov- p. 6515-6522. Morgan, W. J., 1971, Convection plumes in the lower mantle: Nature, v. 230, ince: Journal of Geoli>gy, v. 11, p. 239-282. Faure, G., 1977, Principles of isotope geology: New York, John Wiley & p. 42-43. Alexander, E. C„ Jr., Mickelon, G. M., and Lanphere, M. A., 1978, MMhb-1: Sons, 464 p. 1972, Deep mantle convection plumes and plate motions: American A new *°Ar/39Ar daiing standard, in Zartman, R. E., ed. Short papers Foland, K. A., 1983, AT/ At incremental heating plateaus for biotites with Association of Petroleum Geologists Bulletin, v. 56, p. 203-213. of the Fourth International Conference, geochronology, cosmochronol- excess argon: Isotope Geoscience, v. 1, p. 3-21. 1983, Hot-spot tracks and the early rifting of the Atlantic: Tectonophys- ogy, isotope geology, 1978: U.S. Geological Survey Open-File Report Foland, K.A., and Faul, H., 1977, Ages of the White Mountain Intrusives— ics, v. 94, p. 123-139. 78-701, p. 6-8. New Hampshire, Vermont and Maine, U.S.A.: American Journal of Philpotts, A. R., 1974, The Monteregian Province, in Sorensen, R, ed., The Bell, K., Blenkinsop, J., Cole, T.J.S., and Menagh, D. P., 1982, Evidence from Science, v. 277, p. 888-904. alkaline rocks: New York, John Wiley & Sons, p. 293-310 Sr isotopes for long-l ved heterogeneities in the upper mantle: Nature, Foland, K. A., Under, J. S., Laskoski, T. E., and Grant, N. K.., 1984, Âr/Âr Randall, K. A., and Foland, K. A., in press, Age and time span of enplacement v. 298, p. 251-253. dating of glauconites: Measured 39Ar recoil from well-crystallized spec- of the Pliny Range complex, northern New Hampshire: Geological Billings, M. P., 1956, The gee logy of New Hampshire Part II, bedrock geology: imens: Isotope Geoscience, v. 2, p. 241-264. Society of America Bulletin. Concord, New Hampshire, New Hampshire State Planning and Devel- Foster, J., and Symons, D.T.A., 1979, Defining a paleomagnetic polarity pat- Reid, A. M., 1976, Geology of Mount Megantic: Ministere des Richesses Nat- opment Commission, 204 p. tern in the Monteregion intrusives: Canadian Journal of Earth Sciences, urelles du Quebec, Report ES-25, 59 p. Brooks, C., Han, S. R., and Wendt, I., 1972, Realistic use of two-error regres- v. 16, p. 1716-1725. Seguin, M. K., 1982, Emplacement of the Monteregian Hills of Quebec, geo- sion treatment as app ied to rubidium-strontium data: Reviews of Geo- Gilbert, L. A., and Foland, K. A., ingress, The Mont St. Hilaire plutonic physical evidence: Tectonophysics, v. 86, p. 305-317. physics and Space Ph/sics, v. 10, p. 551-577. complex: Occurrence of excess Ar and short intrusion history: Cana- Shafiqullah, M., Tupper, W. M., and Cole, T.J.S., 1970, K-Ar age of the Chen, J., Foland, K. A., Gilbert, L. A., and Randall, K. A., 1984, Depleted dian Journal of Earth Sciences. carbonatite complex. Oka, Quebec: Canadian Mineralo {ist, v. 10, mantle sources for Mesozoic pi utons of New England and southern Gold, D. P., 1967, Alkaline ultrabasic rocks in the Montreal area, Quebec, in p. 541-552. Quebec: Geological Society of America Abstracts with Programs, v. 16, Wyllie, P. J., ed., Ultramafic and related rocks: New York, John Wiley Steiger, R. H., and Jäger, E., 1977, Subcommission on geochronology: Conven- p. 469. & Sons, p. 288-301. tion on the use of decay constants in geochronology and cos nochronol- Crough, S. T., 1978, Thermal origin of mid-plate hot-spot swells: Royal Astro- Greenwood, R. C., and Edgar, A. D., 1984, Pedogenesis of the gabbros from ogy: Earth and Planetary Science Letters, v. 36, p. 359-362. nomical Society Geophysical Journal, v. 55, p. 451-469. Mt. St. Hilaire, Quebec, Canada: Geological Journal, v. 19, p. 353-376. Uchupi, E., Phillips, J. D., and Prada, K. E., 1970, Origin and structure of the 1981a, Mesozoic hotipot epeirogeny in eastern North America: Geol- Harrison, T. M., 1983, Some observations on the interpretation of Âr/Âr New England Seamount chain: Deep-Sea Research, v. 17, j>. 483-494. ogy, v. 9, p. 2-6, age spectra: Isotope Geoscience, v. 1, p. 319-338. Valiquette, G„ and Pouliot, G., 1977, Geology of Mounts Brome ar d Shefford: 1981b, Reply to Comment on 'Mesozoic hotspoc epeirogeny in eastern Harrison, T. M., and McDougall, I., 1981, Excess Âr in metamorphic rocks Ministere des Richesses Naturelles du Quebec, Report ES-28,99 p. North America': Geology, v. 9, p. 342-343. from Broken Hill, New South Wales: Implications for 40Ar/39Ar age Wen, J., Bell, K., and Blenkinsop, J., 1985, Sr and Nd isotopic relationships, Currie, K. L., 1983, An interim report on the geology and petrology of the spectra and the thermal history of the region: Earth and Planetary the Oka carbonatite complex: Geological Society of America Abstracts Mont Saint Hilaire pluton, Quebec, in Current research, Part B: Geolog- Science Letters, v. 55, p. 123-149. with Programs, v. 17, p. 747. ical Survey of Canadj. Paper 83-1B, p. 39-46. Lanphere, M. A., and Dalrymple, G. B., 1976, Identification of excess40Ar by Wilson, J. T., 1963, A possible origin of the Hawaiian Islands: Canadian Duncan, R. A., 1984, Age progressive volcanism in the New England Sea- Âr/Âr spectrum techniques: Earth and Planetary Science Letters, Journal of Physics, v. 41, p. 863-870, mounts and the opening of the central Atlantic Ocean: Journal of v. 32, p. 141-148. Geophysical Researcl, v. 89, p. 9980-9990. Larochelle, A., 1969, Paleomagnetism of the Monteregian Hills: Further new Eby, G. N., 1984a, Geochronology of the Monteregian Hills alkaline igneous results: Journal of Geophysical Research, v. 74, p. 2570-2575. province, Quebec: Giology, v. 12, p. 468-470. Larson, R. L., and Hilde, T.W.C., 1975, A revised time scale of magnetic 1984b, Monteregian Hills I. Petrography, major and trace-element geo- reversals for the Early Cretaceous and Late Jurassic: Journal of Geo- chemistry, and strontium isotope chemistry of the western intrusions: physical Research, v. 80, p. 2586-2594. Mount Royal, St. Biuno, and Johnson: Journal of Petrology, v. 25, Lowdon, J. A., 1960, Age determinations by the Geological Survey of Canada: p. 421-452. Geological Survey of Canada Paper 60-17, 51 p. 1985, Monteregian Hills II. Petrography, major and trace-element 1961, Age determinations by the Geological Survey of Canada: Geolog- geochemistry, and strontium isotopic chemistry of the eastern intrusions: ical Survey of Canada Paper 61-17,127 p. MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 11,1985 Mounts Shefford, Brome, and Megantic: Journal of Petrology, v. 26, McHone, J. G., 1981, Comment on 'Mesozoic hotspot epeirogeny in eastern REVISED MANUSCRIPT RECEIVED FEBRUARY 7,1986 p. 418-448. North America': Geology, v. 9, p. 341-342. MANUSCRIPT ACCEPTED MARCH 24, 1986

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