DISCUSSIONS 381

exact age of this event difficult to fix precisely, I believe that the concordance of three different dating techniques (Currie et aE. 1986) suggests that it must be in the range 130- 135 Ma.

Gilbert and Foland use their data to deduce a short period of emplacement for the Mont Saint Wilaire complex and therefore a probable cogenetic origin for the different parts of the com- plex. Whatever the merits of the first part of this argument, the second is clearly erroneous. Currie et al. (1986) showed that the nepheline syenite and nepheline diorite display a geo- chemical signature distinctly different from the older, more mafic, nepheline-free rocks that they intrude. Since petrog- raphy and geochemistry clearly demonstrate different proto-

liths and fractionation trends for the nepheline-free and nepheline-bearing parts of the complex (Currie et ul. 1986; Currie L983), assumption of a cogenetic relation is clearly unjustified.

CUWRIE, K. L. 1983. A preliminary report on the geology and petrol- ogy of the Mont Saint Hilaire pluton, Quebec. In Current research, part B. Geological Survey of Canada, Paper 83-lB, pp. 39 -46.

CUWRIE, K. L., EBY, S . N., and SITTINS, J. 1986. The petrology of the Mont Saint Hilaire complex, southern Quebec; an alkaline gabbro - peralkaline syenite association. Lithos, 19: 65-81.

The Mont Saint Hilaire plutonic complex: occurrence of excess 40Ar and short intrusion history:' Reply

K . A. FOLANB Depclrtment of Geology and Mineralogy, Ohio State University, Columbus, OH 432 10, U. S. A.

AND

L I S A A. GILBBERT Eurth Technology Corporation, 3777 Long Beach Boul., Long Reach, CA 90807, U. S.A.

Received November 3, 1986

Accepted November 4, 1986

Can. J . Earth Sci. 24, 381-385 (1987)

We respond to the thoughtful comments and discussion by Currie and appreciate the opportunity to clarify and amplify several points. He objects to our proposals for a short intrusion history and for a cogenetic origin of the various igneous lithol- ogies of the Mont Saint Hilaire complex. The former con- clusion is based upon remarkably uniform biotite ages and discordant amphibole age spectra, while the latter is by infer- ence from the ages and spatial geometry and is supported by isotopic measurements. The issues of the discussion seem well taken in light of his recent paper on Mont Saint Hilaire (Currie et al. 1986), but we stand by our earlier conclusions for the reasons outlined below.

Before addressing whether substantial age differences exist between different portions of the mafic western part of the complex, we point out that there is even apparent disagreement on the relative order of emplacement. Currie (1983) proposed intmsion outward from the core, whereas Greenwood and Edgar (1984) subsequently proposed the opposite trend. This point was neither addressed nor clarified by Currie et al. (1986), who concluded that the central mafic rocks predate the nepkeline-bearing mafic rocks and the younger nepheline syenites by about 10 Ma. While not taking sides on this point, we conclude that the time differences are very small, leaving the issue to be argued on field or perhaps chemical rela- tionships.

Currie proposes that primary biotite occurs only in the nepheline syenite and suggests that our other analyzed biotites are secondary, formed by late metasomatism. We disagree.

'Discussion by K. I,. Currie. 1987. Canadian Journal of Earth Sciences, 24: 380 - 38 1 .

Many mafic rocks, especially nepheline-bearing ones, contain large, separate, and sometimes euhedral (with included apa- tite) biotite grains that are clearly primary. The occurrence of associated unclouded cryptoperthite, clear and fresh nepheline, and fresh olivine in such samples rules out significant deuteric or other hydrothermal processes. Greenwood and Edgar (1984) likewise interpreted the biotite as primary, albeit a late- crystallizing phase.

We are unable to address Currie's suggestion, based on fieldwork, that biotite has formed along the margins of nephe- line syenite dikes. However, we do not ascribe the occurrence of biotite in our samples (collected from outcrops without any sign of dikes) to such an origin. The biotite is evenly dissemin- ated throughout the samples mther than being concentrated in selected zones or margins. Nor do we concur with such an origin for biotite in many mafic rocks that quite commonly contain several percent, exceeding 10 % in some cases (Green- wood 1983; Greenwood and Edgar 1984).

We do agree that many rocks contain biotite as overgrowths on other ferromagnesian minerals, as documented by Currie et al. (1986), which could be attributed to magmatic or deuteric reactions or conceivably metasomatism. Textural rela- tions are often a matter of judgment, but the rather large areas of optical continuity of these overgrowths rather than fine- grained aggregates suggest to us formation at magmatic stages. Moreover, biotite occurring as over- or intergrowths with other femmagnesians is likely to be largely excluded during mineral separation in favor of larger, independent grains. The biotite from amphibole gabbro Q83-80 (Gilbert and Foland 1986), which we regard as giving the only reliable date for this unit, has a low abundance, characteristic of the largely cumulate

Printed in Canada I Imprime au Canada

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CAN. J. EARTH SCI. VOL. 24, 1987

TABLE 1. Additional 40AP9Ar analytical data and apparent ages from the Mont Saint Hilaire complex, Quebec

Temp.a 40Ar/39Arb 37Ar/39Arb 36Ar/39Arb 39Ar 40Ar*d Apparent Agef ("c) meas. con. meas. Fc (%I (%) K/Cae (Ma)

Fusion Total

Fusion

600 675 710 750 790 860 900 920 935 955 975 990

lo00 1025 1050 1075 1100 1150 Fusion Total

Fusion

483-79, amphibole gabbro-amphibole (Run No. 32.9; J = 0.008370)

13.03 0.8283 59.58 0.19 19.49 22.45 1.260 180.9 0.07 32.43

8.386 0.5652 90.62 0.11 35.12 8.140 0.3902 72.42 0.11 38.56

10.23 0.1545 18.22 0.31 28.72 10.55 3.202 32.63 0.14 3.31 7.139 0.08558 11.01 0.23 30.67 6.330 0.02742 9.732 0.58 55.91 5.402 0.01203 9.122 1.83 74.16 5.006 0.00534 9.032 7.70 88.05 5.101 0.00316 8.833 38.85 93.91 5.482 0.02464 8.826 2.37 56.07 5.260 0.00340 8.693 19.20 93.24 5.628 0.01732 8.770 6.29 64.96 5.892 0.01455 8.447 1.99 68.48 5.701 0.00488 8.730 10.00 89.41 6.485 0.01782 8.556 2.10 64.00 6.469 0.01245 8.956 7.95 73.55 5.453 0.01495 9.246 100.0 69.58

(Run No. 32.10; J = 0.008241) 5.517 0.01580 9.360 100.0 68.59

Q83-80, amphibole gabbro-amphibole (Run No. 3248; J = 0.008271)

4.976 0.06613 22.22 1.05 53.57 6.754 0.06166 24.27 1.34 57.67 0.8734 0.01719 14.83 0.64 74.56 0.6242 0.01735 12.85 0.79 71.48 0.5711 0.01500 11.67 1.28 72.47 1.237 0.00756 10.65 9.01 82.99 2.124 0.00653 10.10 5.16 84.82 2.756 0.01486 10.04 3.63 70.39 2.914 0.00534 9.515 2.21 87.24 2.731 0.00851 9.407 0.79 80.05 5.132 0.00574 9.609 19.55 87.81 5.590 0.00367 9.219 7.69 93.09 6.025 0.00419 9.135 17.40 91.92 6.593 0.00302 9.082 12.60 95.66 7.085 0.00383 9.392 16.24 93.88 7.388 0.02286 9.824 0.35 61.15 8.514 0.02248 56.80 0.04 90.38

10.46 0.1461 13.01 0.05 23.34 8.549 0.2075 12.47 0.18 16.96 5.081 0.00738 9.968 100.0 84.53

(Run No. 3247; J = 0.008244) 5.174 0.1822 10.19 100.0 15.95

gabbro. It is frequently associated with other ferromagnesians, so that biotite forming by reaction may constitute a consider- able portion of the separate. Regardless, the highly concordant biotite dates from across the complex suggest that the entire complex cooled as a unit.

None of the data presented by Gilbert and Foland (1986) and Currie et al. (1986) substantiate a significantly older age for any Mont Saint Hilaire rocks. The interpretation (Currie et al. 1986) of the 130- 135 Ma age for central, largely cumulate lithologies seemed quite reasonable without the ability to recognize the excess 4QAr problem revealed by 40Ar/39Ar incre-

mental heating. However, all of the fission-track determina- tions reported by Currie et al. (1986) are within uncertainty of the 124.4 Ma 40Ar/39Ar age of biotites, independent of uncer- tainties in the 238U spontaneous fission decay constant. The K-Ar ages quoted by Currie et al. (1986), 124 + 5 Ma for biotite from a syenite and 134 + 6 Ma for an amphibole from a central gabbro, are also consistent: the former is concordant, whereas the latter may be attributed to excess 4QAr in arnphi- bole, as documented by us originally and discussed below.

Close inspection of the data reported in our paper and in the Depository of Unpublished Data (CISTI, National Research

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DISCUSSIONS

TABLE 1 . (Concluded)

Temp .a 40Ar/3YArb 37Ar/39Arh 36Ar/39Arb 39Ar 40Ar*d Apparent Agef ("(-3 meas. con-. meas. F c (%) ( % KIGae (Ma)

600 675 750 825 900 975

1050 1150 1225 1240 1260 Fusion Total

600 675 750 825 900 975

1050 1150 1225 1240 Fusion Total

483-1, amphibole gabbro-feldspar (Run No. 3241; J = 0.008261) 0.06627 20.42 9.05 0.02752 16.60 6.67 0.0301 1 11.46 7.84 0.02630 10.93 9.55 0.04535 11.70 10.30 0.02967 10.11 3.71 0.02644 9.960 8.13 0.03523 11.86 8.49 0.03193 13.41 6.19 0.07871 13.38 4.88 0.07706 14.56 8.39 0.04364 14.72 16.81 0.04364 13.48 100.0

488-79, amphibole gabbro-py roxene (Run No. 3242; J = 0.008335) 0.5242 71.78 1.71 32.47 0.7404 140.9 1.57 38.89 0.1214 72.19 3.36 66.99 0.0892 36.44 5.10 58.38 0.0489 21.62 3.56 61.38 0.0344 24.03 10.53 73.78 0.0105 12.72 47.46 84.58 0.0671 20.31 17.93 75.11 0.2088 41.58 5.61 62.33 0.3368 52.93 2.38 37.49 1.3038 62.99 0.78 10.95 0.0815 24.02 100.0 57.45

aTemperatures are estimated at + 50°C. hThe isc?tope ratios given are not corrected for interfering Ca- and K-derived argon isotopes. The ?'Ar has been corrected for

decay using a half-life of 35.1 days. cF is the ratio of radiogenic 40Ar to K-derived "Ar. It is corrected for interferences and atmospheric argon using (40Ar/36Ar),,, =

295.5; (39Ar/37Ar),, = 7.330 X (36Ar/37Ar)C, = 2.716 X lom4; and (4°Ar/39Ar)K = 4.51 1 X

ci40Ar* is the percent of total 40Ar that is radiogenic 40Ar. .Weight ratio calculated using the relationship KICa = 0.4S9ArKP7Ar. f Ages were calculated with the following constants: X, = 0.581 X year-'; X, = 4.962 x 10-I0 year1; and 40K = 1.167 x

at. % of K . Uncertainties are quoted at the l a level and do not include uncertainty in J.

Council of Canada) does not substantiate a ca. 130 Ma plateau date for amphibole. On the contrary, the minimum apparent ages of incremental-heating experiments set a younger, maxi- mum age for the amphibole. In particular, two runs for Q83-1 amphibole suggest a maximum of 126 Ma (the average of the last 36% of 39Ar for experiment No. 3144; the average of 16%, the significant highest temperature fractions in run No. 3151).

Subsequent to the preparation of our report, we conducted additional 40Ar/39Ar experiments for four mineral separates previously analyzed by the conventional K-Ar method only. As the data are pertinent to the present discussion, they are here listed in Table 1, and the amphibole spectra are shown in Figs. 1 and 2. (Note the erratum in the column heading of Table 1 of Gilbert and Foland (1986). The correct headings should read as shown on Table 1 herein.) The samples and the analytical techniques are as described in Gilbert and Foland (1986). The pertinent and salient features of these data are the following.

(1) The total-gas ages are consistent with the previously reported K- Ar dates, with the exception of Q83-79 pyroxene.

The difference in this case (330 versus 855 Ma) probably represents variable contents of 40Ar. The anomalously old dates for Q83-1 feldspar and Q83-79 pyroxene can only logically be interpreted as reflecting excess 40Ar.

(2) All samples are characterized by very high apparent ages in lower temperature fractions, and none of the release spectra define a meaningful plateau. These spectra, along with the amphibole spectra previously described, are similar to those known to reflect excess 40Ar (e.g., Lanphere and Dalrymple 1976; Harrison and McDougall 1981 ; Zeitler and Fitz Gerald 1986) for which the minimum apparent age is usually con- sidered a maximum age of the sample.

(3) The additional 40Ar/39Ar amphibole samples are similar to those reported earlier but provide somewhat better resolu- tion. The spectra show a rather consistent decrease in apparent age and reach a minimum when nearly all Ar has been released. No statistically meaningful plateaus are observed, and the experiments, like the previous ones, do not sufficiently isolate the effects of excess 40Ar. However, the minimum apparent ages do approach the biotite 40Ar/39Ar age.

(4) There is a systematic relationship between the amount of

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3 84 CAN. J. EARTH SCI. VOL. 24, 1987

1 1 1 1 1 1 1 I I 20 40 60 80 100

39 % Ar released

FIG. 1. 40Ar/39Ar spectrum for amphibole from amphibole gabbro Q83-79 (No. 32.9).

excess 40Ar (or total-gas age) and the minimum apparent age duting incremental heating. The minimum apparent ages for the feldspar and pyroxene are about 143 and 182 Ma, respec- tively, and obviously have no chronologic significance. For all four amphiboles analyzed with total-gas ages of about 144, 143, 134, and 134 Ma, the respective minima in step heating are roughly 13 1, 129, 125, and 124 Ma. This suggests that the more excess 40Ar contained in the sample, the more difficult it is to isolate, as might be expected. All four amphiboles are from nepheline-free, largely cumulate gabbros, which Currie suggests were formed by an older event. With the exception of a biotite that has a K-Ar age concordant with other units, all examined samples from this unit are seriously affected by excess 40Ar.

In summary, the 40Ar/39Ar data, with the exception of those for biotite, should not be interpreted as providing meaningful ages and do not support an older event. Moreover, the amphi- bole age spectra minima at near 125 Ma is a maximum age of the amphibole gabbro, as suggested initially but not discussed in detail.

The close agreement of all the biotite dates indicates essen- tially simultaneous cooling of the entire complex below the Ar blocking temperature of about 300°C for biotite. It is highly unlikely that emplacement of the late nepheline syenite could have raised the temperature of the entire complex this high if substantial time had elapsed since emplacement of the early phases. Such heating would also have reset the thermally much more sensitive fission-tmck systems, thus voiding the data upon which an older event might be hypothesized. If the amphibole minimum apparent age of about 125 Ma has any significance, any reheating hypothesis must also consider the significantly higher temperatures needed to rest the amphibole K- Ar system. In our opinion, the simplest interpretation is that all units were emplaced within a short interval and cooled simultaneously through the biotite closure temperature. We note that this interpretation is consistent with new 40Ar/39Ar data (Foland et al. 1986a) on several other Monteregian intru- sions, not only in an apparent short interval but also in the time of formation, nearly synchronous at 124 Ma.

Our use of the term "cogenetic" was in general, inferred from the close spatial and time proximity of the vatious lithol- ogies, and would encompass processes such as fractional crystallization, crustal assimilation, and partial melting events.

39 % Ar released

FIG. 2. 40Ar139Ar spectrum for amphibole from amphibole gabbro Q83-80 (No. 3248).

Currie et al. (1986) proposed a different magma for the central, largely cumulate gabbros. While an extensive discus- sion of this is inappropriate here, we do not find their evidence compelling. Greenwood and Edgar (1984) proposed a model by which all units were derived from a common parent. A report on our independent chemical and Sr, Nd, and 0 isotopic studies is in preparation (see also Foland et al. 1986b). The results, which show only small isotopic variations, are consis- tent with derivation from a common parent by fractionation along with small yet significant amounts of crustal contamina- tion. The initial 87Sr/86Sr and r43Nd/'44Nd ratios for various lithologies converge to common values, equal to the lowest and highest observed ratios, respectively. The nepheline- bearing mafic rocks, pyroxene gabbros, and amphibole gabbros are consistent with a common parent with 87Sr/86Sr at approximately 0.7032, and one nepheline syenite is only slightly higher at 0.7033. For these rock types, the highest observed initial 143Nd/144Nd ratios are, repsectively , 0.5 1273, 0.51272, 0.5 1274, and 0.5 1271. These data do not prove that the units are cognetic but do support a hypothesis of generation from a common parent. A priori, we wodld expect magmas derived from different mantle sources at different times to have somewhat different isotopic compositions, in contrast to what is observed.

CURRIE, K. L. 1983. An interim report on the geology and petrology of the Mont Saint Hilaire pluton, Quebec. In Current research, part B. Geological Survey of Canada, Paper 83-IB, pp. 39 -46.

CURRIE, K. L., EBY, G. N., and GITTINS, J. 1986. The petrology of the Mont Saint Hilaire complex, southern Quebec: an alkaline gabbro - peralkaline syenite association. Lithos, 19: 65-81.

FOLAND, K. A., GILBERT, L. A., SEBRING, C. A., and CHEN, J.-F. 1986~. 40Ar/39Ar ages for plutons of the Monteregian Hills, Quebec: evidence for a single episode of Cretaceous magmatism. Geological Society of America Bulletin, 97: 966 -974.

FOLAND, K. A., GILBERT, L. A., and HENDERSON, C. M. B. 1986b. Crustal contamination during genesis of the Mont St. Hilaire alka- line igneous complex, Quebec. Eos, 67: 389.

GILBERT, L. A., and FOLAND, K. A. 1986. The Mont Saint Hilaire plutonic complex: occurrence of excess 40Ar and short intrusion history. Canadian Journal of Earth Sciences, 23: 948 -958.

GREENWOOD, R. C. 1983. Petrogenesis of the gabbroic suite, Mt. St. Hilaire, Quebec. M .S. thesis, the University of Western Ontario, London, Ont.

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DISCUSSIONS 385

GKEENWOOI), R. C., and EDGAR, A. D. 1984. Petrogenesis of the LANPHEKE, M. A., and DALRYMPLE, D. B. 1976. Identifjcation of gabbrc~s from Mt. St. Hilaire. Quebec, Canada. Geological Jour- excess 40Ar by 40Ar/39Ar age spectrum technique. Earth and Plane- nal, 19: 353-376. tary Science Letters, 32: 141 - 148.

HARRISON, T. M., and MCDOUGALL, I. 1981. Excess 40Ar in meta- ZEITLER, P. K., and FITZ GERALD, J. D. 1986. Saddle-shaped morphic rocks from Broken Hill, New South Wales: iinplications 4nAr/"Ar age spectra from young. microstructurally cornplex for 40Ar/"Ar age spectra and the thermal history of the region. potassium feldspars. Geochimica et Cosmochimica Acta, 50: Earth and Planetary Science Letters, 55: 123 - 149. 1185 - 1199.

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