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POTASSIUM-ARGON DATES OF BASIC INTRUSIVE ROCKS OF THE DISTRICT OF MACKENZIE, N.W.T.l
ALICE PAYNE LEECH De#artment of Geology, University of Alberta, Edmonton, Alberta
Received July 31, 1965
ABSTRACT
Samples \stere taken from diabase dykes and related contact rocks and from a differentiated intrusive body of the District nf hlackenzie in the YellowkniIe - Prosperous Lake area, the Lac de Gras arc;*, the Point I ~ k e area, and the Tree River - Coror~ntion~Gtrlf area. In addition, seven1 %?mples were taken from basic intrusions of Ontarlo, and some of the results ot)tained lro~n these are i~lcluded.
lindiornetric dating by the potassium-argon method indicated at least four periods of diabase dyke intnision in the Slave I'rovince of the Precxmbriaii ~hicld about 2 21H)-2 400 rn.);. ago, 1 900-2 100 m. y. ago, 1 l(.K)-I 200 ni.j:, ago, and 600-700 m.y. ago. Scatter in the radiometric dates prevents recognition of other possible evenis by use of thc whole-rock method, where the age of intrusion is older than R4idclle Proterozoic.
Chemical and petro1c)gic st~ldies were rarried out in conjunction with the radiometric dating. Precise determination oi potassium proved dificrrlt; in most cases, a best value for each sample was selectcrl from the res~~l t s of three inde- pendent determinations. There is some suggestion of an increase in the potassium content of dinbase dykes in the Canadial~ aliicld throughout Precambrian time. Alkali-silica ratios show that all of the d i ~ h ~ s e dykes sttidiwi belong to the ~ ~ o r l r i - wide tholeiitic magma type.
INTRODUCTION
Recently, there has been much interest concerning the nature of basic intrusive rocks of the Canadian Precambrian shield; their age of intrusion, their composition, and their tectonic significance. This paper extends the radiometric dating work begun by Burwash et al. (1963) in the District of AIackenzie, and has been taken from a M.Sc. thesis presented in R1lay, 1965, to the Department of Geology of the University of Alberta, Edmonton. Some of the data were published (Payne et al. 1965) as Canadian Contribution NO. 58 to the Upper Mantle Project.
This work began as a study of a differentiated intrusive body a t Yellowknife, in the District of Mackenzie, but was subsequently expanded to include the investigation of the belts of diabase dykes and sills in several other areas. Field work was done near Yellowknife during the summer of 1963, when the author accompanied a departmental field trip to the area. The differentiated intrusion, lying to the east of Yellowknife Bay, was surveyed and sampled, and a 158-ft drill core obtained from it. A few samples were taken from the diabase dykes of the area.
In the summer of 1964, the diabase dykes in the Yellowknife - Prosperous Lake area were sampled more comprehensively. An opportunity was provided to visit prospecting camps a t Point Lake, 200 mi north of Yellowknife, and an unnamed lake 200 mi further north near the Tree River and Coronation Gulf. This resulted in further dyke sampling and an enlargement of the study
'Canadian Contribution No. 99 to the International Upper Mantle Project.
Canadian Journal of Earth Sciences. Volume 3 (1966)
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390 CANADIAN JOURNAL OF EARTH SCIENCES. VOL. 3. 1966
area. In the same year, during a trip through Ontario, samples of basic dykes and sills were collected near Sudbury, north of Sault Ste. Marie, and south of Fort William - Port Arthur.
Some insight into the general usefulness of potassium-argon whole-rock dating of basic rocks is afforded by the more than 60 determinations made. The resulting dates suggest distinct periods of basic intrusion during Precam- brian time, and show the type of results that can be expected from one sample, one dyke, or one set of dykes.
The accurate determination of potassium for radiometric work in these low alkali rocks is a recognized difficulty: determinations of the potassium content of the dykes were made by three different methods and provide a valid comparison of analyses done routinely but carefully by different means.
In conjunction with the age determination work, petrologic studies of thin sections made possible modal estimation of the composition of the rocks and aided in the interpretation of the radiometric dates obtained. Further chemical and X-ray fluorescence analysis established the total alkali and silica contents of the rocks, which were then classified regarding magma type.
AREAS OF STUDY
Sampling of basic dykes and one differentiated intrusion has been accom- plished in four regions of the District of Mackenzie: the Yellowknife - Prosper- ous Lake area, the Lac de Gras area, the Point Lake area, and the Tree River - Coronation Gulf area (Fig. I). These several areas of the Slave Province (Stockwell 1963) resemble one another closely: the rock types are similar in character and the various regions have undergone major periods of meta- morphism and intrusion a t about the same time. The dykes of each area show the same general mode of occurrence and distribution and give indications of several periods of intrusion over large areas.
The general geology of these areas has been described by various authors. In the Yellowknife - Prosperous Lake area, reports by Stockwell and Kidd (1932) and Stockwell (1933) were followed by those of Jolliffe (1936, 1938, 1942, 1945, 1946), Henderson and Brown (1948, 1949, 1950a, 1950b, 1952a, 1952b), and Boyle (1961), among others. Most of these reports describe the diabase or gabbro dykes and the differentiated intrusive body as intruding the older Precambrian volcanic and sedimentary rocks of the Yellowknife group, and the granitic intrusives. The basic intrusive rocks are the latest consolidated rocks, and have been affected by post-dyke faulting and slight post-dyke low temperature thermal events.
In the Lac de Gras area, northeast of Yellowknife, the rocks are of the same type and were described by Stockwell (1933) as similar to his Point Lake - Wilson Island group. These Henderson (1938) renamed the Yellowknife group, but the area was not thoroughly mapped until later (Folinsbee 1949; Moore 1956). Once again, thick series of folded volcanic and sedimentary rocks are intruded by granitic-type rocks, and later by gabbro or diabase dykes of various trends.
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LEECH: POTASSIUM-ARGON DATES
FIG. 1. Map showing sampling areas. The boundary of the 2 500-2 700 m.y. Slave Province is shown by a broken line, the boundary of the Precambrian shield by a solid line.
In the Point Lake area, described by Stockwell (1933) and Fraser et al. (1960), the oldest succession is one of folded volcanic and sedimentary rocks of the Yellowknife group intruded by granite. These rocks are cut by diabase dykes and sills. Only one locality was sampled, in the northeast arm of Point Lake (65'20' N., 113'00' W.).
In the Tree River - Coronation Gulf area, the specific area under study was located a t an unnamed lake, about 8 mi south of Coronation Gulf and 8 mi west of the Tree River. The general area has been mapped on a reconnaissance scale by Fraser (1964), although the particular area was mapped privately by several mining companies. The older rocks, volcanic and sedimentary, are assumed to belong to the Yellowknife group and are cut by granites and the younger diabase dykes.
Radiometric dating of the granites intruding the Yellowknife group rocks in the Yellowknife - Prosperous Lake area (Folinsbee 1955; Lowdon 1961, p. 29; Lowdon et al. 1963, pp. 40, 41) and the Lac de Gras area (Folinsbee 1955) has established that the older Yellowknife rocks were intruded and metamor- phosed a t least 2 400 m.y. ago. A recent date of 2 110 m.y. (Wanless et al. 1965, p. 55) suggests that the Tree River - Coronation Gulf area rocks belong to the Yellowknife group. No dates are yet available from the Point Lake area studied here, although the region is included in the Slave Province an the
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392 CANADI4N JOURNAL O F EARTH SCIENCES. VOL. 3. 1966
basis of radiometric dates obtained from adjacent areas (Lowdon 1961, p. 28) and the similarity of the rocks to the other Yellowknife group rocks.
DIABASE DYKES
In the field and under the microscope, the diabase dykes from all these areas look very similar.
The dykes weather to a rusty-brown color (from a grey-mottled fresh surface), vary in width from a few inches to a few hundred feet, and are steeply dipping or vertical. Most of the dyke contacts show chilling against the country rock and are fine-grained. Some are composite, with internal dykelets chilled against the major dyke. In areas of fine-grained basic volcanic rocks, a contact is very difficult to find: a slight baking of the wall rock near the contact is often all that is apparent. The coarser-grained dyke centers show the obvious charac- teristic diabasic or ophitic texture. hlany dykes are cross-jointed and tend to weather into muskeg or water-filled depressions, and critical outcrops of crosscutting contacts are scarce.
The dykes tend to occur in swarms along various trends and persist along strike over long distances. In the Yellowknife - Prosperous Lake area, three dyke set trends are apparent (Burwash et al. 1963): N. 70"-80" E., N. O0- 30" E., and N. 45"-60" W. These are hereafter referred to as set I , set 11, and set IV respectively (according to the suggestion of Burwash et al. 1963). In the Lac de Gras area, the dominant dyke trend varies from N. to N. 30" W. (set 111, Burwash et al. 1963), although N . 20" W. dykes are most common. There are a few dykes with a N. 0"-30" E. strike (set 11), and a N. 80" E. striking set (set I) crosses the southern portion of the area. These three sets have been mapped in the adjacent Aylmer Lake area to the east (Lord and Barnes 1954). In the Point Lake area, a few dykes striking east and northeast have been mapped, but most trend northwest (N. 20"-30" W.) and are thought to be a continuation of the Lac de Gras - Aylmer Lake set I11 (Payne et al. 1965). West of 115" W. longitude the dyke swarm thins out, but is mapped to the east for some distance. In the Tree River - Coronation Gulf area, the dominant dyke trend is N. 25" W., with a few east and northeast striking dykes. Thus, the northwest-trending dykes appear to occupy a belt 200 mi wide and a t least 350 mi long.
Examination of thin sections of the dykes showed that the dykes cannot be distinguished from each other petrologically. Mineralogically, the dykes are very uniform, with about equal amounts of plagioclase and pyroxene (or a slight excess of plagioclase) making up 80-907J0 of the rock. The plagioclase is twinned and zoned ; most distinct zoning occurs near quartz intergrowths. The pyroxene is present in two varieties, augite and pigeonite. Some of i t is twinned. Olivine occurs in quantities up to lo%, but usually is zoned or heavily altered to serpentine or magnetite. Quartz was found in nearly all the sections examined. I t is always interstitial, usually in a granophyric intergrowth of quartz and albite or orthoclase feldspar. Quartz and olivine occur frequently in the same slide. Apatite crystals and rutile needles were found as accessory
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LEECH : POTASSIUM-ARGON DATES 393
minerals, with magnetite, ilmenite (altered to leucoxene), and hematite. Some samples are rich in carbonate, both primary and secondary.
At the contact, the groundmass is glassy and only the phenoerysts, aligned parallel to the contact, are distinguishable. Whenever possible, in choosing samples for dating, a slice of dyke rock 1 to 3 cm from the contact was used. In this basaltic contact rock, phenocrysts of plagioclase, olivine, and pyroxene are found in a groundmass in which small round flecks of biotite are seen. The phenocrysts form up to 25% of the rock and most of them are smaller than 4 to 1 mm. In the center of the dyke, the grain is medium to coarse, the texture diabasic or ophitic. Many feldspar laths and pyroxene plates exceed 5 mm in length or width. Olivine, if present, generally is highly altered to serpentine or magnetite. Plagioclase is slightly altered to sericite, rarely to epidote, in most samples. In a few i t is completely altered. Most pyroxene is unaltered.
PREVIOUS WORK ON RADIOMETRIC DATING
The first attempt a t potassium-argon dating of the dykes of the district of Mackenzie was made by Burwash et al. (1963). In that study, the dykes of set I , set 11, and set I11 in the regions of Yellowknife - Prosperous Lake, Matthews Lake (in the Lac de Gras area), and Redout Lake (not within the present study areas) were dated. The ages of intrusion suggested were 2 200- 2 300 m.y. for set I , 1 400-1 600 m.y. for set 11, and 1 000-1 100 m.y. for set 111. Set IV was not dated and a possibility of multiple intrusion along the set I trend was noted. Dating by the Geological Survey of Canada (Fahrig and Wanless 1963) indicated ages of 2 165 m.y. for the N. 70"-80" E. swarm, 2 090 m.y. for the N. 0"-30" E. swarm, and 1 315 m.y. for the N. 0"-30" W. swarm. Dates from other basic rocks of the District of Mackenzie, the Muskox complex, and the Coppermine River flows have been reported (Lowdon 1961, p. 22 ; Smith 1962; Wanless et al. 1965 ; Robertson 1964). A few radiometric dates from Point Lake and Coronation Gulf are given by Payne et al. (1965).
T H E YELLOWKNIFE DIFFERENTIATED INTRUSION
This composite intrusive body outcrops to the east of Yellowknife Bay. From the Yellowknife River south to Duck Lake, a distance of almost 7 mi, there is a discontinuous outcrop of this rock mass. A fault cuts off further exposures to the south for 5 mi, where a 3-mi continuation of the body re- appears on the other side of the fault. The intrusive strikes north-south, is rudely stratiform with an apparent low angle dip to the east, and is faulted throughout. Nowhere is a complete section found.
The two main types of rocks composing this body are an olivine-rich rock best described as a picrite (as defined by Smith 1962), and a gabbro. The picrite is the basal layer of the intrusive and the gabbro forms the upper part. The picrite invariably outcrops to the west of the gabbro, and there is a zone of transition between the two rock types in which the percentage of olivine suddenly decreases as one passes from the picrite to the gabbro. The picrite layer dips gently and forms jointed cliffs covered with orange lichen. The
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394 CANADIAN JOURNAL OF EARTH SCIENCES. VOL. 3. 1966
petrology of this body has been described by Hill (1940) and Leech (1965) and appears very like a basic intrusion in east Greenland, recently described by Douglas (1964) as intrusive into the upper part of the Skaergaard intrusion. The age of intrusion of this body has been set a t 1 900-2 100 m.y. (Payne et al. 1965).
SAMPLE TREATMENT
Unweathered and, so far as possible, unaltered material was chosen. Samples were sawn from hand specimens taken a t the contact of the intrusive with the country rock, or from those taken a t other central parts of the intrusive body. In the case of contact samples, thin slices parallel to the contact were cut a t a distance of 1 to 3 cm from the contact. Samples were ground either by hand with a pestle and mortar or by use of a pulverizer (a Willy-Bleuler Swing- mill for one minute), and sieved.
Material from 45-60 mesh (U.S. standard sieves) or 60-120 mesh, or both, was used for the potassium-argon work. Splits of the same material were used for the argon extraction and all three methods of potassium analysis. The split chosen for X-ray fluorescence work was reground to less than 325 mesh in size (one minute in the Willy-Bleuler Swing-mill).
ANALYTICAL PROCEDURES
Three methods were used for potassium determination. These are described in detail elsewhere (Leech 1965) and are in routine use in the Department of Geology, University of Alberta. Briefly, they include ( I ) a gravimetric method involving leaching of potassium and precipitation with sodium tetraphenol borate; (2) a flame photometric method following the leaching of potassium; (3) an X-ray fluorescence method which uses the powdered whole rock and W-1 as a standard.
Sodium was determined in conjunction with the flame photometric deter- mination of potassium, from the same solution prepared for the potassium determinations and by the same method.
The briquettes made for the potassium determinations were used for the silica determinations. A similar method of running the samples was used, with W-1 as a standard. All runs were made in duplicate and an average value was obtained.
The argon extraction system used was a flux-fusion system, modified from, but the same in principle as that described by Goldich et al. (1961). The method has been described in detail by Peterman (1962). The spikes used were prepared and calibrated by Dr. H. Baadsgaard of the Department of Geology. A few mass spectrographic runs were made by the author: the majority were done through the kindness of Drs. Baadsgaard and Cumming of the Departments of Geology and physics, respectively. C
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LEECH : POTASSIUM-ARGON DATES 395
RESULTS
Potassium Content I t was hoped that the three independent methods would produce the same
result for each sample. This was seldom the case, although careful analytical procedure was followed in all cases and for all methods. When i t was obvious that something was amiss during a determination, that sample was discarded and the determination was repeated.
Results are available from 86 samples; 69 of these are from basic intrusions of the District of Mackenzie; 17 are from basic intrusions of Ontario. These Ontario results are included since the location of the intrusion is immaterial in comparing the results of the three types of analyses. In Figs. 2a and 2b, a plot of the percentage of KzO by both flame photometric and X-ray fluorescence methods was made. Since two runs (separated by a time interval of one month) were made by X-ray fluorescence, a range-of-values plot was obtained.
From Figs. 2a and 2b, i t is obvious that the flame photometric method and the X-ray fluorescence method gave similar values and that both were lower than the percentage of KzO obtained from the gravimetric method in the &1.5% KzO content range only. By visual estimation, two best fit lines can be drawn from the data, one for the precipitation versus flame results, the other for precipitation versus X-ray results. Since comparatively few samples had a K 2 0 content from 1.5 to 3.0y0, these lines are less meaningful in this range. However, in lower ranges, the average slope of the two lines is close to unity and the average intercept is 0.07. This conclusion was reached by the statistical "least squares" method of the fitting of a line to a set of experi- mental data.
A recent report by Lachance (1965) concerns the X-ray spectrographic determination of potassium in micas. Although the percentage of potassium was much higher than that of diabase and the material is so different, La- chance's results show the same type of variation as the X-ray results discussed here. He concluded that X-ray results are better than those provided by routine chemical methods, but was able to obtain good agreement by applying a correction factor to iron content. Whether or not i t is possible to apply correction factors to remove the sort of bias found in the data of the present paper is not known, although an attempt was made.
In trying to decide which value for potassium content was nearest to the true value, the following factors were considered.
(1) Precipitation values are best for greater than 0.15y0 KzO when a 1 g initial sample weight is used (since the balance error is f O . l mg).
(2) Precipitation values are reproducible: two determinations on one whole- rock diabase (AK 584) gave 2.021 and 2.000y0 KzO, and on W-1, 0.680 and 0.677% KzO.
(3) Precipitation values were high for W-1, assigned a value of 0.62y0 KzO by Goldich and Oslund (1956) and 0.64% KzO by Fleischer and Stevens (1962).
(4) Flame photometric values for W-1 were 0.624 and 0.640y0 KzO.
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I*= - / 0 9 . .-.:A
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Flame K2O % X-Ray fluorescence KzO
FIGS. 2a and 2b. Relatio~lship of results for the three different methods of potassium deter- mination, in the range 0.0-1.0 and 1.0-3.0% KzO. Solid circles indicate flame values, open circles and terminated lines indicate the X-ray fluorescence values or range of values.
(5) X-Ray fluorescence values were determined using W-1 as a standard, in hopes of compensating for matrix effects over the limited range of diabase composition. Although W-1 was assigned a value of 0.62y0 K20, recalculation showed that no significant difference would result from using 0.64%.
(6) At greater than 1.0% K20 there is too large a variation in X-ray values, but for samples with less than l.Oyo K 2 0 results were reproducible within
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LEECH: POTASSIUM-ARGON DATES 397'
275, when new briquettes were prepared from the original sample and the runs were repeated.
Since some values were of doubtful quality, i t seemed unwise to simply average the results of all the determinations made on a sample; instead, a "best K20" value was chosen. In most cases, this was obtained by discarding clearly unacceptable results, reducing precipitation results by 0.07y0 and then averaging the resulting values. The results of these potassium analyses are shown graphically in Fig. 3, where 86 samples of basic rocks give a mean value of 0.70% '020.
Referring to Figs. 2a, 2b, and 3, 64 of the samples were taken from diabase dykes of the four sample areas described. The results from these samples can be treated individually, grouping the dykes according to area and trend (Table I) on the assumption that all the dykes of a specific set belong to the same period of intrusion. The average K 2 0 content of these 64 samples is 0.6370, slightly lower than the overall average expressed in Fig. 3.
Table I is arranged so that the age of intrusion decreases from top to bottom: dykes of set I , set IV, and set I1 are the oldest, and set I11 and the northeast- trending dyke a t Coronation Gulf are the youngest. The 46 samples of the older dykes have a mean value of 0.56% K20 , which is lower than the mean of
86 somples meon % K2 0 = 0.70
1.4'
1 2 3 4 5 6 7 8 9 1 0 number of samples
FIG. 3. K 2 0 content of basic dykes and sills, including determinations from Ontario diabase dykes.
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TABLE I Average KzO content for dykes in the District of Mackenzie
Number of Average yo Area Age m.y. Trend samples KzO
Yellowknife, Lac de Gras 2 200-2 400 N. 70"-80" E. (set I) 241 0.631
Yellowknife 1 800-2 000 N. 30"60° W. (set IV) 13 46 0.33 0.56 Yellowknife, Lac de 1
Gras 1 800-2 000 N. 0'-30" E. (set 11) 91 0.70) ' I
Lac de Gras, Point Lake, Coronation Gulf 1000-1 250 N. 0"30° W. (set 111) 15 0.91
Coronation Gulf 600-700 N. 30" E. 3 0.41
0.63y0, and much lower than the average value of 0.91y0 K 2 0 in the dykes of set 111. These results suggest that the older Precambrian dykes have a lower potassium content than the younger dykes, as first mentioned by Burwash et al. (1963). Data from the northeast-trending dyke would not support this idea, but insufficient information prohibits comment.
Although all the radiometric results from the Ontario samples are not presented here, i t is interesting to observe that the same type of situation exists (Leech 1965): the younger intrusions apparently contain less potassium than the older intrusions.
Green and Poldervaart (1955) remarked that "there is no consistent variation in the composition of basalt magma with time", presenting tables showing world basalt compositional averages from Cenozoic, Mesozoic, Paleozoic, and Precambrian rocks. In the present study, however, a definite increase in potassium content from Archean to late Proterozoic time seems to be indicated. Future work may show that there is a different sort of time-composition relationship; rather than a systematic variation in the content of a given element, there may be a characteristic compositional range for each episode of basaltic magmatism (see Campbell and Imrie 1965). But if the increase in potassium content of the dykes is real and not imagined, a corresponding decrease in the potassium content of the source is implied. This raises the questions of the degree of mixing or homogeneity of the mantle, the possibility of differential melting in the mantle or shallow differentiation reservoirs, and the problem of assimilation of sial by the rising magma.
Alkali-Silica Proportion Kennedy (1933) recognized two dominant types of basaltic rocks, the
tholeiites and the alkali-olivine basalts, and i t is possible to classify the dykes of the District of Mackenzie and Ontario as either tholeiitic or alkalic in nature, according to their alkali-silica proportion. Macdonald and Katsura (1964) recently defined an empirical boundary line between the tholeiitic and alkalic fields on an alkali-silica diagram, and Fahrig et al. (1965) made the first attempt to classify Canadian Precambrian diabases on this chemical basis. During this study, the sodium and silica contents were conveniently determined in conjunction with the potassium analyses, in the hope of con- firming the findings of Fahrig et al. (1965).
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0 4
FIELD I
6 NIODW b NIODE Trend 1 4 sompkr) ZNIOD to 4 0 ° W Peod l7romples) 8 .N5V - 8 0 P E Trendf l somder)
THOL EIITIC / FIELD
FIG. 4. An alkali-silica plot for dykes of the District of Mackenzie and Ontario shows that all the rocks are tholeiitic in character.
In Fig. 4, the average alkali content has been plotted against the average silica content for each dyke set. From this plot i t can be seen that all the points fall below the line, although, if the values are plotted individually, a few fall in the alkalic field. The collective results, however, show that the dykes may be classed as tholeiitic.
These findings are in partial disagreement with some of the conclusions reached by Fahrig et al. (1965), who tentatively placed the N. 0'-30". W. dykes of the District of Mackenzie and the northwest-trending ("Sudbury") dykes of Ontario in the alkalic field. There is a possibility that the seven northwest- trending dykes of Ontario used in this study were not all taken from dykes known to be "Sudbury dykes", which may explain one discrepancy. On the other hand, the plots shown here for the N. 30" E. ("Abitibi") dykes and for the north-trending ("Matachewan") dykes of Ontario agree with those given by Fahrig et al. (1965).
Examination of the alkali-silica proportion shows that the dykes or dyke sets cannot be distinguished from each other in this way with any certainty a t present.
Radiometric Dates The dating of diabasic rocks has been discussed by Burwash et al. (1963).
The radiometric dates presented here are mostly on whole-rock samples of diabase dykes. For a few samples, biotite or hornblende could be separated to give a mineral date. When possible, samples were taken a t the contact or in the chilled margin zone.
Table I1 lists the results obtained from each sample and includes data on the samples (see also Figs. &lo). The potassium-argon dates have been rounded off to the nearest five million years. The assumed deviation in the
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TABLE I1 Potassium-argon age data t
Radiogenic K-Ar date Sample TY pe K% 40Ar/"K argon yo m.y. -
N. 70"-80" E. trend, Yellowknife - Lac de Gras areas:
AK 515 ct.wr 0.23 0.1824 AK 536 c.wr 0.36 0.1038 AK 579 ct.wr 0.57 0.1644 AK 582 c.wr 0.81 0.0925 AK 583 ct.wr 1 .93 0.1509 AK 586 ct.wr 0.59 0.1019 AK 587 ct.wr 0.16 0.1565
*AK 467 cm.wr 0.29 0.1982 *AK 468 c.wr 0.33 0.1580 *AK 469 c. wr 0.27 0.1406 *AK 470 c.wr 1 .20 0.1731 *AK 471 c.wr 1.62 0.1603 *AK 472 c.wr 0.56 0.0680 *AK 473 ct.wr 0.44 0.1893 AK 584 ct.wr 2.02 0.1572 AK 580 gr.hb 0.40 0.1710
*AK 261 c.hb 0.25 0.2690 *AK 446 gr.bio 6.34 0.1852 *AK 450 cm.wr 0.52 0.1947 AK 466 cm.wr 0.57 0.1508 AK 514 c.wr 0.46 0.0752
0.0839 *AK 442 c.wr 0.42 0.1924 *AK 447 c.wr 0.43 0.1351 *AK 453 c.wr 0.43 0.1296 *AK 4408 ct.wr 1.02 0.2560 *AK 443 5 gr.bio 7.62 0.2790 AK 518 c.bio 4.82 0.2846
N. 30°40" W. trend, Yellowknife area:
AK 574 cm.wr 0.20 0.1532 AK 575 c.wr 0.33 0.1420 AK 576 ct.wr 0.45 0.1301 AK 577 gr.hb 0.64 0.0995 AK 523 ct.wr 0.22 0.1842 AK 531 gr.hb 1 .01 0.1448 AK 537 c.wr 0.61 0.1064 AK 585 c.wr 0.21 0.2029 AK 589 ct.wr 0.09 0.3088 AK 590 cm.wr 0.19 0.1632 AK 592 cm.wr 0.80 0.1305
N. 0"30° E. trend, Yellowknife - Lac de Gras areas:
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LEECH : POTASSIUM-ARGON DATES
TABLE I I (Concluded)
Radiogenic K-Ar date Sample Type K% 40Ar/40K argon % m.y.
N. 0"-30" W. trend, Lac de Gras, Point Lake, Coronation Gulf areas: *AK 444 ct.wr 0.63 0.087 95.0 *AK 445 c.wr 0.87 0.0904 94.5 *AK 448 c.wr 0.66 0.0695 89.7 *AK 452 ct.bio 0.95 0.0788 94.7 AK 513 c.wr 1.35 0.0793 92.4 AK 597 ct.wr 0.50 0.1041 95.3 AK 598 ct.wr 0.45 0.1046 93.8 AK 600 ct.wr 0.66 0.1033 91.8 AK 604 ct.wr 1.54 0.0903 95.6 AK 610 ct.wr 1.02 0.677 98.7 AK 613 ct.wr 0.60 0.1032 88.8
Yellowknife differentiated intrusive:
N. 30" E. trend, Coronation Gulf area:
AK 601 gr. bio 5.48 0.0470 98.4 AK 602 ct.wr 0.81 0.0490 98.9 AK 603 c.hb 0.16 0.0462 63.4 AK 606 ct.wr 0.21 0.0495 74.5
*Work on these samples not done entirely by the author. Wonstants: A. = 0.589 X 10-'Q/vear. AR = 4.76 X 10-'Q/vear. 4QK/K = 0.01181 atomic . . . .
percent. &See Rilnvash el al. (1963) for snatial relations of these dvkes. ~- - - . ~ - KOTE AS indicated in-th; c ~ l & n ~ " ~ y p e " . the sample GaLeither whole rock (wr) or a mineral
separate. biotite (bio) or hornblende (hb). The diabase dyke samples were taken from the dyke contact (ct). the chilled dyke margin (cm), or the dyke center (c) ; other samples were from granite a t the intrusive contact (gr), hornfels at the contact (hf), and the gahhroic (g) or the ultrabasic (ub) differentiate of the Yellowknife intrusive.
dates is f 5y0. Duplicate runs for two samples (AK 514 and AK 474) indicate that this is reasonable precision ; the reproducibility of 40Ar/40K determinations has been discussed by Baadsgaard et al. (1957). Dates from the samples with low potassium content (less than 0.20y0) have the largest error, owing to the difficulties experienced in determining the potassium, and in these cases an error of &lo% is more realistic. The date obtained from sample AK 589 has been rejected because of the extremely low (0.0970) KzO content.
Figure 5 presents the dates given in Table I1 in histogram form. In the upper column, 66 determinations from rocks of all areas are given. In lower columns, these dates have been separated according to area and trend of the dykes.
Figures 6, 7, 8, and 9 show the sample locations and the radiometric dates obtained from the samples. Abbreviations were used on the maps as follows: gr indicates that the sample was from granite a t the contact ; w, that a whole- rock dyke sample was used ; b or hb, that mineral separates, biotite or horn- blende, were dated; c, that the date was obtained from a sample taken from the dyke center rather than the dyke contact.
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FIG. 5. Potassium-argon dates from basic intrusive rocks of the District of Mackenzie.
The dykes of set I, set 11, and set IV are the oldest dykes dated. Since so many of the dates obtained from set I fall in the 1 900-2 100 m.y. range, there is a strong suggestion of more than one intrusion along this trend, possibly a t the same time as the intrusion of the set I1 and set IV dykes, or the Yello\v- knife differentiated intrusion.
The dykes of set I1 and set IV seem to be a conjugate set. If they were intruded a t about the same time, then the field relations described by Henderson and Brown (1952b) and Wilson (1949) are explained: these investigators found that the dykes of set I were cut by dykes of set IV in most cases, but that the opposite relationship is seen in a few places. Here again there is the possibility of multiple intrusion along these trends.
Dates from the dykes of set I11 and the northeast-trending Coronation Gulf dyke give the most consistent data, along with the dates from the Yellowknife differentiated intrusion. The latter gives three dates ( 2 015 m.y., 1 925 m.y., and 2 090 m.y.) that are reasonably concordant. All four dates (from two whole-rock samples, one hornblende sample separated from a center dyke sample and one biotite date from the dyke contact) from the Coronation Gulf dyke fall in the range from 650 m.y. to 700 m.y.
From Fig. 5 , i t is apparent that there is considerable variation in dates for a given area and trend. Dates given on the maps of Figs. 6 , 7, 8 and 9 (es-
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LEECH: POTASSIUM-ARGON DATES
FIG. 6. A map of the Yellowknife - Prosperous Lake area illustrates the variation in dates from dykes of three sets. In the oldest set, the same dyke may show a variation of over 1 000 m.y. C
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- -
FIG. 7. TWO major dyke sets dominate the Lac de Gras area. Dates from the older north- east-trending dykes show much more scatter than those from the younger northwest-trending dykes.
pecially Fig. 6) show that there is also this variation in dates for a given dyke, along both width and length. The older the dyke, the greater the variation, presumably because of the greater number of post-dyke events that may have caused loss of argon. For example, the first 14 samples listed in Table I1 (AK 515 to AK 473 inclusive) are taken from the same dyke in the Yellowknife area. The radiometric dates range from 1 925 m.y. all the way to 900 m.y. If the six dates from two parallel dykes to the south are considered, the range increases to become 2 310 m.y. to 900 m.y. In the younger dykes (i.e. those of set I11 and the Coronation Gulf dyke), the "scatter" in dates is much less.
Across the width of a dyke, the central whole-rock samples usually give lower radiometric dates than the contact samples. This is best illustrated by
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LI*:ECII : POTASSIUM-ARGON DATES
FIG. 8. Dates from contact samples of a dyke from the northwest-trending set a t Point Lake are not significantly different from these obtained from the northwest-trending dykes of the Lac de Gras area.
the dyke profile taken across the 300-ft-wide dyke striking N. 70'-80' E., north of Prosperous Lake. Figure 10 shows the dates obtained from this profile, collected by Drs. R. A. Burwash and F. A. Campbell of the Department of Geology, University of Alberta. A mineral date from the dyke to the south of this dyke (AK 261) indicates a probable age of intrusion of a t least 2 300 m.y., as does a mineral date from a dyke outcropping on an island in Great Slave Lake (AK 518). Therefore, either there was more than one period of intrusion along this trend, or the contact whole-rock dates are generally lower by 300-400 m.y. than the actual time of intrusion of the dyke indicated by mineral dates. This latter situation probably applies in the case of older dykes, such as the set I dykes, more than for younger dykes. In accepting the older dates given by mineral separates, i t is assumed that biotite and hornblende have not gained argon. Dates from the country rock adjacent to the dyke should be accepted with care, as complete outgassing does not always occur more than a few feet (or even inches) away from the contact.
Several reasons may be given for this scatter in dates, most probably caused by loss of argon by diffusion. Post-dyke fauIting or low temperature thermal events may have effected this. There is ample field evidence of post-dyke faulting, especially in the Yellowknife - Prosperous Lake area (see Fig. 6). In this area too, in one locality east of Ryan Lake, an aplitic vein with comb quartz and some sulfide mineralization cuts a diabase dyke.
There is a second possibility. The grouping of the scatter dates, best seen in the combined dates column of Fig. 5, corresponds to periods of major
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dhr PHSL
FIG. 9 A map of the Tree River - Coronation Gulf area shows dates obtained from the dykes of the northwest-trending set found to the south and east, and dates from a younger, northeast-trending dyke.
orogeny in the Precambrian shield (Stockwell 1963), and i t is possible that "echoes" of these major events taking place in other shield areas caused the argon leakage.
The mineral source of the potassium is a third factor that can be considered in interpreting these varied dates. In examining the thin sections of samples dated, i t was found that biotite is generally abundant in the chilled dyke margins but scarce in the centers of most dykes. Center samples with a high percentage of potassium were found to contain a high granophyre content or to be highly altered and sericitized, or both. Many of these samples managed to retain more argon than fresh unaltered samples of the same dyke. For example, sericitized AK 470 and AK 471 gave older dates than the fresh unaltered AK 472 (see Figs. 6, lo) , and AK 586 gave an older date than AK 579 (see Fig. 6). Argon would be expected to be retained in biotite, hornblende, or sericite, while the potassium feldspar in the granophyric intergrowths probably accounts for much argon leakage. Recent studies by Amaral et al.
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FIG. 10. Radiometric dates obtained from a dyke north of Prosperous Lake. Samples taken a t the contacts of the dyke yielded higher dates than center dyke samples, and are closer to the possible age of intrusion of this dyke 2 200 m.y. ago.
(1966) show that potassium feldspar is indeed present in these intergrowths in basaltic rocks.
DISCUSSION OF RESULTS
The geologic age sought here is the time of intrusion of the dykes. Because no major metamorphic events have occurred since dyke emplacement in the areas under discussion, the radiometric (apparent) date will be the same as the age of intrusion and cooling of the dyke if certain conditions are satisfied. These have been discussed by Baadsgaard et al. (1964).
From the radiometric dates presented here, periods of initial i*trusion are indicated 2 400 m.y. ago, 2 100 m.y. ago, 2 000 m.y. ago, 1 250 m.y. ago, and 700 m.y. ago (see Fig. 5). Allowing the limits of f 5 y o , the 2 100 m.y. and 2 000 m.y. periods of intrusion fall in essentially the same range. These con- clusions are reached on the assumption that all the dykes of a given trend were emplaced during the same period of intrusion. No definite field evidence is available, and the age data alone do not solve this problem.
Assignment of an age of intrusion to the oldest dykes, those of sets I , 11, and IV, was most difficult. If only the mineral dates of set I are considered, the indicated event began about 2 400 m.y. ago. The chilled margin whole-rock samples give dates varying from 1 930 m.y. to 1 225 m.y. and the center dyke whole-rock samples gave still younger ages (1 905 m.y. to 900 m.y.). Because of this extreme scatter in dates, i t was not considered feasible to simply average them to obtain an "age of intrusion" ; instead, the oldest dates of each set were accepted as indicative of the time of initial intrusion.
Although no mineral dates are available from the dykes of sets I1 and IV, the oldest dates of 2 000 m.y. indicate an earlier age of intrusion than that suggested by Burwash et al. (1963). Assuming f 5y0 limits, these dykes appear
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to have been intruded about the same time as the Yellowknife differentiated intrusion (2 100 m.y. to 2 000 m.y. ago). I t should be emphasized that these "ages of intrusion", assigned to these intrusive bodies older than 1 600 m.y., have been set up arbitrarily on the basis of the oldest dates obtained for each dyke swarm and that future findings may alter the situation.
Dates from the younger dykes of set I11 show little scatter and those from the northeast trending Coronation Gulf dyke show less. If the oldest dates are again considered, periods of intrusion were initiated 1 250 m.y. ago and 700 m.y. ago. Because the dates from these rocks are more closely grouped, i t is possible to average the dates and obtain a meaningful figure for an average age of intrusion: 1 155 f 100 m.y. for the set 111 dykes and 675 f 20 m.y. for the Coronation Gulf dyke.
These dates may be compared with some of those obtained for several basic intrusions of similar age in the District of Mackenzie and in other regions of Canada. Burwash et al. (1963), Fahrig and Wanless (1963), and Fahrig et al. (1965) reported dates obtained from the f 2 400 m.y. old dykes of the District of Mackenzie, Ungava, and western Quebec. Van Schmus (1965) found that the Nipissing diabase of Ontario is 2 155 m.y. old, and there is a possibility that the Sudbury nickel irruptive may be this old (Fairbairn et al. 1960; Faure et al. 1962). With respect to the younger intrusions, Burwash et al. (1963), Fahrig and Wanless (1963), and Fahrig et al. (1965) assigned ages from 1000 m.y. to 1315 m.y. to the dykes of set 111; Lowdon (1961, p. 22), Smith (1962), and Wanless et al. (1965, p. 67) reported ages of 1 155 m.y. and 1095 m.y. for the Muskox complex of the Coppermine area. Dates ranging from 735 m.y. and 918 m.y. (Wanless et al. 1965, pp. 60-61) to 1150 m.y. (Robertson 1964) have been obtained from the Coppermine River basalts, which may not all be of the same age. Leech et al. (1963, p. 55) reported ages of 635 m.y. and 640 m.y. for sills of Victoria Island.
Thus i t can be seen that there were a t least three periods of intrusion; 2 200-2 400 m.y. ago, 1 000-1 200 m.y. ago, and 600-700 m.y. ago. This last event may represent one of the final events of Precambrian time.
The broad range of scatter dates obtained in this study shows the type of data which may be expected from whole-rock samples taken from old dykes of ancient areas where there has been much tectonic noise. The Yellowknife - Prosperous Lake area is one such area and the Porcupine - Timmins area is another. In the Porcupine - Timmins area, dykes known as "Matachewan" gave whole-rock dates of 1 930 m.y., 1 760 m.y., 1 500 m.y., and 1 325 m.y. One gave a date of 1 290 m.y. where cut by a northeast-trending "Keweena- wan" dyke of 1 220 m.y. The oldest date obtained here indicates intrusion a t about the same time as the set I1 and set IV dykes of the District of Mac- kenzie, but the range in dates is difficult to interpret, as i t may be the result of argon loss or of multiple intrusion along this trend, or both. The first possibility is supported by the date of 2 485 m.y. reported by Fahrig et al. (1965) for the dykes, but field relations in Ontario tend to support the latter idea: personal communication with Dr. S. A. Ferguson of the Ontario Depart-
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ment of Mines indicates that along the east boundary of Deloro township, O.D.M. Map 47a is in error and that the north-striking dyke cuts the north- east-striking diabase. Further, a t the same location, a small dykelet strikes east-west and is the latest of the three. Campbell and Imrie (1965) point out one example in the Quemont - Noranda area where a north-trending dyke both cuts and is cut by east-west trending dykes. In these areas of northern Ontario then, there may be two generations of north-trending diabases or two generations of east-west diabase, or both. I t is possible that the same sort of situation exists in the Yellowknife - Prosperous Lake area.
Since such scatter is observed here, i t is advisable to consider the results obtained by others who have attempted radiometric dating of older Precam- brian basic intrusive rocks. The potassium-argon dates obtained by McDougall et al. (1963) from the British Guiana dolerites are the most interesting in com- parison with the results presented in this paper because they show the same sort of scatter. McDougall attributes this to a loss of radiogenic argon by diffusion, explaining that the geological evidence suggests that emplacement of the dolerites was not spread over 500 m.y. as the dates indicate. Further, he suggests that the argon loss occurred a t low temperature, probably below 200 "C. As with the Canadian samples, the potassium-argon dates do not conclusively indicate several periods of intrusion, although the possibility is there.
Altogether, these results show that the potassium-argon whole-rock method. must be used with care in dating these old basic intrusive rocks. Even when a number of samples are dated and chilled margin material is chosen, the results may not be reliable. The rubidium-strontium isochron method may or may not be more useful: Van Schmus (1965) was able to establish that the Nipissing diabase of Ontario was intruded 2 155 m.y. ago and metamorphosed 1 700 m.y. ago, but McDougall et al. (1963) discussed the possibility of Sr leakage in the Guiana dolerites. McElhinny and Opdyke (1964), Allsopp (1965), and Evans and Tarney (1964) likewise reported a range in dates of up to 700 m.y. in K-Ar and Rb-Sr dating of basic rocks in their various areas.
On a small scale, in contributing to the understanding of geologic events, an understanding of the ages of intrusion of Precambrian basic rocks is a necessity. On a larger scale, this knowledge is important with respect to its application to the problems of continental drift. The possibility of long-distance dyke correlation was suggested by Fahrig and Wanless (1963) and discussed by Payne et al. (1965). In addition, since Precambrian time was of such long duration, the ages of Precambrian basic intrusive rocks are useful with respect to paleomagnetic work. The paleomagnetism of the Precambrian dolerites of Rhodesia was discussed by McElhinny and Opdyke (1964) and the significance of this was further commented upon by Gough et al. (1964), who were able to suggest a polar wandering curve relative to Africa for the early Precambrian. A similar paleomagnetic study of the diabase dykes of the Canadian Precam- brian shield was made by Fahrig et al. (1965), in which the paleomagnetic findings are related to the potassium-argon dates. Paleomagnetic work is being
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done on the dyke systems of Finland: a recent study of the remanent mag- netization of Jotnian olivine dolerites has been made by Neuvonen (1965).
CONCLUSIONS
In this paper, several aspects of potassium-argon radiometric dating of Precambrian basic intrusive rocks have been discussed: the difficulties of potassium analysis, the advisability of using various types of samples available, and the possible causes of the scatter in dates obtained from the older Pre- cambrian dykes. In a further effort to understand the nature of these rocks and to interpret the dates obtained from them, some compositional studies were made. From the results of this study, i t is possible to conclude that:
(I) There were a t least three periods of basic intrusion in the District of Mackenzie 2 200-2 400 m.y. ago, 1000-1 200 m.y. ago, and 600-700 m.y. ago. Another period of intrusion may have occurred 1 800-2 000 m.y. ago.
(2) Dating of mineral separates or chilled margin samples is recommended. (3) A method of readily and accurately determining potassium in basic
intrusive rocks is required. (4) There is a suggestion of an increase of potassium content from Archaean
to Late Proterozoic time. (5 ) Classification of the dykes according to magma type by the alkali-silica
method indicates that all the dyke swarms are tholeiitic in character. (6) The potassium-argon whole-rock method is most applicable where the
age of intrusion of a dyke or dyke swarm is Middle Proterozoic or younger, or where there has been little tectonic activity since the time of initial in- trusion. The effect of tectonic noise is to cause a scatter in dates that prevents recognition of subsequent events. As far as Precambrian basic intrusive rocks are concerned, the older they are the more difficult the discovery of age of intrusion. This may be restated as Leech's Law,
"Updating and uplifting have the same effect; old dykes and old ladies give young ages".
ACKNOWLEDGMENTS
The author particularly wishes to thank Dr. R. E. Folinsbee of the Depart- ment of Geology for his many kindnesses and invaluable assistance during the writing of my thesis and this paper. The very great help and cooperation of Drs. H. Baadsgaard and R. A. Burwash of the Department of Geology, and Dr. G. L. Cumming of the Department of Physics, is also very much appre- ciated. Drs. W. F. Fahrig and R. K. Wanlessof the Geological Survey of Canada reviewed the manuscript and offered many useful suggestions. The trip to Point Lake and Coronation Gulf was provided by Nahanni Mines Ltd., Toronto, Ontario, and Precambrian Mining Services Ltd., Yellowknife, N.W.T. These companies supplied transportation, food, shelter, and maps, and are sincerely thanked. Field trips to Yellowknife were provided through grants by the Boreal Institute, University of Alberta, and Eldorado Mining and Refining Limited, Saskatchewan. Dr. S. A. Ferguson of the Ontario Department of Mines kindly supplied many of the Ontario samples.
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LEECH: POTASSIUM-ARGON DATES 411
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*Now Alice Payne Leech.
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