athapuscow 1985

U-Pb zircon ages from Athapuscow aulacogen, East Arm of Great Slave Lake, N.W.T., Canada

S. A. BOWRING' AND W. R. VAN SCHMUS Department of Geology, University of Kansas, Lawrence, KS 66045, U.S.A.

AND

P. F. HOFFMAN Geological Survey of Canada, Precambrian Geology Division, Ottawa, Ont., Canada KIA 0E4

Received February 20, 1984 Revision accepted June 18, 1984

Athapuscow aulacogen is an Early Proterozoic intracratonic basin located in the East Arm of Great Slave Lake between the Slave and northwest Churchill provinces. Athapuscow aulacogen comprises three stratigraphic sequences, the Wilson Island Group, the Great Slave Supergroup, and the Et-Then Group. New U-Pb zircon ages provide constraints on the development of the aulacogen.

The Blachford Lake Intrusive Suite consists of an older alkaline phase (Hearne Channel Granite) dated at 2175 5 7 Ma and a younger peralkaline phase (Thor Lake Syenite) dated at 2094 + 10 Ma, confirming the suggestion that the two phases may not be related. A felsite from the Wilson Island Group has an age of 1928 ? I I Ma. The Wilson Island Group is intruded by epizonal granites (Butte lsland Intrusive Suite), one of which has an age of 1895 + 8 Ma. The Wilson Island Group and the Butte Island Instrusive Suite are entirely allochthonous with respect to the Slave craton. Rocks of the Great Slave Supergroup overlie mylonitized Wilson Island Group rocks and both were involved in northeast-directed thrusting. The Compton laccoliths intrude rocks of the Great Slave Supergroup, postdate thrusting, and are about 1865 Ma old.

The Blachford Lake lntrusive Suite is significantly older than both the rift sequence in Wopmay Orogen (ca. 1900 Ma) and the Wilson Island Group; it probably is genetically unrelated. The age of the Wilson Island Group and Butte Island Intrusive Suite is considerably younger than previous estimates and is close to the minimum age of rifting in Wopmay Orogen. The Compton laccoliths are very similar to intrusive rocks in the Great Bear Magmatic Zone of Wopmay Orogen and may be related to east-dipping subduction beneath the aulacogen.

The new ages strengthen the correlations between Athapuscow aulacogen and Wopmay Orogen and suggest a link with events in the Trans-~"dson Orogen to the south.

L'aulocogene d'Athapuscow est un bassin intracratonique datant du Protkrozoi'que infkrieur localis6 dans le Bras oriental du lac de Great Slave entre le nord-ouest de la province de Churchill et la province de Slave. L'aulacogbne d'Athapuscow renferme trois skquences, le groupe de Wilson lsland. le supergroupe de Great Slave et le groupe d'Et-Then. De nouvelles dkterminations d'Lge par U-Pb de zircon prkcisent la pkriode de dkveloppement de I'aulacogene.

La sCquence intisice du lac Blachford cimprend une phase alcaline plus ancienne (granite de Hearne Channel) datCe A 2175 + 7 Ma et une phase peralcaline plus jeune (syCnite de Thor Lake) datee a 2094 ? 10 Ma, ce qui corrobore la suggestion que les deux phases ne sont probablement pas reliCes. Une felsite du groupe de Wilson Island est datke a 1928 ? I I Ma. Des granites Cpizonaux (skquence intrusive de Butte Island), dont un est Lgk de 1895 ? 8 Ma, pknetrent le groupe de Wilson Island. Le groupe de Wilson Island et la skquence intrusive de Butte lsland sont intkgralement allochtones relativement au craton de Slave. Les roches du supergroupe de Great Slave postdatent le charriage et sont Lgkes environ de 1865 Ma.

La skquence intrusive de Blachford Lake est significativement plus ancienne que la skquence du rift de l'orogenese de Wopmay (ca. 1900 Ma) ainsi que le groupe de Wilson Island; il n'y a probablement pas de relation gCnCtique. L'Bge du groupe Wilson lsland et de la sequence intrusive de Butte Island est considkrablement plus jeune que les Lges rapportks antkrieurement et se rapproche de l'lge minimum du rifting associk a l'orogenese de Wopmay. Les laccolites de Compton ressemblent beaucoup aux roches intrusives de la zone magmatique de Great Bear de l'orogenese de Wopmay et ils peuvent &tre associks h la subduction de pendage est s'ktendant sous l'aulacogbne.

Les kcentes dkterminations d'Lge renforcissent les corrClations entre l'aulacogene d'Athapuscow et I'orogenkse de Wopmay et indiquent une relation avec les CvCnements de l'orogenese trans-hudsonienne au sud.

[Traduit par le journal]

Can. J. Earth Sci. 21, 1315-1324 (1984)

Introduction Since 1966 many studies have elucidated the structure, stra-

tigraphy, magmatism, geochronology, and paleomagnetism of Athapuscow aulacogen. The ages reported here are the first published U- Pb zircon ages and have important implications for the timing and development of Athapuscow aulacogen and its relationship to nearby Wopmay Orogen and the Trans- Hudson Orogen of the Churchill Province. Reliable U-Pb zircon ages for the rocks of the aulacogen, as well as for the

'Present address: Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, U.S. A.

rocks in the surrounding orogens, are fundamental to formu- lating tectonic models for their evolution and in understanding general crustal growth. In addition, U-Pb ages will be important in interpreting the wealth of paleomagnetic data available for the rocks of the aulacogen.

Athapuscow aulacogen is an Early Proterozoic intracratonic basin located in the East Arm of the Great Slave Lake between Slave Province and the northwest Churchill Province (Fig. 1). Hoffman (1973) and Hoffman et al. (1974) proposed that the East Arm area represents a failed rift or aulacogen related to the rifting of the Slave craton and the development of a west-facing passive margin sequence, now preserved in Wopmay Orogen (Hoffman 1980). Essential to this model is the fact that the

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CAN. J. EARTH SCI. VOL. 21, 1984

FIG. 1. Archean and Early Proterozoic tectonic elements of the northwestern Canadian Shield showing the location of Wopmay Orogen, Athapuscow aulacogen, and the Trans-Hudson Orogen. Tectonic zones of Wopmay Orogen are: (1) autochthonous cover, (2) foreland thrust and fold belt, (3) metamorphic-plutonic hinterland, (4) Great Bear Magmatic Zone, (5) Hottah Terrane. Zones 2 and 3 constitute the Calderian Orogen. Black triangle is the Blachford Intrusive Suite and black diamond is the Bigspruce Complex.

Great Slave Supergroup can be correlated in detail with the Coronation Supergroup and that in all probability the aulacogen merges with the orogen beneath the Western Canada Basin (Hoffman et al. 1970; Fraser et al. 1972; Hoffman 1973).

The geology of the East Arm is shown in Fig. 2. There are three major stratigraphic sequences present in the aulacogen (Wilson Island Group, Great Slave Supergroup, and Et-Then Group) that record several periods of sedimentation, magmatism, and deformation. The regional geology and the three- fold stratigraphic framework were first described by Stockwell (1933, 1936a,b). The oldest sequence, the Wilson Island Group, was studied and mapped by E. W. Reinhardt (1969, unpublished maps). The sedimentology of the two younger sequences was studied in detail by Hoffman (1968, 1969).

Mapping of the aulacogen at 1 : 50000 scale by Hoffman et al. (1977) led to the discovery of large nappes that were emplaced from the southwest at the end of Great Slave Super- group deposition. Concepts on the evolution of the aulacogen were further refined by Hoffman (1981), who proposed a six- stage evolutionary model for its development, in which the timing and orientation of magmatic and structural events in the aulacogen were related to collisional events in Wopway Orogen and the Churchill Province.

Geochronologic studies Numerous geochronologic studies of the rocks in Atha-

puscow aulacogen and the surrounding area have been at- tempted and were reviewed by Goff et al. (1982), Frith (1980), and Ghandi and Loveridge (1982). Available ages for the rocks of the aulacogen are presented in Fig. 3. Rb- Sr and K- Ar systematics are highly susceptible to disturbance during low- grade metamorphic events, and as can be seen in Fig. 3 the ages are variable and with one exception are younger than the ages determined in this study.

We report new U-Pb geochronologic data for the major igneous events in the development of the East Arm and com- pare the ages with those of events in the surrounding orogens. This study is part of a much larger project that is concerned with the geology and geochronology of Wopmay Orogen. Rocks dated in this study are two phases of the Blachford Lake Intrusive Complex, a felsic intrusive from the Wilson Island Group, a granite that intrudes the Wilson Island Group, and two of the quartz-monzonite laccoliths that intrude the Great Slave Supergroup.

Analytical procedures Isotopic analyses (Table 1) were carried out at the Isotope

Geochemistry Laboratory at the University of Kansas. Zircons were dissolved and Pb and U separated using procedures modified after Krogh (1973). Aliquots of the dissolved samples were spiked with a mixed 208Pb-235U tracer solution. Isotope ratios were measured on a 23 cm radius Nier-type 60" sector,

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BOWRING ET AL

1 Pethe1 Grow 1 itanocnel la

Group LO

TABLE 2. Summary of zircon ages

Fit* Sample Unit/location n (%) Age (Ma)* Model

79- 13 Hearne Channel Granite 4 15 2175T5 I (62"3.2'N, 1 12'45.6'W)

8 1-20 Thor Lake Syenite 4 18 2094+10 1 (62"6'N, 112"35'W)

80-31 Wilson Island Group felsite 5 99 1928+11 1 (6I048.5'N, 112"53.01W)

80-28 Post-Wilson Island Group granite 5 8 1895k5 1 (61°40.9'N, 113"19.3'W) 1895+8 2

79-10 Laccolith, Stark Lake 6 <I 1872*8 1 (62"30.9'N, 1 10°0.O'W) 1867222 2

80-34 Laccolith, Christie Bay 5 39 1861*17 1 (62'24.1 'N, 1 10°59.3'W)

*Regression, probability of fit, and uncertainties after Ludwig (1980, 1982). Uncertainties given at 95% confidence levels for model 1 and at 2u for model 2.

Ages

-4- Rb-Sr

lsochron

-0- U-Pb

Zlrcon

--C

K-Ar

-2- Pb-Pb

Whole-Rock ~sochron

Bars are Uncertalntles

1 1 , I .

----t

1600 1800 2000 2200 AGE (Ma)

FIG. 3. Chart summarizing published radiometric ages for the East Arm region. See text for comment. Sources of data: (1) 2057 ? 56 Ma, Davidson (1978); (2) this work; (3) 1846 * 24 Ma, Frith (1980); (4) 1810 + 19 Ma, Goff et al. (1982); (5) 1832 ? 10 Ma, Baadsgaard et al. (1973); (6) 1805 ? 18 Ma, Ghandi and Loveridge (1982); (7) 1804 ? 23 Ma, Cumming (1980); (8) 1845 2 95 Ma, Lowdon et al. (1963); (9) 1795 k 55 Ma, Lowdon et al. (1963); (10) 1630 2 50 Ma, Wanless et al. (1970); (1 1) 1809 k 30 Ma, Goff et al. (1982); (12) 1677 + 99 Ma (unpublished GSC age, cited by Ghandi and Loveridge 1982); (13) 1969 ? 82 Ma, Ghandi and Loveridge (1982).

solid-source mass spectrometer. Pb and U analyses were carried out using silica gel and phosphoric acid on Ringle Re fil- aments. A mass fractionation correction of 0.15% bias per mass unit was applied to all data. Radiogenic 206Pb and 207Pb were calculated by correcting measured ratios with measured blank Pb and with Stacey and Kramers' (1975) model Pb corresponding to the age of the zircons for the original nonradiogenic component. All samples are sufficiently radiogenic that reasonable uncertainty in the composition of nonradiogenic Pb does not contribute significant uncertainties to the resulting age. During the course of these analyses analytical blanks aver- aged 0.25 ng 206Pb and less than 0.2 ng 238U. Decay constants used were 238U = 0.15513 X year-' and 235U = 0.98485 x year-'.

Uncertainties on the 207Pb/206Pb and the U/Pb determinations are estimated from the long-term reproducibility of standards and are estimated to be +0.1% for 207Pb/2"Pb and

20sPb/206Pb at the 2u level. Relative precision for the 206Pb/2""~b varies with the 204Pb content but is better than 5 1% for samples having 206Pb/2@'Pb of 1000. In any case the abso- lute uncertainty in the 204Pb content contributes less than +0.1% uncertainty to the calculated ages.

Zircon analyses reported here were obtained during the period 1979- 1983, during which time the uncertainty associated with U/Pb ratios has decreased from + 1 .O% to 0.5% at the l o level. Most of the zircon suites analyzed from the East Arm are all relatively discordant and exhibit scatter exceeding that which can be accounted for by present analytical uncertainty; the scatter is probably due to complicated lead-loss histories and (or) gain of uranium. Thus, to account for the variability in our measurement uncertainty and to increase the probability of fit for discordia lines, a conservative estimate of + 1 .O% uncertainty in the U/Pb ratios at the l o level has been adopted.

Concordia intercepts were calculated using the regression and error analysis method of Ludwig (1980, 1982), which is modified from York (1969). One sigma uncertainties used were -+ 1.0% in 207Pb/235U and 206Pb/"8~ ratios, with a correlation coefficient of 0.990 (corresponding to an uncertainty of +O. 14% in radiogenic 207Pb/206Pb). Results are reported using Ludwig's (1982) model 1 solution for all samples and also given for his model 2 solution if the probability of fit was less than 10% (Table 2). Model 1 ages are cited throughout the text.

Blachford Lake Intrusive Suite The Archean rocks along the northern margin of the East

Arm are intruded by a major alkaline-peralkaline complex of plutons, the Blachford Lake Intrusive Suite, which has been described in detail by Davidson (1978, 1982). The Blachford Suite is composed of two parts (Fig. 2), an older, aluminous western portion that includes the Caribou Lake Gabbro, White- man Lake Quartz Syenite, Hearne Channel Granite, and Mad Lake Granite, and a younger peralkaline pluton consisting of the Grace Lake Granite and the Thor Lake Syenite. K-Ar mineral ages from both parts of the Blachford Lake Suite range from 2170 to 2130 Ma (Wanless et al. 1979) and are anal- ytically indistinguishable from unpublished Rb- Sr whole-rock isochron ages from the Whiteman Lake Quartz Syenite, Mad Lake Granite, and Hearne Channel Granite (Davidson 1982). The younger peralkaline units do not yield reliable Rb-Sr

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1320 CAN. J . EARTH SCI VOL. 21. 1984

0.5 pyramidal type (similar to type A of Pupin 1980). Bipyramidal zircons are common in alkaline rocks (Poldervaart 1956; Speer 1982; de Saint-Andre et al. 1983) and are thought to form at

3 0.4 - relatively low temperatures (Pupin 1980). The Bokan Moun- m m

tain peralkaline granite in Alaska (de Saint-Andre et al. 1983) N

3 and the Blachford Suite are very similar in that both have a

(D intense albitic alteration and U-Th mineralization associated

0 Cu with late stages of peralkaline magmatic activity. De Saint-

Andre et al. (1983) suggested that in the altered phase of the Bokan Mountain peralkaline granite similar bipyramidal and

A: T=2175- '5 Ma 6: ~ = 2 0 9 4 * l o Ma xenomorphic zircons may be related to the circulation of hydro-

thermal fluids related to albitic alteration and the low-grade U-Th mineralization. The high measured 208Pb/206Pb ratios

207~b/235 u (Table 1) in the zircons from the altered svenite of the Blach- ford suite and from the altered phase of the Bokan Mountain

Frc. 4. Concordia diagram showing U-Pb isotopic data for (A) ~~~~i~~ suggesi a ~h-~i~. source and a relationship the H e m e Channel Granite (79-13) and (B) the Thor Lake Syenite between crystallization of the zircons and ~h mineralization. (8 1-20).

isochrons, but the data allow ages as much as 200 Ma younger than the rest of the complex (Davidson 1982).

Hearne Channel Granite The Hearne Channel Granite is a medium-grained, sub-

equigranular granite that crops out in the southwest part of the Blachford Lake Suite. The sample (79- 13) selected for U-Pb geochronology was collected from the southwest comer of the pluton (Fig. 2). The granite is dominated by large, composite feldspar grains that have anti-perthitic plagioclase cores and perthitic rims. Large subhedral quartz grains make up 10- 15% of the rock. Large (average 2 mm) euhedral zircons are abun- dant and readily visible in thin section.

Five fractions of zircon separated from the Hearne Channel Granite yield an age of 2175 ? 5 Ma (Fig. 4). This is in good agreement with an unpublished zircon age of 2185 ? 5 Ma (A. Davidson, personal communication, 1983) for the Whiteman Lake Quartz Syenite. The most concordant of the four points is the least magnetic fraction, which was handpicked and air abraded following the method of Krogh (1982). The youngest portion of the Blachford Suite consists of the peralkaline Thor Lake Syenite and the Grace Lake Granite. Associated with these two phases are dykes and veins of pegmatite that are mineralogically related to the host rocks (Davidson 1982). An area of the Thor Lake Syenite has been intensely altered to an albite-rich rock and contains concentrations of fluorite, carbonate, quartz, and zircon, with rare minerals rich in Nb, Be, Li, Th, Y, U, and the rare earth elements (REE's) (Davidson 1982).

Zircons were separated from a drill core sample (81-20) of altered syenite supplied to the authors by A. Davidson. The altered rock consists of aggregates of radiating albite crystals with cross-cutting veins of carbonate and zircon. Zircon makes up 10-20% of the rock and occurs both as xenomorphic grains and as small, euhedral, bipyramidal crystals that are brown and strongly magnetic. Analysis of four fractions yielded a chord with an upper intercept of 2094 ? 10 Ma (Fig. 4). The zircons are younger than those in the Hearne Channel Granite by as much as 100 Ma and it is therefore possible that the peralkaline portion of the Blachford Suite is not genetically related to the older aluminous phase.

Zircons from the altered syenite are unusual in habit and isotopic composition but are similar to zircons reported from other alkaline rocks. The zircons from the altered syenite are dominated by two varieties, a xenomorphic type and a bi-

Wilson Island Group The Wilson Island Group is exposed principally within a

large nappe in the southwest half of the aulacogen (Fig. 2). The nappe is cut by the McDonald-Wilson Fault and is overlain by sedimentary rocks of the Et-Then Group. The nappe is structurally isolated and is composed exclusively of Wilson Island Group rocks and a small high-level intrusion (the Butte Granite). Wilson Island Group rocks are nowhere observed in depositional contact with the Archean basement, although preliminary data from detrital zircons in an arkose of the Wilson Island Group indicate a large component of Archean age.

Wilson Island Group rocks and adjacent Archean gneisses have undergone intense subhorizontal ductile shear on the Petitot Islands. This shearing is coextensive with a 20 km wide mylonite zone that trends along the southeast border of the aulacogen (Hoffman 1981). The Great Slave Supergroup is not mylonitized and locally contains clasts of mylonitized Wilson Island Group. Metamorphic grade of the Wilson Island Group is greenschist to amphibolite facies, and the rocks have been deformed into a series of northeast-trending steep-limbed folds. The relative ages of metamorphism and mylonitization have not yet been determined; it is not known whether metamorphism and folding predate the Great Slave Supergroup deposition and subsequent nappe emplacement. The geologic history of the Wilson Island Group and its relationship to the Archean basement prior to nappe emplacement are also poorly understood.

The stratigraphy of the Wilson Island Group has been dis- cussed by Stockwell (1933, 1936), Reinhardt (1969), and Hoffman et al. (1 977). The oldest part consists mainly of basalt and rhyolite flows with conglomerate containing volcanic, granitic, and gneissic clasts. On Wilson Island this sequence is overlain by a thick sequence of quartzite. In the Basile Bay area a sequence of impure dolomite, argillaceous quartzite, and argillite, with basal flows of porphyritic basalt, lies strati- graphically above the sequence on Wilson Island (Hoffman et al. 1977). It is not known what genetic relationship rocks of the Wilson Island Group have, if any, with rocks of the Great Slave Supergroup, although Yeo (1976) suggested that the Wilson Island Group represents a rift assemblage in a precursor trough coincident with the aulacogen.

On Wilson Island a felsic hypabyssal intrusive rock (sample 80-31) intrudes metasedimentary rocks and lavas of the Wilson Island Group, and it is thought to be comagmatic with the rhyolite lavas. The rock is a pink aphanitic felsite with less than

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FQG. 5. Concordia diagram showing U-Pb isotopic data for the Wilson Island Groupe felsite (80-31).

10% phenocrysts of quartz and feldspar in a groundmass of fine-grained, intergrown quartz and alkali feldspar with about 5% small glomeroporphyntic clots of piapioclase.

Analyses of five fractions of zircon are shown in Fig. 5. The five points yield an age of 1928 ? 11 Ma, which we interpret as a minimum age for the Wilson Island Group. The ape is younger than all previous estimates of the age of the Wilson Island Group and is older than the two published Rb-Sr whole- rock isochrons (see Fig. 2). Goff et al. (1982) obtained a Rb - Sr whole-rock isochron age of 18 10 r 19 Ma for basaltic lavas in the Wilson Island Group and stated that the age represents a later metamorphic event rather than the age of ex- trusion. Goff et al. (1982) stated that the Wilson Island Group is probably older than 2200 Ma, based in part on the obser- vation that the Simpson Island dyke and the Blachford Suite, which are about 2200 Ma, lack the penetrative deformation of the Wilson Island Group. However, because the Wilson Island Group is structurally isolated and allochthonous with respect to the Blachford Suite, intensity of deformation is an unreliable indicator of relative age.

Butte Island Intrusive Suite On the Isles du Large, the Butte Island Intrusive Suite, a

complex of epizonal granodiorite and monzogranite, intrudes Wilson Island Group rocks. On Butte Island the granites con- tain large blocks of Wilson Island Group quartzite. Several samples of these granites were collected for zircon geochronology but results are available only for one (80-28, Fig. 2).

The granite is a medium-grained monzogranite to granodiorite. As viewed in thin section, the granite consists of a few epidotized plagioclase phenocrysts surrounded by graphic in- tergrowths of quartz and microcline. Between 3 and 5% ragged biotite and chlorite grains are also present. Sample 80-28 has been sheared and the groundmass is almost completely re- crystallized. This rock is a fine-grained mosaic of quartz and microcline with 5- 10% clots of biotite intergrown with muscovite, although a few blocky remnant domains of graphically intergrown quartz and microcline are preserved. A weak foli- ation is defined by alignment of the muscovite and biotite. In the centres of some of the biotite-muscovite aggregates are relict amphiboles that have been replaced by magnetite and are rimmed with muscovite.

Analyses of zircons separated from sample 80-28 result in a strongly discordant array on a concordia plot (Fig. 6). In- creased concordancy for two fractions was obtained using the air abrasion technique of Krogh (1982), resulting in an upper

FIG. 6. Concordia diagram showing U-Pb isotopic data for the post-Wilson Island Group granite (80-28).

intercept age of 1895 + 5 Ma (Fig. 6). Preliminary work on other samples suggests that the more magnetic zircons have experienced an isotopic disturbance sometime after crystallization that may be explained by a gain in uranium. Work is underway at this time to more fully understand this phenomenon. Nevertheless, we believe an age of 1895 + 5 Ma represents the true age of the Butte Island Intrusive Suite, which is internally consistant with the age of 1928 + 11 Ma obtained on the Wilson Island Group felsite.

Compton laccoliths The Compton Intrusive Suite consists of more than 30 calc-

alkaline laccoliths that occur along the length of the East Arm (Fig. 3). Compositionally the laccoliths range from diorite to quartz monzonite (Hoffman et al. 1977). Most of the laccoliths were intruded along the base of the Stark megabreccia; their floors tend to conform to the top of the Pethei Group and their roofs bulge irregularly upward into the megabreccia (Hoffman et al. 1977). Several laccoliths locally transgress nappe boundaries and are therefore younger than the thrusting. The intrusions are offset along transcurrent faults and are overlain unconformably by the Et-Then Group, which was deposited contemporaneously with transcurrent faulting.

The calc-alkaline laccoliths are similar in form, composition, and associated mineralization with early Great Bear Magmatic Zone intrusions in Wopmay Orogen (Badham 1973, 1975, 1978a,b; Badham and Morton 1976; Hoffman and McGlynn1977; Hildebrand 198 1). Hoffman and McGlynn (1977) suggested that the laccoliths were genetically related to the Great Bear Magmatic Zone, which developed 1850- 1870 Ma ago (Bowring and Van Schmus 1982).

Two laccoliths were sampled for zircon geochronology (79-10, 80-34), and their locations are shown in Fig. 2. Both laccoliths are quartz monzonites and consist of 30-50% large (6-8 mm), sericitized plagioclase phenocrysts in a finer grained groundmass of subhedral to anhedral quartz and alkali feldspar, p e n amphibole. and biotite. Zircons from the two laccoliths yield ages of 1861 -+ 17 and 1872 k 8 Ma (Fig. 7). These ages overlap within statistical uncertainty, and a mean of 1865 2 15 Ma is probably a reasonable estimate of the age of laccolith emplacement.

Discussion The new U-Pb zircon ages reported here are important in

understanding the timing and development of Athapuscow aulacogen and its relationship to Wopmay Orogen and the Trans-

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FIG. 7. Concordia diagram showing U-Pb isotopic data for the Compton laccoliths.

Hudson Orogen of the Churchill Province. The zircon data support the temporal stratigraphic correlations between the East Arm and Wopmay Orogen that were made without the benefit of zircon geochronology (Hoffman 1969, 1973, 1981) and raise interesting questions regarding the age and significance of the Wilson Island Group.

The zircon data from the Blachford Suite indicate that the alteration associated with the peralkaline phase of magmatism is distinctly younger than the alkaline phase. However, until more data are available for the area south of the East Arm it is difficult to put these ages into a regional perspective. Two other alkaline intrusions in the area are of similar age to the older part of the Blachford. The Simpson Island dyke is a northeast- trending dike that intrudes the Archean basement 35 km south of the Blachford Intrusive Suite and has been dated by the K-Ar method on biotite at 2200 Ma (Burwash and Baadsgaard 1962) and 2170 Ma (Leech el (11. 1963). The Big Spruce Complex (Fig. 1) is a large intrusive body of alkali syenite to carbonatite that has a Rb-Sr whole-rock isochron age of 2184 +- 39 Ma (Martineau and Lambert 1974).

Hoffman (1980) suggested that the peralkaline-alkaline magmatism ca. 2200 Ma in the southern Slave Province was related to rifting and breakup of the craton. The rifting event is thought to have led to the eventual development of the west- facing passive margin in Wopmay Orogen and the two failed rifts preserved in the East Arm and in Kolohigok Basin. U-Pb ages from the rhyolites in the rift sequence (Akaitcho Group) of Wopmay Orogen cluster around 1900 Ma (Bowring and Van Schmus 1982) and are consistent with the age of the correlative Union Island Group being between 1928 and 1860 Ma (see below). Thus, there is a considerable period of time between the alkaline-peralkaline magmatism and the first datable event in the Akaitcho Group, especially when compared with the rapid evolution of Wopmay Orogen (Hoffman and Bowring 1984). If the alkaline-peralkaline magmatism is related to rifting another possibility must be considered: the complexes are related to a rifting episode that predates the development of the passive margin in Wopmay Orogen and its products are either not preserved or have not been recognized. An obvious alternative is that the alkaline-peralkaline complexes are not related to a rifting event, but represent instead a period of anorogenic magmatism.

The significance of the age of 1928 ? 11 Ma obtained for the felsite that intrudes the Wilson Island Group supracrustal rocks is uncertain. The age may be interpreted as the approximate

time of deposition of the Wilson Island Group or it could be interpreted as a much younger intrusive unrelated to the Wilson Island Group. The latter is not considered likely; however, further work is planned to resolve this problem. The age of 1895 ? 5 Ma for the Butte Island Intrusive Suite is consistent with the Wilson Island Group being 1928 Ma.

If 1928 + 11 Ma is the age of deposition for the Wilson Island Group, this constrains the maximum age of the basal Great Slave Supergroup (Union Island Group) to be less than or equal to 1928 Ma, and strengthens the correlation between the Union Island Group and the 1900 Ma Akaitcho Group. However, the possibility that the Union Island Group and Wil- son Island Group are correlative cannot be ruled out. An age of 1928 Ma also provides a maximum age for the mylonitization of the Archean and Wilson Island Group along the southern margin of the aulacogen, which may be related to a collision between the northwest Churchill and Slave provinces (Gibb and Thomas 1977).

The age of about 1865 2 15 Ma for the Compton laccoliths confirms that they are time correlative with similar intrusions in the Great Bear Magmatic Zone of Wopmay Orogen, which is consistent with an east-dipping subduction zone beneath the Great Bear Magmatic Zone during this time as originally proposed by Hoffman and McGlynn (1977). However, the presence of the laccoliths in the aulacogen 100-200 km from the magmatic arc of Wopmay Orogen is unexpected. Similar intrusions are absent in the adjacent older crustal provinces, suggesting that the laccoliths owe their presence to a zone of weakness such as a pre-existing suture between the Slave Prov- ince and the northwest Churchill Province that was sub- sequently reactivated during the development of the aulacogen. Hoffman (1980) suggested that a flat subduction zone beneath the Slave craton could result in the absence of an as- thenospheric mantle wedge and resultant arc magmatism ex- cept in the aulacogen. In any case, the age of the laccoliths constraints the northwest-directed thrusting and nappe development to be older than about 1860 Ma. The development of the northwest-directed nappes cannot be directly linked to any events in Wopmay Orogen. However, as suggested by Hoffman (1981) and Hoffman et al. (1983), thrusting in the aulacogen may be related to a collision in the Trans-Hudson Orogen at about 1865 Ma (Ray and Wanless 1980; Lewry 1981; Lewry er al. 1984). The thrusting in the aulacogen occurred between the two major collision events in Woprnay Orogen (Hoffman and Bowring 1984) and probably before development of the Great Bear Magmatic Zone.

The U-Pb zircon age for the Compton laccoliths is of interest in that it suggests that the many inconsistent Rb-Sr, K- Ar, and Pb-Pb ages obtained on various rocks of the Wilson Island Group and the Great Slave Supergroup (Fig. 3) have been reset to "ages7' that have questionable geologic significance. This phenomenon has been observed on a regional scale throughout Wopmay Orogen (Easton 1983) and in other Precambrian terranes (Page 1978; Bickford and Mose 1975; Van Schmus et al. 1975) and points out the lack of understanding regarding the response of Rb-Sr systematics to low- grade metamorphic events.

Conclusions The U-Pb geochronology for rocks of the East Arm of Great

Slave Lake is very important in understanding the development of Athapuscow aulacogen and its relationship to Wopmay Orogen and the Trans-Hudson Orogen. The Blachford Lake

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Suite is significantly older than the initial rift sequence in Wopmay Orogen and thus it is likely that the alkaline complex is unrelated to the development of Wopmay Orogen. The ten- tative age of 1928 M a for the Wilson Island Group is much younger than previous estimates and is close in age to the 1900 M a minimum age of the Akaitcho Group in Wopmay Orogen. The Wilson Island Group and Butte Island Granite are entirely allochthonous and their genetic relationship to the Great Slave Supergroup or Wopmay Orogen is uncertain. The mylonitization of Wilson Island Group rocks along the southern boundary of the aulacogen raises the possibility that a collision between the Slave Province and northwest Churchill Province occurred between 1928 M a and Great Slave Supergroup deposition (pre-1860 Ma). The age of about 1865 M a for the Compton laccoliths strengthens the correlation with the Great Bear Magmatic Zone and constrains the timing of northwest- directed thrusting in the East Arm. The thrusting took place between the two major collisions in Wopmay Orogen and is probably synchronous with the broad northeast-trending cross- folds in the foreland of Wopmay Orogen that predate the Great Bear Magmatic Zone. This deformation may be the result of a collisional event in the Trans-Hudson Orogen to the southeast.

As the geology and geochronology of the western Canadian Shield become more well known (Bowring and Van Schmus, in preparation; Hoffman and Bowring 1984; Lewry et al. 1984), it is becoming clear that crustal growth between 1.9 and 1.8 G a may be viewed in terms of rapid assembly and inter- action of crustal blocks. Combined geologic and U-Pb geochronologic studies are important tools in understanding the formation and zonation of Proterozoic orogens and their interrelationships.

Acknowledgments This work was supported by a University of Kansas general

research fund grant and National Science Foundation (NSF) grants 79-19544, 81-18234, and 83-031 10 to Van Schmus. W e thank Bill Padgham and the geology office of the Depart- ment of Indian and Northern Affairs Canada (DIANA), Yellowknife, Northwest Territories, for support of fieldwork. G. M . Ross and R. S. Hildebrand provided thoughtful reviews.

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