C. Contributed Papers on Northern Studies

Saskatchewan Geological Survey 109

110 Summary of Investigations 1994

Paleomagnetic Test of Missi Conglomerate Clasts, Flin Flon Domain, Trans-Hudson Orogen (Part of NTS 63K-13) 1

D. T.A. Symons 2 and M. T. Lewchuk 2

Symons, D.T.A. and Lewchuk, M.T. (1994): Paleomagnetic test of Missi conglomerate clasts, Flin Flon Domain, Trans-Hudson Orogen (part of NTS 63K-13); in Summary of Investigations 1994, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 94-4.

Paleomagnetic analysis has been completed on 76 specimens from 58 clasts from two sites in the Missi Formation. Site 1 in the Little Cliff Member just north of Flin Ron, records a remagnetization at - 1838 Ma that reflects cooling of magnetite and pyrrhotite after amphi­bolite grade regional metamorphism. Site 2 in the basal Beaverdam Member of low greenschist grade just south of Flin Flon appears to have been remagnetized at - 1855 Ma through hydrothermal hematization. These re­sults rule out the presence of later regional charac­teristic remnant magnetization (ChRM) overprints in the amphibolite and lower grade rocks of the Flin Flon arc.

A paleomagnetic conglomerate test can help identify the age of a ChRM component in a study area. If con­glomerate clasts retain random ChRM directions, then the clasts have not been remagnetized and the ChRM of the host unit is primary (Graham, 1949). Conversely, if the clasts' ChRMs are uniformly directed, then they have been remagnetized by a regional metamorphic,

Figure 1 - Location of the study area at Flin F/on in the Flin Aon Domain of the Trans-Hudson Orogen. HB. Hanson Lake Block; C-SBZ, Churchill-Superior boundary zone.

( t ) Funding for this project was provided for by an NSERC research grant .

contact metamorphic or regional fluid flow event. If re­magnetization has occurred, then adjacent units are likely to have been reset by the same event. In his re­gional paleomagnetic study of rock units in the Flin Flon Domain and Hanson Lake Block (Figure 1), Park (1975) found that many units had been remagnetized. In con­trast Mackay (1992) and Symons (in press) have ar­gued that the massive Boot Lake and Reynard Lake complexes near the town of Flin Flon retain primary remnances. This study also examines clasts from two sites in Missi conglomerates near Flin Flon (Figure 2).

• Boundary intrusions Missi Formation

Little Cliff member • sandstone O conglomerate

Beaverdam member D sandstone ~ conglomerate

Figure 2 - Geological map with site locations in the Flin Flon ba· sin, modified from Stauffer (1990).

(2) Department of Geology, University of Windsor, Windsor, Ontario, N9B 3P4.

Saskatchewan Geological Survey 111

1. Geology

The Flin Flon basin straddles the Manitoba-Saskatche· wan border as part of the Flin Flon arc terrane in the western part of the Flin Flon Domain of the Paleopro­terozoic Trans-Hudson Orogen (THO) (Figures 1 and 2). The oldest rocks are tholeiitic to calc-alkaline vol­canic and volcaniclastic rocks of the Amisk Group that give U-Pb zircon ages ranging from 1886 ±2 Ma to 191 O Ma (Stauffer et al., 1975; Gordon et al., 1990; Stern et al., 1993). They represent ocean ridge, intra­oceanic island arc, and back-arc extrusives (Gaskarth and Parslow, 1987; Bailes and Syme, 1989).

After 01 deformation, the Missi Formation was depos­ited unconformably upon a regolith up to 1 O m thick atop the Amisk metavolcanics; locally, a thin hematitic zone caps the regolith (Figure 2). The Missi comprises two upward-fining conglomerate, pebbly sandstone, and sandstone sequences. The lower is the Beaverdam Member, the upper is the Little Cliff Member; together they exceed 2000 m in thickness. Each member re· cords fluvial deposition starting as a proximal fan and evolving into a braided-stream environment (Stauffer, 1990). Based on the youngest age found for rounded zircons by U-Pb zircon dating, deposition of the lower member began after -1857 ±2 Ma and the upper mem­ber after 1847 ±2 Ma (Ansdell, 1993). Deposition of the Missi ended by 1842 ±3 Ma when it was cut by the Boundary intrusions (Heaman et al., 1992). The _clasts are dominantly mafic volcanics with common fels1c vol­canics and some granitoids.

During and after deposition of the Missi Formation, sev­eral episodes of plutonism occurred within the Flin Flan arc, starting with the 1860 ±7 Ma Annabel Lake grano­diorite pluton. At 1850 ±1 Ma the Reynard Lake grano­diorite was emplaced, about coeval with the 1850 ±2 Ma Wekach Lake metagabbro, and followed by the 1842 ±3 Ma metagabbros of the Boundary intrusions, the massive 1838 ±2 Ma Boot Lake gabbro and Phan­tom Lake granite complexes, and finally the widespread 1790 to 1770 Ma pegmatites (Gordon et al., 1990; Reilly, 1990; Ansdell and Kyser, 1991; Hearnen et al.,_ 1992). Regional metamorphism increased from prehrnte­pumpellyite facies at the south end of the Flin Flon ba­sin near site 2, to amphibolite grade at the north end near site 1 (Digel and Gordon, 1991). Reilly et al. (1993) define seven deformation events, 01 to 07, for the Flin Flon arc. 02, 03, and 04 record the peak of metamorphism prior to 1838 Ma, and 05 to 07 record lesser events that ended -1 no Ma.

2. Experimental Methods

At sites 1 and 2 (Figure 2), oriented drill cores were ob· tained from 30 and 28 clasts in the basal conglomerate of the Little Cliff and Beaverdam members, respectively. The conglomerate units dip at -55° to the northeast and east, respectively. One or two specimens were sliced from each core. The Little Cliff clasts are larger with half yielding two coherent specim~ns: The Beave~­dam clasts are smaller with only two yielding two spec1· mens and they are more friable also so that most

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needed to be glued. Each specimen was alternating field (AF) demagnetized in nine steps up to 120 mT or thermally demagnetized in nine steps up to 600°C. The ChRM directions were determined using orthogonal vec­tor plots and the Kirschvink (1980) least-squares method.

3. Results

The 45 specimens from 30 site 1 clasts give NAM inten­sities ranging from 1.9 to 62.3 x 10-2 Alm (amperes per metre) with a median of 5.1 x 10-2 Alm. Except for one specimen, AF step demagnetization at 120 mT reduces their ChRM intensity (J12o/JNRM) ratio to <0.22 with a mean of 0.13 ±0.08 (Figure 3a, b). Thermal step demag· netization also reduces the NAM intensity rapidly in many specimens with some unblocking in the 250° to 330°C range of pyrrhotite, some in the 450° to 585°C range of magnetite, some in both pyrrhotite and magnet­ite (Figure 4) and, rarely, some above 600°C in the range to 675°C of hematite {Figure 5). Most ChRM di­rections are inclined moderately to steeply downward, but broadly scattered (Figure 6a). The mean of the mean angular deviation (MAD; Kirschvink 1980) values for the 46 specimen ChRM directions is 4.3°. The 30 clast mean ChRM directions give a site mean of 27.4°, 79.4° (ags=9.5°, k=8.7) (declination, inclination, radius of cone of 95 percent confidence, precision parameter of Fisher (1953)}. At high AF coercivities and unblock­ing temperatures, after the ChRM directions have bee_n defined, many specimens appear to start on a great cir­cle track towards a second direction which would greatly randomize the population (Figure Sb). Neverthe­less, the length of the resultant vector for the 30 cl~sts is 25.67 which is very much larger than the comparison statistic of 8.80 at the 95 percent confidence level (Wat­son, 1956). This shows that the population has a non-

a b w.u N. U

20 s N E 120

0.8

NRM

E, 0 S, O

c w.u

s , , I .,

NRM-120 I

I

c/-120

l_NRM

I

I

E. D

0.8

N

Figure 3 - Orthogonal AF step demagnetization plots of repre· sentative specimens from: a) site 1, #11-2, Jo=1 .01 x u:r-2 Nm; b) site 1, #26·2, Jo=3.69 x 10-2 Nm; and c) site 2, #27-1, Jo=4.41 x 10-2 Alm. Circles denote projections onto the horizon­tal plane (N,E,S,W) and triangles the vertical P_lane_ (U,D). Axial unit is the Jo value. Some step values shown m m1J/Jtesla.

Summary of Investigations 1994

a b c N,U W,U w.u

w E s N s N 570 540

330 330

250 250

0.8 0.8 0.8

NRM NRM NRM S, D E, D E, D

1.0 J Jo

0 600 Figure 4 - Orthogonal thermal step demagnetization plots of representative specimens from site 1: a) #10-1, Jo=5.58 x 10-3

Alm; b) #14·1, Jo=3.91 x 10-6 Alm; and c) #19-2, Jo=3.59 x 10-3 Alm. Conventions as in Figure 3 except step values in degrees Celsius (°C). Below is the J/Jo versus °C plot with the diagnostic pyrrhotite (P) and magnetite (M) unblocking tempera­ture ranges.

random preferred direction, meaning that the clasts have been remagnetized since deposition.

The 30 specimens from 28 site 2 clasts have quite uni­form ChRM properties. The NAM intensities range from 1.3 to 9.5 x 10-2 Alm with a mean of 4.3 ±1.8 x 10-2 Alm. AF demagnetization at 120 mT gives J120/JNRM ra· tios of >0.35 with a mean of 0.63 ±0.16 (Figure 3c). Thermal step demagnetization results in slow intensity decay to 600°C with 0.65 ±0.06 of the JNRM still remain­ing, showing that the ChRM is carried by hematite (Fig­ure Sc). Further, the ChRM directions are easily defined with a mean MAD angle of 3.1°. The 28 clast directions form a coherent cluster with a mean of 123.9°, 71 .2° (ass:::3.1 °, k=76.5) (Figure 6b). Their resultant vector of

Saskatchewan Geological Survey

a b c N,U W,U N, U

330 450 w E s N w E

600 600

330

250 0.8 0.8 250

NRM NRM S, D E.D S, D

1.0 J Jo

0 600 Figure 5 - Orthogonal thermal step demagnetization plots of representative specimens from: a) site 1, #22-1, Jo=2.88 x 10-3

Alm; b) site 1, #30-2, Jo=S.09 x to-3 Alm; and c) site 2, #18·1, Jo=3.49 x 10-2 Alm. Conventions as in Figure 4 . Figures a) and c) show residual hematite-borne ChRM at 600°C and b) shows an underlying earlier ChRM in a clast above soo•c.

26.65 far exceeds the critical value of 8.50, showing with >>95 percent confidence that the clasts were re­magnetized after deposition.

In addition to having very different magnetic mineralogy and demagnetization characteristics, the two site mean directions are significantly different at >>95 percent con­fidence (McFadden and Lowes, 1981). This shows that they were formed at different times in separate remag­netizing events.

4. Pole Positions

The pole position obtained from the site 1 mean ChRM direction is located at 72.4°W, 70.8°N (dp=17.2°, dm=18.1 °). This pole is like only one other pole so far within the Flin Flon arc, namely, the concordant Boot

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a

30 clasts Site 1

• 270 + +

•

•

28 clasts Site 2

270 + +

b

0

+ • •

:~ + • •••

+

+

• + ': + '6l' •••• • • • +

+

180

•

+ 90

+ 90

Figure 6 - equal-area stereogram showing the clast mean direc­tions for a) site 1 and b) site 2. Down(up) vectors are solid (open) symbols. The site mean (x) is circumscribed by its cone of 95 percent confidence.

LakEH'hantom lake pole (Mackay, 1992) (Figure 7). This complex of massive unmetamorphosed gabbro and granite has been dated at 1838 ±2 Ma (Heaman et al., 1992). It contains vertical late-phase diabase dykes that show that the complex and its pole have not been tilted (Thomas, 1989). Thus, the obvious conclusion is that the Missi conglomerate was metamorphosed to lower amphibolite grade and tilted during the main 03 and 04 deformation events before 1838 Ma, and that cooling after peak metamorphism at site 1 substantially reset the magnetization at -1838 Ma. In turn this im­plies that ~ 1838 Ma rock units in the Flin Flon arc, with equivalent or lower grades of regional metamorphism, will retain an equivalent or earlier ChRM. In other words, the ChRM carried by the Reynard Lake pluton could be primary at 1853 Ma as Symons (1993) argued and the ChRM carried by Amlsk volcanics and other sites could be a secondary metamorphic remanence as Park (1975) argued, but both must predate 1838 Ma.

For site 2, the pole is located at 69.4°W, 29.6°N (dp=4.8°, dm=-5.5°). This pole is not a close match for either the Reynard Lake complex or Flin Flon mean pole (Figure 7). Note, however, that all three are about equidistant from the Flin Flon Domain, implying that

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Figure 7 - Pole positions for the Missi conglomerate clasts from site 1 (MCt) and site 2 (MC2) compared to those from the Lynn Lake gabbro pipes (LL; Dunsmore and Symons, 1990), Molson dykes (MO; Zhai et al., 1993), Flin Flon Domain com­posite (FF; Park, 1975), Reynard Lake pluton (Re; Symons, in press) and Boot Lak~Phantom Lake complex (BP; Mackay, 1992; Symons, 1993).

they were formed at a similar paleolatitude. Also, note that the site 2 pole is about halfway between the -1890 Ma Lynn lake gabbros pole and -1883 Ma Molson dykes' pole from domains surrounding the Flin Flon Domain and the 1838 Ma Boot lake-Phantom lake pole in the domain. This suggests an age of -1855 Ma for the site 2 ChRM. The speculation is, therefore, that the Missi conglomerate at site 2 is record­ing an - 1855 Ma event, and that local vertical-axis rota­tion(s) during the 03 or 04 events within the Flin Flon Domain account for the difference between it and the Reynard lake pole. Observing that the Beaverdam con­glomerate rests on a thick hematitic regolith and that the ChRM at site 2 is carried by abundant hematite, it appears that hydrothermal fluids saturated, altered, and deposited hematite in the conglomerate at - 1855 Ma.

5. Conclusions

A paleomagnetic conglomerate test on clasts from the basal Beaverdam Member of the Missi Formation is overwhelmingly negative. The ChRM of the clasts re­sides in hematite. It is speculated that hydrothermal flu­ids at -1855 Ma deposited hematite derived from the underlying regolith during orogenesis to reset the mag­netization.

A second test on clasts from the base of the upper Lit­tle Cliff Member of the Missi Formation gives a strongly negative conglomerate test also. This ChRM resides in

Summary of Investigations 1994

magnetite and pyrrhotite. Its pole position shows that cooling occurred after amphibolite-grade metamorphism at -1838 Ma and, therefore, surrounding rocks of equivalent or lower-grade metamorphism in the Flin Flon arc likely retain an equivalent or earlier ChRM.

6. Acknowledgments

The authors thank T.B. Symons for field assistance and specimen preparation, J. Kulhanek for measuring assis­tance, K.E. Ashton for collecting advice, and the Natural Sciences and Engineering Research Council of Canada tor research funding.

7. References Ansdell, K.M. (1993): U-Pb zircon constraints on the timing

and provenance of fluvial sedimentary rocks in the Flin Flon and Athapapuskow basins, Flin Flon Domain, Trans­Hudson Orogen, Manitoba and Saskatchewan; Geol. Surv. Can., Pap. 93-2, p49·57.

Ansdell, K.M. and Kyser, T.K. (1991): Plutonism, deformation, and metamorphism in the Proterozoic Flin Flon greenstone belt, Canada: Limits on timing provided by the single-zir­con Pb-evaporation technique; Geot., v19, p518·512.

Bailes, A.H. and Syme, E.C. (1989): Geology of the Flin Flon­White Lake area; Man. Dept. Energy Mines, Geol. Rep. GR 87·1, 313p.

Digel, S. and Gordon, T.M. (1991 ): Prehnite-pumpellyite to am­phibolite facies metamorphism near Flin Flan, Manitoba; Geol. Surv. Can., Pap. 91-1C, p165-172.

Dunsmore, D.J. and Symons, D.T.A. (1990): Paleomagnetism of the Lynn Lake gabbros in the Trans-Hudson Orogen and closure of the Superior and Slave cratons; in Lewry, J.F. and Stauffer, M.A. (eds.), The Early Proterozoic Trans­Hudson Orogen of North America, Geol. Assoc. Can., Spec. Pap. 37, p215-228.

Fisher, A.A. (1953): Dispersion on a sphere; Proc. Roy. Soc. London, Series A, v217, p295·305.

Gaskarth, J.W. and Parslow, G.R. (1987): Proterozoic volcan­ism in the Flin Flon greenstone belt, east-central Saskatch­ewan, Canada; in Pharoah, T.C. et al. (eds.), Geochemistry and Mineralization of Proterozoic Volcanic Suites, Geol. Soc. Lon., Spec. Publ. 33, p183-200.

Gordon, T.M., Hunt, P.A., Bailes, A.H., and Syme, E.G. (1990): U·Pb ages from the Flin Flan and Kisseynew belts, Mani­toba: Chronology of crust formation at an Early Proterozoic accretionary margin; in Lewry, J.F. and Stauffer, M.A. (eds.), The Early Proterozoic Trans-Hudson Orogen of North America, Geol. Assoc. Can., Spec. Pap. 37, p177. 199.

Graham, J.W. (1949): The stability and significance of magnet­ism in sedimentary rocks; J. Geophys. Res., v54, p131-167.

Heaman, L.M., Kama, S.L., Ashton, K.E., Reilly, B.A., Slimmon, W.L., and Thomas, D.J. (1992): U-Pb geochro­nological investigations in the Trans-Hudson Orogen, Sas­katchewan; in Summary of Investigations 1992, S~skatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 92-4, p120-123.

Saskatchewan Geological Survey

Kirschvink, J.L. (1980): The least squares line and plane and the analysis of paleomagnetic data; Geophys. J. Roy. Astr. Soc., v62, p699-718.

Mackay, C.D. (1992): Paleomagnetism of the Boot Lake-­Phantom Lake intrusive complex, central Saskatchewan· unpubl. B.Sc. Thesis, Univ. Windsor, 26p. '

McFadden, P.L. and Lowes, F.J. (1981): The discrimination of mean directions drawn from Fisher distributions; Geophys. J. Roy. Astr. Soc., v67, p19-33.

Park, J.K. (1975): Paleomagnetism of the Flin Flon--Snow Lake greenstone belt, Manitoba and Saskatchewan; Can. J. Earth Sci., v12, p1272·1290.

Reilly, B.A. (1990): Bedrock geological mapping, Mystic Lake-­West Arm, Schist Lake area (part of NTS 63K-12); in Sum­mary of Investigations 1990, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 90-4, p25-35.

Reilly, B.A., Thomas, D.J., Slimmon, W.L., Ashton, K.E., and Heaman, L.M. (1993): Structural history of the Flin Flon Domain in Saskatchewan; in Hajnal, Z. and Lewry, J. (eds.), LITHOPROBE, Trans-Hudson Orogen Transect, Rep. 34, p98·102.

Stauffer, M.A. (1990): The Missi Formation: An Aphebian mo­lasse deposit in the Reindeer Lake zone of the Trans· Hudson Orogen, Canada: in Lewry, J.F. and Stauffer, M.A. (eds.), The Early Proterozoic Trans-Hudson Orogen of North America, Gaol. Assoc. Can., Spec. Pap. 37, p121-141.

Stauffer, M.A., Mukherjee, A.C., and Koo, J. (1975): The Amisk Group: An Aphebian (?) island arc deposit; Can. J. Earth Sci., v12, p2021-2035.

Stern, A.A., Lucas, S.B., Syme, E.C., Bailes, A.H., Thomas, D.J., Leclair, A.D., and Hulbert, L. (1993): Geochronologi­cal studies in the Flin Flon Domain, Manitoba· Saskatchewan, NATMAP Shield Margin Project area: Results for 1992-1993; Geol. Surv. Can., Pap. 93-2, p59-70.

Symons, D.T.A. (1993): Paleomagnetic studies in the Trans­Hudson Orogen, emphasizing results from the Reynard Lake pluton; in Hajnal, Z. and Lewry, J. (eds.), LITHO­PROBE, Trans-Hudson Orogen Transect, Rep. 34, p72·76.

____ (in press): Paleomagnetism of the 1850 Ma Reynard Lake pluton: Limit on accretion of the Flin Flon Domain to the Superior craton; Can. J. Earth Sci.

Thomas, D.J. (1989): Geology of the Douglas Lake-Phantom Lake area (part of NTS 63K-12,-13); in Summary of Investi­gations 1989, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 89-4, p45·54.

Watson, G.S. (1956): A test for randomness of directions: Mon. Not. Roy. Astr. Soc., Geophys. Supp., v7, p160·161.

Zhai, Y. and Halls, H.C. (1993): Age and tectonic setting of the Molson dyke swarm: A paleomagnetic study; in Hajnal, Z. and Lewry, J. (eds.), LITHOPROBE, Trans-Hudson Oro­gen Transect, Rep. 34, p77-83.

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