The Scimitar Complex: Initial SHRIMP U-Pb and Nd Isotope Data from Granitoid Rocks1

Kevin M. Ansdell 2 and Richard Stern 3

i\nsdcll, K.M. and Stern. R. ( I 997): The Scimitar Complex : Initial SHRJ MP U-Pb and Nd isotope data from granitoid rocks; i11 Summary of Investigations 1997. Saskatchewan Gcologiral Survey. Sask. Energy Mines. Mist:. Rep. 97-4.

I. Introduction

Recent interest in the Scimitar Complex (Figure I ; e .g. Ansdell , 1995) has focused on understanding its relationship with the paragncisses of the Kisscynew Domain , which are thought to have originally hcen

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deposited in a hack-an: environment hetwccn about 18.50 and 1840 Ma (Ansdell et al.. 199.5 ; Ansde ll and Stern , in press). If the supracrustal rocks of the complex arc stratigraphically intercalated with Kisseynew gne isses then they may provide information on the origin of the Kisseynew hasin (e .g . Zwanzig,

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55"30'

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1997). Detailed lithological and structural mapping of the complex (Ashton et al., l 996a, b) suggests, as origina lly proposed hy Pearson ( 1973), that it like ly represents the northeaste rn extension of the Glennie Domain. and comprises rocks similar in origin and metamorphic grade to those exposed in the Attitti Block. itself an extension of the Flin Flon Domain (Ashton et al .. 1996a, b). This would imply that Scimitar rocks are older than paragneisses of the Kisscyncw Domain. The nature of the contact between the complex and the Kisseynew Domain has not been fully resolved , although there is evidence that it may, in places, be tectonic ( Ashton et al.. I 996b). Gneissic granodiorites, common in the central portions of the complex. were sampled to obtain a minimum age for the supracrustals. The U-Ph age data reported were obtained using the SHRIMP II, recentl y acquired by the Geological Survey of Canada (Ste rn, 1996). The spatial resolution afforded hy this instrument allows determinatio n of potential zircon metamorphic ages as we ll as c rysta llization in these upper amphibolite grade rocks.

Figure l · Simplified lithntectnnic map of the Tra11s-H11dso11 Oroge11, eastern Saskatchewan, showing the location of the Scimitar l,ake (A) and Wintega Lake (B) areas (!rum A11sde/J, /995).

Rocks of the ce ntra l and southern Re indeer Zone. including the Scimitar Complex, were deformed and metamorphosed during collision with the Archean

'. LITIIOl'RORE Publica1ion No. IOOO. Projecl funded hy NSERC- LITHOPRORE gr:ml to K.M. Ansdd l. · Dcpan111cn1 of Geologica l Scii;nc.-s. Uni vcrs11y of Sa,ka1chcw::m. l 14 Scicncc Placc. Sasb1oon. SK S7N 5E2. 1Gco!og ical S urwy of Ca nada. 601 Boolf1 Stree1. Ot1awa. ON KIA OE~.

Saskmcllt'wan Geolo~ical Sun•ey 145

Saskatchewan Craton (Ansdell et al., 1995), and the most recent structural interpretations suggest that Reindeer Zone rocks were transported in a southerly direction over the Archean basement (Lewry et al., 1990) between about 1830 and 18 IO Ma ( e.g. Heam an et al., 1995). Direct determinations of the character and age of the craton have been made on samples from structural inliers in the Reindeer Zone (Heam an et al., 1995; Chiarenzelli et al. , 1996) and from scattered drill holes which intersect the sub-Phanerozoic basement in southern Saskatchewan (Collerson et al., 1988; Collerson, 1989). The predominance of 2.5 Ga zircons in gneissic rocks from the exposed Shield distinguishes the craton from the Hearne and Superior cratons. The extent of the Saskatchewan Craton beneath the allochthonous Paleoproterozoil: rol:ks of the Reindeer Zone, and the Phanerozoic cover is more difficult to constrain. Seismic images of the crust obtained during LITHOPROBE transects ( e.g. Lucas et al. , 1993) shows a highly reflective crustal culmination interpreted as Archean basement. Nd and Pb isotopic determinations of post-l:ollisional intrusive rocks provide another indirect method of traci ng buried Archean crust, by measuring Archean crustal contamination (Bickford et al., 1992; Steinhart et al., 1997). These studies have concentrated on relatively undeformed pegmatitcs and leucogranites, typically yield U-Pb zircon ages of between 1770 and 1780 Ma (Bickford et al., 1987; Chiarenzelli, 1989; Ashton et al., 1992), and which are likely derived by partial melting during crustal thickening.

Ashton et al. ( 1996b) suggest that highly strained rocks along the southern margin of the Scimitar Complex may mark the reappearance of the Pelican Dccollement Zone, which bounds Archean Saskatchewan Craton rocks in the Pelican Window. This may imply the presence of craton rocks at the surface or sub-surface in the complex. Jn this study, a relatively undeformed pcgmatitic granite from Wintcgo Lake on the southern margin of the complex was sampled, and its

neodymium isotopic composi tion determined to source the magma.

2. SHRIMP U-Pb Analysis

The pink-weathering K-feldspar-bearing quartz monzodioritic phase of the grano<liori tic gneisses on the west arm of Scimitar Lake was sampled (sec Figure 2 in Ansdell, 1996 for location o f KA95-49) to ohtain the age of these intrusive rocks, and thereby a minimum age for the supracrustals in the central Scimitar Complex.

Zircons were separated using conventional c rushi ng, shaker table, and magnetic and heavy liquid separation techniques at the University of Saskatchewan. Thiny­fi vc zircon grains selected from the most non-magnetic fraction we re mounted on an epoxy d isk (GSC mount JP42), and internal complexities observed using back­scattered e lectron and l:athodoluminesl:ence imagery. The differences in grey level emphasize variations in chemical composition, and provide evidence for either complex primary growth or redistribution of components in the internal portions o f many zircons, and for potential new zircon growth around grain margins. Z ircons were analyzed using the SHRIMP II at the Geological Survey of Canada in Ottawa usi ng the techniques detailed by Stern ( 1997). In thi s study, most analyses were performed using a spot size of 30 µm, although IO µm spots were used to analyze rims on two grains.

Six zircon grains were analyzed, four of which were analyzed in two or more locations, to determine the age of central portions and rims. Mult ispot analysis from a single gra in also potentially improves precision o f calculated ages. The 207Pb/2'"'Ph ages for sing le spots range from 1813 ±26 Ma to 1872 ± 18 Ma, and U concentrations from I 30 to 455 ppm (Table I). The zircon grains are from upper amphibolite grade rol:ks,

Table 1 - SHRIMP compositional and isotope data from Scimitar Lake metagranodiorite (Sample number KA95-49; UTM 644250£; 6193900N).

Spot u Pb 1117 Ph/2°"Pb No. (ppm) (ppm) Th/U .1114Pbt11"'Pb l111!pb/21"'Pb 2'"'Pbtn•u 1<11Ph/23~U M pbf'"'Pb Agt:(Ma)

44. 1 208 74 0.398 0.0000 1 0 .11 97( 14) 0.3376(56) 5.329( 109) 0.11451(112) 1872 ±18 44.2 2 11 78 0.429 0.00001 0.1 256(23) 0.3453(64) 5.420(124) 0. I 1386( 131 ) 1862 ±21 54.1 455 168 0.280 0.00001 0.(>9 12(28) 0.3557(77) 5.6 14(157) 0.11448( 172) 1872 ±27 3. 1 266 96 0.437 0.00018 0. 1289(24) 0.3374(62) 5.22J( l 18) 0. 11 226( 123) 1836 ±20 :u 255 81 0.46 1 0.00017 0 .1304(26) 0.2978(41) 4.673(89) 0.1 1382( 1:12) 186 1 ±2 1 3.3 235 87 0.412 0.000 18 0. 1248(23) 0.348 1 (54) 5.408(99) 0.11 265(93) 1843±15 H 267 91 0.384 0.00017 0.1180(25) 0.32 10(56) 5.032( 106) 0.11}68(113) 1859 ±1 8 4. 1 215 78 0.483 0.00020 0 .1342(27) 0.3374(55) 5.29 1( 11 0) O. l 1374( 122) 1860 ±20 4 .3 142 47 0.3 17 0.00034 0.0925(31) 0.3215(55) 4.912(116) 0. 11080( 155) 181 3 ±26 7.1 134 43 0.322 0 .00027 0.0887(34) 0 .3105(52) 4.757(1 11 ) 0. 11114( 169) 18 18 ±28 7.2 130 42 0.29 1 0.0003 1 0.080:\()2) 0.3 137(60) 4.847( 121) (l.11208( 155) 1834 ±25

29.] 292 94 0. 159 O.Cl0032 0 .0455(22) 0 .32J7(43) 5.001 (88) 0. I 1205( I 12) 1833 ±18

Notes: Spot number r.:fcrs to gr.tin number on mount and analytical spot on gr.tin. Data reduction procedures are outl ined in Stern ( 1997). Error on isotope ratios (refers to last significant figures) and age are quoted at I sigma.

146 Summary of In vestigations /997

and are internally complex. Two spots in grain 4 yielded ages of I 8 I 3 and 1860 Ma, both of which arc concordant. The most reasonable interpretation is that the I 8 I 3 Ma age, which is similar to the age of peak regional metamorphism throughout much o f the Reindeer Zone (e.g. Gordon et al., 1990), represents the age of Pb loss during metamorphic recrystallization and the 1860 Ma age is the emplacement age. Ansdel I and Stern (in press) suggest that detrital zircon grains, which exhibit similar internal variations, contain domains that have, at leas!, undergo ne partial Pb loss during peak metamorphism. A similar process likely operated within the whole population of zircons, thus the original emplacement age is modified by partial Pb loss to yield younger 107Pb/2""Pb ages. Typically, spot analyses from a population of igneous zircons are pooled 10 yield the age of the rock. In this study, the older spot analyses are used to derive an age of 1859 ± 10 Ma (Table I ; Fig ure 2), whereas the younger spot analyses are assumed to result from Pb loss during regional metamorphism.

The granodiorite is similar in age 10 numerous granitoids in the Flin Flon and Glennie domains which postdate intraoceanic accretion, and which developed as part of extensive successor arcs on the Flin Flon­Glcnnie protocontinent (Lucas et al., 1997). The age of the Kisseynew paragneisses adjoining the Scimitar Complex arc constrained by detrital zircons as young as 1844 Ma (Ansdell and Stern, in press). Therefore, the supracrustal rocks of the Scimitar Complex predate deposition in the Kisseynew basin, and arc probably similar in age Lo the volcanic and sed imentary rocks of the Flin Flon Domain , as suggested by Pearson (1 973) and Ashton et al. ( I 996a).

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3. Nd Isotope Analysis

The medium-grained central portion of a 5 metre wide relatively undeformed pcgrnatit ic granite, trending about 160°, was sampled for Nd isotopic analysis (sec Figure 3 in Ansdell, 1996 for location of KA95- I OOB). The granite has a di stinctive pegrnatitic margin of quartz, K-fcldspar, biotite, and garnet. The granite is assumed to be one o f the post-tectonic 1770 to 1780 Ma suite.

The Nd isotope analysis was performed at the University of Saskatchewan. The sample was spiked with an appropriate amount of a mixed 149Sm- 1

~0Nd

tracer solution prior to digestion in HF-HN03 in screwtop Savillex. Sm and Nd were separated using standard cation-exchange procedures, prior to loading on outgasscd rhenium fi laments with H3PO, and 2.5N HCI. Isotopic analyses were performed in static mode on a Finnigan MAT 261 mass spectrometer, and corrected for mass fractionation by normalization to 1" Nd/144Nd=O. 721 9 , and " 1Sm/'~1Sm::c:0.5608 l . Analyses of Ames Nd standard yielded a 143Nd/ 144Nd value of0.512127 ±0.0000 15, and Ames Sm standard "

9Smr 2Srn value of 0.5 16884 ±0.000012. The ENd values were calculated using the following reference values: 14 1Nd/144Nd CHUR (prcsent)::::0.512638 and 147Sm/'44Nd CHUR (prescnt)=0. 1967. Procedural blanks are about I 00 pg Nd and 30 pg Sm. The data for this sample is shown in Table 2.

The Nd isotopic composition o f the pegmatitic granite at the time of format ion can be compared directly with the Nd iso topic composi ti on of potential source rocks (Figure 3). The eNd value at 1780 Ma, the assumed age of the pegmatitc, is -7.5, which is similar to the values

obtained by Bickford et al. (1992; -8.2 to -1 3 .4) for pegmatites and leucogranites crosscutting the Archean inliers in the Glennie Domain. The arc and ocean floor volcanic rocks that comprise the dominant portion of the Paleoproterozoic Trnns-Hudson crust have significantly higher ENd values, from Oto +5 .5 (e.g. Chauvel et al. , 1987; Stern et al., 1995a,b). Grani toid rocks in the volcano­plutonic domains of the Trans-Hudson O rogen have similar values suggesting that these magmas were derived either from

0.30 Igneous age "' 1859 ± 1 O Ma the mantle or by melting o f thickened arc crust which likely included only slivers of older crust (e.g. Whalen et al., 1997).

4.4 4.6 4.8 5.0 5.2 5.4 5 .6 5.8

207 Pb/ 235U

Figure 2 - Concordia plot of analyses from the Scimitar Lake granodiorite gneiss. Error polygons are at I sigma. The po/ygo11s used for regression and age determination are shaded.

Saskatchewan Geological Survey

Archean continental crust at 1780 Ma would likely have exhibited eNd values of about -IOto-20.

147

Table 2 - Isotope and compositional data for Wintego /.ake pegmatitic granite (UTM 636/00E; 61596.'iON).

Sample Nd (ppm) Sm (ppm)

KA95- IOOB 3 1.84 6.80 0. 129 18

Thus, the simplest interpre tation of the Nd isotopic data o btained from the Wintego La ke pcgmatitic granite is that this magma was derived from, or significantly interacted with, o lder Archean crust. Turbiditic (Burntwood Group) and lluvial (Missi/Ourom Group) rocks in the region are also potential so urces; however, dc trita l zircon and limited Nd isotopic analysis suggests that these sedimentary rocks derived most of their detritus fro m Palcopro terozoic rocks. The regio nal implicatio n is that the southern part of the Scimitar Complex is underlain by Archean crust of the Saskatchewan Craton, and that the buried Archean craton extends farther northeast than previously recognized.

4. Acknowledgments

Samples were collected with the assistance of Curtis Kelln. Zircons were separated by Grant M ourre and Melissa Kujawa. The first author would like to thank Richard Siem at the Geological Survey of Canada SHRIMP Laboratory for help in everything from mo unt

+10 Flin Flon mantle

ENd at 1780 Ma

0.5 l 1463 :t0.000006 -7 .5 ±0. 1

pre paratio n, 10 data reduction, 10 ha rdware troubleshooting. Nd isotope data at the University of Saskatchewan were obtained with the assistance of Diane Fox and Chris Holme.Jen.

5. References Ansdell , K.M. (1995): Observations in the Scimitar and

Wintego lakes area, Trans-Hudson Orogcn. Saskatchewan; in Summary of Investigations l 995. Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 95-4, p162- 167.

_ ___ _ ( 1996): The Scimitar Lake gneisses: Preliminary data on metamorphic conditions; LITHOPROBE Trans­Hudson Orogcn Transect, Rep. 55, p 143- 15 1.

Ansdell , K.M. and Bleeker, W. (1997): The margin of the Superior Craton: Neodymium isotope constraints on the evolution of the Thompson Nickel Belt , Manitoba; Geol. Assoc. Can.-Miner. Assoc. Can .. Abst. Vol.. v22, pA-4-5.

Ansdell. K.M. , Lucas, S.B .. Connors. K .. and Stern, R.A.

I

'" DEPLETED MANTLE

( 1995): Kisseynew metasedimentary gneiss belt, Trans-Hudson Orogen (Canada): Back-arc origin and collisional inversion; Geol. , v23, pl039- 1043. ~s

I I

.J I 0 1 I I

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-15

-20

1700

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Glennie pegroatites

1900

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I 2100 2300

Age (Ma)

2500 I

2700

Figure 3 - Epsilon Nd-age plot showing the relationship between the Wintego Lake pegmatite and the isotopic composition of potential source rocks. Age of pegmatites assumed to be ca. 1780 Ma (Bickford et al., 1987). Flin F/011 mantle, and arc and ocea11floor rocks from S tem et al. ( /995a, b) and Whalen et al. (1997); Glennie pegmatites from Bickford et al., (1991); typical Archean crust from Bickford et al., (1992) and Ansdell and Bleeker (1997).

148

Ansdell, K.M. and Stern , R.A. (in press): SHRIMP geochronology in the Trans-Hudson Orogen: Detrital zircons in turbid iti c and flu vial sedimentary rocks; LITHOPROBE Trans-Hudson Orogen Transect.

Ashton. K.E .. Hartlaub, R.P., Therens. C., and Legault , A. ( I 996h): The Scimitar Complex- Kisseynew Domain boundary in the Wintego Lake area (part of 63M- l 0); in Summary of Investigations 1996, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 96-4, p29-37.

Ashton. K.E., Hunt, P.A., and Froese, E. ( 1992): Age constraints on the evolution o f the Flin Flon volcanic belt and Ki sseynew gneiss belt, Saskatchewan and Man itoba; in Radiogenic Age and Isotopic Studies: Report 5, Geol. Surv. Can .. Pap. 9 1-2. p55-69.

Summary of lnvestiKations I 997

Ashton. K.E. . Therens, C. , and Legaul t, A . ( 1996a): Geology of the Scimitar Lake area (part of 63M- 15). cast-central Sci mitar Complex; in Summary of Investigations 1996. Saskatchewan Geological Survey, Sa~k. Energy Mines. Misc. Rep. 96-4. p22-28.

Bickford . M.E .. Collerson, K. D .. Lewry, J.F .. and Orre ll, S.E. ( 1992): Pegmat1tes and leucogranites as probes of crust beneath allochthonous orogcnic rocks in the Glennie and La Ronge domains; in Summary of Investigations 1992, Saskatchewan Geological Survey. Sask. Energy Mines, Misc. Rep. 92-4. p124-1 29.

Bickford. M.E .. Yan S\:hmus. W.R., Coller~on, K.D .. and Macdonald, R. ( 1987): U-Pb zircon geochronology project: New results and interpretations; in Summary of Investigations 1987. Saskatchewan Geologh.:al Survey, Sask. Energy Mines, Misc. Rep. 87-4. p76-79.

Chauvcl, C. , Arndt, N.T., Kielincszuk, S., and Thom, A. ( 1987): Formation of 1.9 Ga old continental crust. I; Nd isotopic data; Can. J. Earth Sci. , v24, p396-406.

Ch iarcnzclli . J . ( I 989): The Nistowiak and Guncoat gneisses: Implications fo r the tectonics of the Glennie and La Ronge domains, northern Saskatchewan. Canada; unpubl. Ph.D. thesis. Univ. Kansas , 229p.

Chiarcnzell i, J.R., Asplcr, L.B., and Villeneuve, M. ( 1996): Characterization, origin , and Paleoproterozoic history of the Saskatchewan craton and possible implications for Trans-Hudson Orogen; LITHOPROBE Trans-Hudson Orogen Transect. Rep. 55, p26-38.

Collcrson, K.D. ( 1989): Sm-Nd isotopic constraints on the age of buried Precambrian basement in central and southern Saskatchewan: Impl ications for diamond exploration; in Summary of Investigat ions 1989, Saskatchewan Geological Survey. Sask. Energy Mines. Misc. Rep. 89-4, p l 68- 171.

Collcrsnn, K.D .. Van Schmus. R.W . . Lewry, J.F.. and Bickford, M.E. ( 1988): Buried Precambrian basement in south-cent ral Saskatchewan: Provisional results from Sm-Nd model ages and U-Pb zircon geochronology; in Summary of Invest igations 1988, Saskatchewan Geological Survey. Sask. Ene rgy Mines. Misc. Rep. 88-4, p l42- 150.

Gordon. T. M .. Hunt . P.A., Bailes . A.H .. and Syme, E.C. ( 1990): U-Pb ages from the Flin Flon and Kisscynew belts, Manitoba: Chronology of crust formation at an Earl y Proterozoic accrctionary margin; in Lewry, J .F. and Stauffer, M.R . (eds. ), The Early Proterozoic Trans­Hudson Orogen of North America. Geo!. Assoc. Can .. Spec. Pap. 37, p l 77-1 99.

Heaman. L.. Ashton, K.E .. and Lewry, J.F. ( 1995): U-Pb geochro nology o f the Pelican Window: Preliminary results; LITHOPROBE Trans-Hudson Orogen Transect, Rep. 48. p l43- 147.

Lewry, J .F .. Thomas, D .. Macdonald, R., and Chiarenzclli , J . ( 1990): Structural relations in accreted terranes of the Trans-Hudson Orogen. Saskatchewan: Telescoping in a collisional regime?; in Lewry, J.F. and Stauffer, M.R. (eds.). The Early Proterozoic Trans-Hudson Orogen of North America, Geol. Assoc. Can .. Spec. Pap. 37. p75-94.

Lucas, S.B. , Green. A .. Hajnal, Z., White, D . . Lewry. J ., Ashton. K., Weber, W .. and Clowes, R. ( 1993): Deep

Suskatchewan Geological S11rVt1y

seismic profi le across a Proterozoic colli sion zone: Surprises at depth; Nature. v363, p339-342.

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Pearson, D.E. (1973): The geology of the Scimitar Lake area (east hall). Saska tchewan; Sask. Dep. Miner. Rcsour. , Rep. 156. 17p.

Steinhart, W.E .. Bickford, M.E .. Lewry , J .F., and Mock, T.D. ( 1997): Common Pb and Sm-Nd study of the Glennie. Hanson Lake, and La Ronge domains, and the Hearne province, Trans-Hudson Orogen: Implications for tec ton ic evolution; Geol. Assoc. Can.-Mincr. Assoc. Can .. Abst. Vol .. v22, pA-142.

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_ _ _ _ ( 1997): The GSC Sensitive High Resolution Ion Microprobe (SHRIMP) : Analytical techniques of zircon U-Th-Pb age dete rminations and performance evaluation; in Radiogenic Age and Isotopic Studies: Report IO. Geol. Surv. Can. Current Research 1997-F, pl -32.

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Whalen. J.B., Syme, E.C.. and Stern. R.A. ( 1997): Temporal geochemical and Nd isotopic variations in granitoid magmatism within the Flin Flon belt, Trans-Hudson Orogen; Geol. Assoc. Can.-Miner. Assoc. Can .. Abst. Vol. , v22. pA- 157.

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