- 23 -

La Bonge Project

by J, F. Lewry, A. Abraham and G. B. Wilson

..... ,

Recent work (Lewry and Sibbald, 1980; Ray and Wanless, 1980 ; IBwr:y, 1980; ~et.al., sul:xnitted) indicates that greater understand­ing of the eastern La Ronge dorrain and its' relation to the Glennie Lake danain (Fig. 1) is critical to any conprehensive interpreta­tion of geotectonic evolution in the Hudsanian orogen of the western Clmrchill Province. Fieldwork airred at such understanding was begun in the past field season.

General recormaissanae w::>rk airred at familia­rization with regional geology included re-examination of road sections along the I.a. Ronge-Maclennan Lake , Stanley Mission and Anglo-Rouyn roads and of shoreline sections in the northern part of Lac La Ronge , otter, Devil and Dickens Lakes. Detailed re-mapping was begun in the Otter-Dickens Lake vicinity (Fig. 2).

"" liO •.•

·~ .... I f

' :, • ;

(: !..: I f,O ~"' ------- - ---· f) !.(:

NORTH'ERN SASKA TCHEW AN

- - - MAJOR SHEAR 7.0NES

---O THER DOMAIN BO UNDARIES

Fig. 1 - Major features of the Saskatchewan Shield and location of area di scussed in text.

* University of ~gina. This project is partially fi.mded by the Saskatchewan Geological SUrvey in 1980-81.

L -"/("'

Fig. 2 - Generalised geology of area discussed in text. 'V' pattern, dominant meta­volcanics of the Western La Ronge doma i n; stipple, meta-arkos ic rocks and r elated pegmatite; horizontal dashes , predominant metasediments of the Eastern La Ronge and Glennie Lake domains; vertical dashes, poss ible oli s tostrome unit (Unit 7 of Padgham, 1966); crosses, undifferentiated granitoid rocks; 'squiggly' pattern, major zones of early shearing and myloniti sation .

Otter-Dickens Lake Vicinity

'Ihe general objectives of this re-IMwing include the detennination of stratigra~ic relations, depositional envirorim;mts, structu­ral geonetry,sequence and strain, and the sequence of netarrorphic and intrusive events.

- 24 -

'Ihe rock units have been well established by previous ~rk. Only a summry of this ~rk, together with sorre arrendrrents resulting from the present re-napping is given here; Three rrajor lithological belts are distinguished; from west to east and from higher to laver structural position these are:

1. The ~tavolcanic Belt 2. The ~ta-Arkosic Belt 3 . The Biotite Psanmite-~taconglarerate

Belt

The ~tavolcanic Belt conprises basic to acidic lavas and pyroclastics, proximal volca­niclastic to epiclastic sedi.rrents and local basic to ul trarna.fic minor intrusions. These are intruded by generally dioritic to granitic plutons of variabl e age relative to regional deforrrational sequence. In the Devil-Dickens Lakes area the eastern part of the belt includes locally pilla.ed basic-intenrroiate lavas, varied agglorrerates, epiclastic conglo­merates, finer grained pyroclastic and epi­clastic rocks, indetenninate highly tectonised homblendic, amphibolitic and feldspathic rocks , and gabbroic to dioritic minor intrusives.

The ~ta-Arkosic Belt consists nainly of rredium grained granulose to granitoid red-pink to buff-grey, potassiun feldspar-rich muscovite psamnites containing little or no biotite or other mafic minerals. Variants r ich in sillinanite-quartz faserkiesel sheathed in retrograde muscovite characterize the eastern part of the belt. Transposed prirrary layering is best preserved on the western side of the belt. Thin pebbly to conglorreratic units appear sporadically and are even rrore wide­spread than indicated by Padgham (1960, 196 3) ; one particularly well-exposed occurrence near the northeastern end of Dickens Lake contains plentiful well-rounded quartz and red granite pebbles. Minor arkosic 'silts tone' and hoO'lblendic or diopsidic calc-silicate units also occur.

Available data suggests derivation from dominantly granitic near-source areas and deposition under shallow water marine to fluviatile conditions. A possible ' exogeo­synclinal' environnent is tentatively favoured.

The Biotite Psanmi.te-~taconglonerate Belt a:mprises a mixed assenblage of "biotite gneisses" (Padgham, 1960, 1963), netaconglo­merates and homblendic gneisses. The biotite gneisses are predominantly psamnitic to semi­pelitic rretagreywackes consisting of plagio­clase feldspar, quartz and generally 10 to 15 percent biotite, with local minor hornblende. They are typicall y potassium feldspar-poor and contrast sharply in mineralogy with the

meta-arkoses. Thin pelitic interlayers, a:mronly sillirranite-bearing, are a:xmon, as are grey-green diopsidic, CXJITTTOnly gameti­ferous, calc-silicate l ayers . lt:lre 'arkosic ' zones containing significant potassium feldspar and locally muscovite and/or silli­manite faserkiesel occur: sare of these appear identical to the main 11Bta-arkose, others are gradational biotite-bearing types. The rreta-arkosic rocks may be infolded strati­graphic equivalents of the rrain meta-arkosic belt; the gradational biotite-bearing types are thought to be integral parts of the biotite psanmi.te sequence.

Hornblendic irembers range from arrphibole­bearing biotite gneisses, transitional to the biotite psanmi.tes, to prani.nently laminated and banded felsic, hornblendic and anphibolitic gneisses . Some appear to be sirrply calcareous greywackes, others may inchrle epiclastic to volcaniclastic tuffs and possibly local metalavas.

Pebbly to conglom:?ratic biotite psanmi.te units display all gradations from sparsely pebbly psanmi.te to coarse polymictic col::ble conglom­erates. The conglorrerate clasts appear generally well rolIDded, though original shape is usually obscured by extrene deformation. The pebbles consist mainly of quartz and varied granitic to granodioritic rocks: mafic gneiss cl asts are l ocally ablmdant, occurring as highly defonred lenticular streaks in the matrix.

Regular layering on a scale of centinetres to over one metre is characteristic of the bio­tite psanmi.te-conglorrerate sequence, as it is of homblendic gneisses . Inconclusive indi­cations of rhythmic layering and graded bedding were locally observed.

The biotite psamnite-netacongl onerate sequence is interpreted as proximal greywackes and conglorrerate fan deposi ts with subordinate intercalated volcaniclastic and epiclastic rocks . All appear predaninantly volcanogenic in provenance. The nore arkosic lenses may indicate stripping of spatially related plutons .

Junction Relations

M:>st of the junction zone betv.een the 1reta­volcanic and meta-arkosic belts is intensely deformed and also intruded by earl y , deformed 'quartz-eye' biotite granite and later pegma­tite sheets. Felsic gneiss intercalations within the rretavolcanics between Dickens Lake and Devil Lake , identified as roota-arkose by Padgham (1963), are all interpreted as de­fonred granitic intrusive bodies by th1;:

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present writers. Rocks of the junction zone possess intense to locally suanylonitic foliation and pronounced rodding and streaking lineation generally plunging down the dip of foliation, generally obscuring original depositional relations. I.Deal interbanding of ' silty' neta-arkose and hornblendic gneiss occurs on a small i s l and adjacent to the junction in the southern part of Dickens Lake. On the west shore of Devil Lake there is an apparent continuous passage from hornblendic and anphibolitic gneiss southwards into inter­layered hornblende gneiss, f elsic gneiss, silty biotite rreta-arkose and thence to the main biotite-poor, muscovite-bearing neta­arkoses. To what extent this junction passage is tectonically rrodified is uncertain.

Passage from the rreta-arkosic to the biotite psanmite-mataa:>nglorrerate belt, though locally an abrupt tectonic discontinuity , generally occurs via a well-defined transition zone . This is characterized by a variety of biotite­and plagioclase-bearing 'neta-arkoses ' and abnormally potassium-feldspar-rich ' biotite psanrni.tes' intenrediate in character bet-ween typical rreta-arkose and biotite psamn:i.te; many of them are prominently ronglareratic. Such rocks are l ocally interbanded with homblende­rich ' psanrni.tes' and anphibolitic gneiss . W:?ll exfX)sed sections of the transition zone occur east of Dickens Lake and on the north shore of Johnson Bay (Otter Lake} . Much of the road section for several kilaietres south of Missinipe also exposes the transition zone, an unrecognized fact which has l ed to problems in previous location of major lithologic boundaries.

Correlation and Age Relations

The rretavolcanic, rreta-arkosic and biotite psamnite- conglorrerate belts closely resenble the Wasekwan Group, Sickle C,roup and Burnb-lOod Supergroup, respectively, of the Lynn Lake area of Manitoba (Ccm{:bell, 1972; Zwanzig, 1977). M:lreover there is strike a:mtinuity of the three major asserrblages between Lynn Lake and the southern La Fonge dooain, albeit sonewhat tenoously as regards the rreta-arkoses. At Lynn Lake, the Wasekwan rretavolcanics and Burnb-lOod Supergroup rocks of partiall y equivalent age are overlain stratigraphically by the Sickle Group (meta-arkoses and rretaoonglorrerates}, in places with significant unconfonnity.

In Saskatchewan however, S01Ie writers (SiJ:bald, 1977; Johnston, in prep.) have argued, on the basis of sparse sedinentary younging data , that the rreta-arkoses of the eastern La Fonge domain are older than the main rretavolcanic sequence, i .e. that the general structural position of the three lithological groups represents the true

stratigraphic relation . Present work casts doubt on this interpretation:

a) Well-preserved crossbedding in IW=ta-arkose indicating younging away fran the rreta­volcanics has been discovered less than 30 m from the junction with rretavolcanics on the south shore of Dickens Lake. This datum i s oonsidered to take precedence over other docunented indications of way­up observed farther away from the junction.

b) The main rreta-arkosic belt in the Otter Lake area occupies the core of a ccrcplex Imjor D synform overturrted to the south­east; ~e rretavolcanics overlie the IW=ta­arkoses on the northwestern, structurally inverted l:i..rrb of this synform.

Structure

Structural georretry of the southeastern La Ponge danain is dominated by nunerous short wavelength, dod:>ly-plunging, tight to iso­clinal overturned major~:;> folds about axial surfaces striking approximately northeast and diwing noderately to steeply to the northwest. 'Ihese folds deform a strongly developed D penetrative regional foliation and genera11y ooplanar transposed primary layering. The foliation is axial planar to locally observed coeval D minor isoclines defined by defamed primary ±ayering. Major o1 fold closures are not presently docurrented but their presence is strongly indicated by 'abberrant' outcrop patterns in the hinge region of the major o

2 overturned synform southwest of Otter Lake, in the area east of Dickens Lake and less persuasively e lsewhere.

o1 and o2 s tructures are refol ded by brpersis­tent but widely developed asynnetric open t o closed major and minor o

3 folds developed

about steep northerly to northnorthwesterly trending axial surfaces, as in the Otter Lake area.

The nost prominent linear fabric elerrent in nost rocks of the area a:irrprises strongly developed mineral aggregate streaking, rodding, mineral lineation and often extrare linear extension of rretaconglcrrerate and agglarerate clasts plunging rroderately to steeply to the west or northv.est, approxirrately down the dip of the o1 foliation , normal to the regional structural grain and at a high angle to o2 fold axes. In places such lineation is derron­strably refolded by o2 minor folds suggesting that it is in large part of D age. However its gearetric and strain relation to o1 folds is presentl y not established, nor is tne possibility that sone ccnponent of the bulk strain extension may be of o2 age.

- 26 -

Numerous zones of major tectonic disoontinuity subparallel to regional strike and to o2 fold trends are developed in the eastern La Ronge domain . In many places these appear to represent linb slides or attenuated fold lirrbs of D age, though further detailed mawing and tabric studies are required to confirm this thesis. In other cases the anplification of o3 major folds away from tectonic d.isconti­ntti.tles suggests that the latter may have acted as zones of detachrrent during this fold event: the sense of asymretry of the D folds indicates possible dextral strike displJcenent of this age along such detachrrent zones. They have also been the loci of later brittle fault ll'OVerrel1 ts •

Strain Estimates

The shape of defamed ronglarerate pebbles and other strain indicators suggests exceed­ingly high finite strains in the eastern La Ronge danain. Previous v.urk (Padgham, 1960 , 1963) allied to preliminary quantitative strain ItEasur€'IIEilt and nore general visual estimates by the present writers, indicate that extensional strains in excess of 5:1 in the dominant do..mdip lineation direction are camon and that extensions may locally exceed 10: 1. conparable magnitudes of finite shortening normal to the o1 foliation are also indicated. Such estimates based on pebble deformation probably represent mini.mum values of bulk strain in the rock as a whole . These data, plus the possibility of slide or thrust stacking acoonµmying o2 folding suggest extrene telescoping of the sedinen­tary prism represented by rocks of the eastern La Ronge danain . It is speculated that pre­sent width may be as little as one tenth of original width of the depositional basin/ prism.

Structural Correlation With '!he Glennie Lake [X)main

Air photo interpretation and regional caipi­lation, allied to previous v.urk (Lewry, 1977) , all™s general interpretation of the nature of structural transition and =rrelation of the deformational events be~ the eastern La Ronge and Glennie Lake dooains .

D:Jminant o2 folds of the eastern La Ronge domain tena. to beoooe flatter lying to the east, and in the Guncoat Bay-Glennie Lake area are refolded about n-.o roughly orthogonal later fold sets . '!he earlier of these is devel oped about steep north-northwesterly trending axial surfaces (e.g. the Wapassini synform of Lewry , 1977) and can be correlated with the generally l ess persistent, shorter

wavelength D structures recognized in the eastern La R6nge darain. '!he later (D

4) _ fold

set (e .g. the " 0 311 Glennie Lake-Knight Lake

antiform of Iewfy, 1977) are large wavelength , northeast-trending upright folds characteris­tic of the Glennie Lake danain and have no evident correlatives in the eastern La Ronge danain. The o

1 fold event of the eastern La

Ronge domain is not recognized in the Glennie Lake domain.

Certain problems of supracrustal correlation noted in the Glennie Lake atta (Lewry, 1977, p.33) might be explained by juxtaposition of autochthonous supracrustal belts of the Glennie Lake darain and allochthonous D (and/or D1?) fold/ thrust sheets driven ~ast­wards from the eastern La Ronge dcrrain .

These data suggest significantly different tectonic evolution and strain history in the eastern La Ronge and Glennie Lake domains, possibl y indicating a f undarrentally different crustal structure and depositional history prior to Hudsonian tectonism.

The Stanley Shear Zone

Preliminary re-investigation of the terrain lying adjacent to the Stanl ey 'fault ' included examination of lakeshore e)(!X)Sures in the northern part of Lac La Ronge and detailed road-logging of the ne;,,rly caipleted Stanley Mission road.

The Stanley Shear zone as here defined is a north-northeasterly trending ' straight belt' extending through the eastern part of Lac La Ronge and distinguishable as a marked aero­rragne tic and structural feature as far north as the northeastern end of Gunooat Bay. I t is fran l to 4 km in width and is characterized by strongly linear structural, aeromagnetic and other geophysical trends . Markedly divergent southwest- trending structural and aeromagnetic trends in the western part of Lac La Ronge appear truncated by (or curve into) the ~stern margin of the Stanley Shear Zone; less marked structural discordance with rocks both to east and west of the shear wne i s evident elsewhere .

Rocks within the Stanley Shear Zone are pre­daninantly quartz dioritic to granitic ortho­gneisses (and possible paragneisses) with subordinate banded hornblendic to anphibolitic gneisses, calc-si licate rocks and psamnitic to pelitic gneisses . AJ.l are intensely tecto­nized; foliation varies from a rredium to fine grained gneissi c fabric to protanylonitic or blast.oJ1¥lonitic in character. True blasto­mylonites and augen blastanylonites, transi­tional to ~at coarser grained tectonites,

- 27 -

occur extensively throughout the straight zone, though forming a subordinate part of the belt as a \o.hole. 'Ihey are particularly well exposed on the Stanley Mission road in the Mc:Caffrey Lake vicinity: here streaky to intensely laminated dark grey to black aphanitic bl asto­mylonitic gneisses CX)ntain variable pror:ortions of lensoid streaks and augen of cream to buff feldspar and l ocally large lensoid blocks of less defamed granitoids. 'llle mylonitic foliation is extensively recrystalliz.ed, in places overgrown by equant idioblastic garnet porphyroblasts an::i locally refolded by open to tight ductile flexural folds. All these fea­tures predate brittle fracturing, brecciation and retrogression along late fault zones.

Blastomflonitic to intense tectonite fabrics were noted in a nunber of subsidiary zones striking parallel to general southwesterly­trending structures to the west of the Stanley Shear zone. One such zone coincides with the narked aeranagnetic and structural disconti­nuity which runs just to the northwest of Sulphide Lake in the Stanley (West Half) Area (Forsythe, 1968).

These data suggest that the Stanley Shear Zone was initiated as a high strain zone of intense ductile to plastic shearing at an early stage of Hudsonian tectonism and is not rrerely a belt of postcrystalline brittle fault devel op­ment. Early mylonite develoµient predates prograde ~tanorphism at least within upper­nost greenschist to lCMer anphibolite facies. Observed relationships appear sarrewhat similar to those dOCUil'el1ted in the Tabbernor belt (Macdonald, 1975; Sibbald, 1980) . To the north of Gunooat Bay h:Mever the Stanley Zone appears to be manifested only as a zone of l ate faulting an::i is not evident as a marked structural or aeronagnetic straight zone.

Age Dating

'I\..Q suites of rocks were rollected from the Glennie lake area for Rb/Sr \o.hole rock isochron dating. One was of early quartz dioritic to granodioritic gneisses (Unit 9 ,

E

A \ower Afl,,01> 1c u1111 ' cla s 1 ic:. & VOIC<1n,Cs

::: :: ::;:; :: :::=:= ::,:•••' ''•••:BillJ : : : : : : ; : : : : : : : :

4

: ' • • ai-ic ' • • •: • : : ; : : :

1n1t1n;.1on s

,------.., ------------- ....._ _____ _ Ect sl tn n mar g in

o i Wnfl~"lon 8 dom;,.ir, ! C :.mp.bcll R Gp

. . . • . . . • J

:~~~ ~ ~~~~~~~ ~~~~~~~~- ,~---.~ .. ~!!!J~ci,,~~~

:;:::::;::: ::;::ii~ Cunt ,nen t<1I li t hosphere

Qee,1n1c l 1t ha Spht>re

-------

A.r e t:omplf!a.

M ud1at ik d U fflll!n

Wo\ 1,'\SI0 11

Pt>lnr L Comphtll

I r Rollt'nSIOl\4'1 I I do ,n:1i,1 '

I I

I I

we& hll n La RonQ" dom:tin

I J

I e11-1;1e r n '. ~it Ro.n901

{1oma,ft I I I

G1Esm 11e l dOm a 1n

(microconl1nent?)

Lewry, 19 77) in the Dirks Lake vicinity, the :.~'sti~~ (~~· ~-·r·ij~I~~~~~~~~,~~ other of biotite granodioritic gneisses (Unit'"''- : : : . ; . ; . : . .. , . ·It . . · ·: :_.·. c ',, ~- ;>.:~i ,:.:., ., · •. :· • • _ 10, I.an:y, 1977) frcm the south shore of : ; : : : : : : : : : : : : : : : : : ·. · .. ·.-::: . .;:.,.$~;~/{i:'i{f}" . . :r;:: ·:: : ::. : Planinshek Lake. These represent a further ~::::;:; :::: ::: :·::.1::.:; >~}~:::~./r· -'-: :::::::;: ·:· atterrpt to establish the age of gneisses in ~ 1, . .-: · •• ,.__ • • · •

the Glennie Lake danain. NW

Discussion

W'.>rk recently in print or subnitted for publi­cation (Lewry and Sibbald, 1979, 1980; Ray and Wanless, 1980; ~, 1980; I£!wry et. al., subnitted) permits the develoµrent of a tenta-

----- --- ----·--- - ........ - -

fi g. 3 - Schematic crustal sect ions showing possible plate tectonic evolution of the Rottenstone, la Ronge and Glennie lake domains in sequence from A to£ (after Lewry et. a1 . in press) .

SE

- 28 -

tive preliminary rrode l of geotectonic evol u­tion of the La Ronge and Gl ennie Lake dorrains (Fig. 3) , the principal features of which are as folla-rs :

a) The Rottenstone-Western La Ronge volcano­plutonic arc evol ved above a subducting, northwesterly-dipping l ithospheric plate, the CX)nvergent pl ate nargin lying sal\;!­

where to the southeast of the La Ronge dorrain.

b) The Gl ennie Lake donain CX)nstituted a Sll'all microo:mtinental bloc within the subducting plate M1id1 eventually collided with the southern part of the La Ronge arc caiplex.

c) colli sion culminated with overriding of the t.rend1 zone and telescoping of the arc­trend1 gap sed.immtary prism (represented nCM by the biotite psanm.ite-conglarerate belt and exogeosynclinal rreta-arkose belt). Arc-trench sedirrents and thrust slices were 'back-driven' eastwards over the advancing microcontinental bloc as semi­recurrbent sheets .

d) The nain arc-microcontinent CX)lli sional suture is na-r represented by the Stanl ey Shear Zone . To the north of Gun(X)at Bay even this shear junction nay be overridden by allochthonous sheets derived from the eastern La Ronge domain. Related rrajor thrust zones are evident in parts of the eastern La Ronge domain.

References

Carrpbell , F.H.A. (1972): Stratigraphic and Structural Studies in the Granville Lake­Lynn Lake Region . Manit. Dep. Mines Nat. Resour., Mines Branch, Geol. Pap . 71-2A.

Forsythe, L .H. (1968): The Geol ogy of the Stanley area (~st half) , Saskatchewan. Sask. I:ep. Miner . Resour. , Report 115 .

Johnston, W. G.Q . (in prep.}: The Geology of the Southend area, Saskatchavan. Sask. Dep. Miner. Resour., Report 145.

Macdonald, R. (1975): Semi- reconnaissance in three areas , Pelican Narro...rs (63M}. Sask. c.eol. Surv., sumnary Report, p . 53-57.

Lewry, J . F. (1977): The Geology of the Glenni e Lake Area, Saskatchewan. Sask. Dep. Miner. Resour., Report 143.

(1980): The age and geological history of the ~Haston, Peter Lake and Rottenstone domains in northern Saskat­dlewan: Discussion. Can. J. Earth Sci. (in press).

---~ and Sibbald, T.I.I. (1979): A review of pre-Athabasca basenent geology in northern Saskatchewan: In: G.R. ParslCM, (Editor) , Uraniun Expl orat i on Techniques . Sask. Geol. Soc., Spec. Publ., 3: p. 19- 58.

---- and Sibbald, T.I.I. (1980): Therno­tectoni c evolution of the Qmrchill Province in northern Saskatchewan. Tecto­nophysi cs , 68: p . 45-82.

Ie,;ry, J . F. , Stauffer, M. R. and Fumerton, S. , (sul:initted). A COrdilleran-type batholithic belt in the Olurchill Pro­vince in northern Saskatchewan. Pre­carrbrian Research.

Padgharn, W. A. (1960) : The geology of the Otter Lake area (~st half), Saskatchewan. Sask . Dep. Miner. Resour. , Report 41.

(1963): The geology of the Otter Lake area (east half), Saskatchewan. Sask. Dep. Miner. Resour. , Report 56.

(1966) : The geology of the Guncoat Bay area, Saskatchewan. Sask. Dep. Miner. Resour ., Report 78.

Ray, G.E . and Wanless, R. K. (1980): The age and geol ogical history of the W::illaston, Peter Lake and Rottenstone dona.ins in northern Saskatchewan. Can. J. Earth Sci. , 17, p. 333- 347.

Sibbald , T.I.I. (1977) : The geology of the Mil ton I sland area (~st half) Saskat­chewan . Sask. Dep. Miner. Resour. Report 153.

(1980) : The geology of the Sandy Narrows area (east hal f) , Saskat­chewan. Sask. Dep. Miner. Resour. , Report 170 .

Zwanzig , H.V. (1977): Geol ogy of the Fox Mine area. Mani t . Miner. Resour. Div. , Geol. Surv. I:ep . , Fiel d Activities, 19 77. p . 21-26.