An evolutionary model of the western Churchill province and western margin of the superior province in canada and the north-central United States—discussion

183

~~cfo~o~~~~ic~, 131 (1986) 183-197 Eisevier Science Publishers

B.V.,Amsterdam- Printed in

The Netherlands

AN EVOLUTIONARY MODEL OF THE WESTERN CHURCHILL PROVINCE AND WESTERN MARGIN OF THE SUPERIOR PROVINCE IN CANADA AND THE NORTH-CENTRAL UNITED STATES-DISCUSSION

3.F. LEWRY

‘, K.D. COLLERSON

’ ~eFort~enr

of Geology, Un~ve~si~ of legion

’ Dep~~f~e~f

of Geology, ~~juer~j?~ of Kunsus, Lawrence, KS 66845 (Li.S.A. j

(Received

January

3, 1986; accepted

‘, M.E. BICKFORD

’ and W.R. VAN SCHMUS

*

Regina, Susk. S4S OA? ~C~nu~~~

April 16, 1986)

Greater understanding of the unexposed, south-trending “Dakota Segment” (Lewry, 1984a) of the Trans-Hudson Orogen, which links exposed parts of the Precambrian Shield in northern Saskatchewan/Manitoba with Early Proterozoic terranes in the midcontinental U.S.A., is critical to overall interpretation of Early Proterozoic crustal evolution in North America. The compilation and interpretation of geophysical data for this segment, by Green et al. (1985a, b), is a most welcome and important ~nt~bution to such understanding. We submit, however, that some of their subsurface domainal extrapolation constitutes an overly enthusiastic interpretation of geophysical data unconstrained by geological evidence and thus provides a rather tenuous underpinning for significant elements of their geotectonic model. Moreover, parts of their interpretation are refuted by data which has become available since Green and co-workers’ manuscript went to press and which highlight some of the problems inherent in poorly constrained geophysical interpretation. We concede the inherent difficulties in subsurface extrapolation of lithotectonic domains lying between the Superior and Wyoming-Churchill. cratonic platforms and have no major quarrel with Green et al. regarding location of significant geophysical boundaries. However, their inte~retation of the litholo~~al character of some of the domains lying between those boundaries can legitimately be questioned. Green et al. (1985a) interpret the geophysical “domain” lying between the well-defined southern extensions of the Churchill-Superior Boundary Zone and Tabbernor Zone entirely as a continuation of the Flin Flon-Snow Lake arc and further suggest that it is a dismembered southern extension of the La Ronge arc, displaced along the Tabbernor Zone (see Fig. 1). While this is an intriguing possibility, we consider it to be highly speculative: nor do we consider it essential to the more important aspects of their geotectonic model. The north-south trending magnetic pattern which character&es this subsurface zone differs somewhat from the aeromagn~tic signature of the exposed Flin Flon-Snow Lake Belt, a feature 0040-1951/86/$03.50

Q 1986 Elsevier Science Publishers

B.V.

%=-__..Highly Reworked Oceanic Material $$$$$ -former open oceans, fore - arc % basin or back - arc basins .~$sz Glennie Lake Domain @@!&$ ... ... o&in unknown E.l Peter Lake Complex ~:~:~:~:~I origin unknown I:.:.:.:.:. Fig. 1. (insert Saskatchewan

w,.w+v~Major

by the Editor).

Tectonic

based on Sibbald

units

et al. (1976)

(includes

the Wdlaston,

Flon-Snow

FRB-Fox

bett;

NF-Needle

perior

craton:

1’--. Virgin I-t’-c‘-

R-S1

SR--Seat River

Ronge-Lynn

Wathaman-Chipenyan

Lewry (19X1) and Green

domain;

Indian TB--Thompson

b’R -Virgin batholith.

Inferred

River

belt: shear

geological

domain;

PL --Parker

Lakes belt;

S-- Stanley

and tectonic

I+- -

~~-Nejanilini

Shear zone; fault/fold Wollaston

environments

---Flin

Lake block;

Lake shear zone:

TFF-Tabbcrnor zone:

Lake zone E%-~1.

HI-R -- fianson

M-Mudjatik

and

(lYRO)-(Refer-

C‘ZZ-Cree

River domains):

Lake domain:

granulites:

of Manitoba

(1977. 1980), Lewry

and Broughton craton:

and Virgin

Lake belt:

PB --- Pikwitonei

-Reindeer-South

River

domain:

Mudjatik

Shield

and Sihhald

CC---Churchill

River belt; Gf.---Glennie

I-R- IL -La

---Peter Lake complex;

basin:

Nejanilini.

Falls shear zone:

Canadian

(1977). Lewry

(1978). and Macdonald

AB-Athabasca

Seal River,

Lake belt:

KG--Kisseynew domain:

et al., 1985a).

on the exposed

McRitchie

et al. (1978, 1981. 19X3), Bailes and McRitchie ences in Green

faults

PLC

SC‘-Suzone: domain:

are mostly from

et al. (1985a).

conceded by Green et al. The aeromagnetic trends presumably reflect orientation of major structural “grain” and give little reliable information regarding tithological character of the domain. We thus see little justification for their proposed extrapolation. Moreover, their ~nte~retation involves grave problems regarding the present,

185

and initial, nature and orientation of the domain boundary between the arc and the Kisseynew Domain. This boundary is generally considered to mark transition from the arc to forearc/remnant basin. Green et al’s (1985a) interpretation seems to require either extreme rotation of this domainal boundary from a north-south, to its present east-west, orientation, or insertion of their indicated ad hoc transform offset. There is little persuasive geological evidence for either scenario. Subsurface drill core control for the region lying between extensions of the Churchill-Superior Boundary and the Tabbernor discontinuity is available only in southeastern Saskatchewan and neighbouring North Dakota. In this region, several basement samples yield Archean ages (Peterman and Hedge, 1964; Peterman and Goldich, 1982), suggesting either that highly reworked parts of the Superior craton extend much farther west than Green and coworkers indicate, or that at least the southern part of this geophysical entity is underlain by an exotic sliver of reworked Archean crust, rather than Early Proterozoic arc rocks. Although current data suggest that the Flin Flon and La Ronge-Lynn Lake arcs are approximately coeval, there seems no good reason to believe that they were originally continuous. However, such a possibility cannot be discounted even if the Flin Flon Domain was a short east-west trending arc segment, as suggested by its present orientation and structural geometry (Lewry, 1981). Current re-examination of basement cores within Green and others’ proposed extension of the Clennie Domain (Collerson and Lewry, 1985; Collerson et al., in prep.) suggests the presence of si~ific~tly contrasting internal lithologicaitectonometamorphic components. One hole immediately west of the extension of the Tabbernor zone intersects unretrograded granulite facies felsic gneisses which yield a concordant ca. 1784 Ma U-Pb zircon age (Bickford and Van Schmus, 1985). Another hole in the same region intersects very high-grade pelitic gneisses. Farther west, in the Choiceland area, most holes intersect low-grade metasediments characterised by thick iron formation. The boundary between these contrasting subdomains may be marked by the prominent magnetic low (fig. 4, Green et. al., 1985a) to the west of their proposed location of the Tabbernor zone. Neither assemblage is typical of the southernmost exposed part of the Glennie Domain and the pro~nent east-west discontinuity just south of the Shield edge, noted by Green and others, may indeed mark a significant domainal junction. We would therefore share Green and co-workers’ doubts about the southern extension of this domain. We doubt that geophysical distinction between the La Ronge and eastern Rottenstone Domains (southern part of Green and others’ Reindeer-South Indian Lake belt) can be maintained very far south of the Shield edge. There is no great lithological difference between these two belts in the southern part of the exposed Shield. However, the eastern part of the Rottenstone Domain in this area is marked by the “Birch Rapids Straight Belt” (Lewry, 1984b), a prominent zone of high strain, localised faults and shear zones which is characterised by pro~nent straight magnetic lows. We suggest that the major magnetic low id~tified by Green and

1X6

others

to the south might be a subsurface

not necessarily

reflect a distinct

extension

lithological,

of this structural

as distinct

belt and does

from structural,

entity.

Core data (Collerson and Lewry, 1985, and in prep.) do not support Green and others’ thesis that the magnetically “flat” area in south-central Saskatchewan is an extension

of the

basement

material

Reindeer-South from

Indian

belt

the Regina-Moose

(Rottenstone

Domain).

Jaw area comprises

Available

poorly

to mod-

erately foliated, megacrystic and non-megacrystic monzogranites similar to those forming much of the Wathaman Batholith, and atypical of rocks characterising the eastern dioritic

tonalite-migmatite and supracrustal

complex of the Rottenstone rocks are only encountered

Domain. Tonalitic-granoeast of the interpreted ap-

proximate position of the North American Central Plains conductivity anomaly, which may indeed be directly or indirectly related to the ancient margin of the Churchill-Wyoming continental platform. The majority of core samples available from southwestern Saskatchewan, an area interpreted by Green and others to be an extension of the Wyoming Craton, in fact comprise essentially undeformed anorogenic granites and rhyolites; two drill holes in the area intersect low-grade by older basement. equivalent

quartzites. While the region may indeed be underlain to that of the Wyoming Craton (an interpretation we

would favour), these data again illustrate cal interpretation. Green Segments

the limitations

of unconstrained

geophysi-

and others’ geotectonic model for evolution of Thelon and Dakota of the Trans-Hudson Orogen, involving as it does a more comprehensive

development of the interpretation of Lewry (1981), generally accords with our prejudices. We consider overall interpretation of the Dakota Segment as evolving via a temporally complex, essentially transpressive regime to be an important advance complex

in geotectonic lithostructural

the latter preclude

and

Fox

modelling. In particular. we favour their interpretation of relations along the Thompson Belt and differences between

River

early compressive

prior to transform/ of evolution

Domain.

transcurrent

of this

relationships

between

the adjacent

Churchill

We note

shortening

boundary

displacement: as a viable

the Churchill-Superior Province.

that

Lewry’s

and overthrusting

However,

(1981)

along

it merely emphasised explanation Boundary recent

data

model

“aberrant”

and domains

require

Belt

the latter phase

of present Zone

did not

the Thompson

within

some changes

to

other parts of their interpretation. Recently available U-Pb zircon age dates and Nd isotopic data from the Glennie Domain suggest that interpretation of this terrane as incorporating Archean continental crust (Lewry, 1981) must be re-evaluated. Zircons from “older” orthogneiss components, previously considered representative of probable Archean basement, have yielded only Hudsonian U-Pb ages little different from those obtained from volcanics and plutons of the La Ronge arc (Van Schmus and Bickford, 1984; Bickford et al., 1984). Moreover, calculated cNd values for plutonic and supracrustal rocks in the domain are strongly positive, suggesting derivation from a depleted

187

mantle source with no significant conta~nation by an older crustal component (Chauvel et al., in press). Although structural relations between the Glennie and adjacent domains are consistent with the postulated collisional model of Lewry (1981), these data suggest that the exposed part of the Glennie Domain must be interpreted as part of another dismembered Early Proterozoic arc terrane, or as a package of entirely allochthonous sheets (cf. Lewry, 1984b), rather than an Archean microcontinent. Similarly, although U-Pb zircon data support interpretation of the Sahli monzocharnockite as reworked Archean basement, metarhyolites elsewhere in the Hanson Lake Block, previously dated as Archean (Coleman, 1970), yield Hudsoni~ U-Pb zircon ages. Such data require some modification of all presently published evolutionary models, including that of Green and coworkers. The time framework for orogeny proposed by Green and others (their fig. 4) is not fully consistent with recent U-Pb zircon chronology (Bickford et al., 1984; Van Schmus and Bickford, 1984). Green and others suggest initiation of arc volcanism as greater than 1.9 Ga whereas the oldest ages we have obtained for volcanic rocks in the La Ronge Domain, Glennie Domain or Hanson Lake Block are about 1.88 Ga. More importantly, they suggest that orogeny continued until about 1.6 Ga: we find no support for this in ages of rocks in this region. The youngest age we have found thus far, in the exposed part of the Trans-Hudson Orogen in northern Saskatchewan, is about 1.83 Ga for late- or post-kinematic, little deformed cross-cutting plutons. However, the ca. 1780 Ma age obtained from felsic granulites in drillcore west of the Tabbernor Zone probably represents a metamorphic age and indicates that high-grade thermotectonism continued at least to this date. Although Green and others do not identify the source of their estimates for age of erogenic events, we presume they have relied on some of the published Rb-Sr age data (e.g., Bell and Macdonald, 1982). We believe that Rb-Sr ages of many rocks in the region have been disturbed or reset by subsequent events and most likely do not record either primary igneous crystallisation or times of main erogenic tectonometamorphism. Our points of disagreement with Green and others’ inte~retation are mostly details and in some cases are based on data only recently available. Nonetheless, we consider them significant details. Aside from presenting important new data constraining interpretation in the region under discussion, we would iike to stress the point that geotectonic modelhng based on geophysical interpretation, insufficiently constrained by geological and geochronological data, is at best a speculative and dangerous venture. REFERENCES Chauvel.

C., Amdt,

continental Bell,

K.

and

Saskatchewan.

XT.,

Todt,

W., Kelinzcuk,

crust in Saskatchewan, Macdonaid,

R.,

1982.

Canada:

S. and

Thorn,

(1) Nd isotopic

G~hronoio~cal

Sask. Geol. Suw. Summ. Invest.,

A., in press.

Formation

data. Can. J. Earth.

calibration

1982, pp. 17-22.

of

the

of 1.9 Ga

Sci.

Precambrian

shield

in

Coleman.

L.C..

1970.

Saskatchewan. CoIIerson,

K.D.

correlations Annu.

Rb/Sr

and

Lewry,

J.F.,

with Iithotectonic

Meet.. Fredricton,

Collerson,

isochrons

for

1985.

(zircon)

Orogen. Bickford,

Precambrian

The Precambrian

elements

rocks

in, the

1985, Progr. Abstr.,

geochronology

Can. J. Earth

Hanson

Lake

area,

and

Trans-Hudson

southern

Saskatchewan:

Orogen. GAC-MAC

Joint

p, Al 1.

M.E. and Schmus,

of basement

cores

from

W.R., in prep. Nd isotope syhtsmatics the Dakota

segment

and

of the Trans-Hudson

Sci.

M.E. and Van Schmus,

in Northern

of central

in the southwestern

K.D. and Lewry, J.F., Bickfnrd.

U-Pb

some

Can. J. Earth Sci., 7: 338-345.

W.R., 19X5. Preliminary

Saskatchewan:

new and

revised

results.

U-Pb

age data for the Trans-Hudson

Sask.

C&I.

Surv

Summ.

Invest..

Orogcn 1985. pp.

63-66. Bickford,

M.E., Van Schmus, W.R.. Lewry.

for portions

of the Hudsonian

Am.. 97th Annu. Green,

A.G., Hajnal,

and

western

Green,

Meet., Reno, Abstr.

A.G.. Weber.

Lewry, Lewq,

W. and Hajnal.

Nature, GAC-MAC

Peterman.

Annu.

and

Churchill

Province

the north-central

United

States.

of Proterozoic

terrains

beneath

the Williston

Z.E. and

margins

and suspect

Meet., London,

Lac La Ronge

S.S., 1982. Archean

tectonics

In the Lower

Abstr.,

in the western

Churchill

Proterozoic

Trans-Hudson

p. A64.

and Wapawekka

rocks of the Churchill

Basin Symp. Regina-Sask.

U.S. Gcol. Surv.. Profess.

and

terrancs

1984. Progr.

Hedge, C.E., 1964.Age of basement

areas.

W.R.

collisional

areas

(NTS 73P/731).

Sask.

1984, 34-41.

4th Int. Williston

Van Schmus.

model of the western

in Canada

arc-microcontinent

compilation,

Z.E. and Goldich.

and adjacent

Province

Z.. 1985b. Evolution

Proterozoic

J.F.. 1984b. Bedrock

Dakota.

Geol. Snc.

294: 69-72.

GcoI. Surv. Summ. Invest., Peterman.

report.

13: 624-62X.

J.F., 1984a. Continental

Orogen.

a preliminav

Progr.. p, 445.

of the Superior

J.F., 1981. Lower

Province.

R.. 19X4. U-Pb zircon gcochronology

in Saskatchewan:

116: 281-322.

Basin. Geology, Lewry,

J.F. and Macdonald. terranes

Z. and Weber, W., 1985a. An evolutionaq

margin

Tectonophysics,

erogenic

Bickford.

Geol. Surv. Summ. Invest.,

M.E.,

rocks from the Williston

Pap., 475D:

1984. U-Pb

basement,

Williston

Basin, North

(ieol. Sot., Spec. Pub]. pp. 11-12.

(Zircon)

Basin of North

Dakota

DlOO-DlO4. Gcochronology

in Saskatchewan.

Sask.

1984. pp. 81-87.

AN EVOLUTIONARY MODEL OF THE WESTERN CHURCHILL PROVINCE AND WESTERN MARGIN OF THE SUPERIOR PROVINCE IN CANADA AND THE NORTH-CENTRAL

A.G. GREEN

‘, Z. HAJNAL

I Geokogicul Surrq~ of Cunudu.

UNITED STATES-REPLY

’ and W. WEBER Eurth Ph~sia

’

Brutwh.

1 Obsertwron~ Crewent,

Obtund

One.

KIA

0 Y3

(Catmfu) ’ Deportnwtlt

oj Geologid

.’ Geolniqicul Smllces (Received

March

Sciences,

Universtt?, of Suskarchewun,

Brunch, Depurmlent

11. 19X6: accepted

qf Ettergv ad

.su.klroatl.

Mittes. Winnipeg

susx. 1 S’N 0 l+Yi (~‘unuiful Mutvr.. R3C 4E3 (Cum&)

April 16, 19X6)

During the course of our investigations for the Green et al. (1979. 1985a,b) papers we reviewed in detail the extensive literature on the geology and geochronol-