Late Archean (3.0-2.5 Ga) greenstone belts are a major component of the Superior Province of the Canadian Shield where alternating, metavolcanic - rich and metasedimentary - rich subprovinces form a prominent central striped region bordered in part by high-grade gneiss subprovinces, the Pikiwitonei and Minto in the north, and the Minnesota River Valley in the south. The high-grade gneiss subprovinces are characterized by granulite facies gneiss of plutonic and supracrustal origin, and by abundant plutonic rocks. Minnesota River Valley has rocks older than 3.5 Ga; absolute ages of Pikiwitonei and Minto rocks are unknown but Minto does have north-south structural trends distinctive from the dominant east-west structures of Superior Province. A discussion follows

Card, K. D.

English

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N86-23096 17
TECTONIC SETTING AND EVOLUTION OF LATE ARCHEAN GREENSTONE
BELTS OF SUPERIOR PROVINCE, CANADA: K.D. Card, Geological Survey
of Canada
Late Archean (3.0-2.5 Ga) greenstone belts are a major component of the Superior
Province of the Canadian Shield where alternating, metavolcanic - rich and
metasedimentary - rich subprovinces form a prominent central striped region bordered in
part by high-grade gneiss subprovinces, the Pikiwitonei and Minto in the north, and the
Minnesota River Valley in the south. The high-grade gneiss subprovinces are
characterized by granulite facies gneiss of plutonic and supracrustal origin, and by
abundant plutonic rocks. Minnesota River Valley has rocks older than 3.5 Ga; absolute
ages of Pikiwitonei and Minto rocks re unknown but Minto does have north-south
structural trends distinctive from the dominant east-west structures of Superior
Province.
Volcano-plutonic subprovinces of'
Superior Province consist of generally narrow;
sinous greenstone belts bordered and intruded
by voluminous plutonic rocks, including
tonalitic gneiss, synvolcanic plutons, and
younger foliated to massive, generally
composite plutons, ranging from quartz
dorite to granite and syenite. Supracrustal
rocks of the greenstone belts include
komatiitic, tholeiitic, calc-alkalic, and rare
alkalic volcanics with volcanogenic clastic
(wacke, conglomerate) and chemical (iron
formation, chert) sediments. Most,
greenstone belts consist of several lensoid/
overlapping piles each on the order of 100 km
in maximum dimension and approximately 5
to 10 km thick and commonly comprising!
several volcanic cycles. Some cycles consist of a lower komatiitic -tholeiitic basalt
sequence, a middle tholeiitic basalt -andesite sequence, and an upper calc-alkalic dacite-
rhyolite-andesite sequence. Other cyles are bimodal tholeiitic basalt-dacite (rhyolite)
sequences. Minor alkalic and shoshonitic volcanics and associated alluvial/fluvial
sediments are present in some belts where they unconformably overlie older volcanics
and synvolcanic plutons. In term of rock types, sequences, and overall configuration,
many Superior Province greenstone belts are closely comparable to modern island arcs.
Superior Province greenstone belts typically have upright folds with curved,
bifurcating axial surfaces, steep foliations and lineations, and major domal culminations
and depressions, the products of polyphase deformation. Some belts, however, display
low angle foliations and faults, overturned sequences, and recumbent and downward -
facing structures suggestive of thrust-nappe style tectonics (1,9,10).
The enclosing gneissic and plutonic rocks display domal structural patterns, again
the product of polyphase deformation involving recumbent folding and diapirism.
Metamorphic grade in the greenstone belts is generally subgreenschist to greenschist in
the central parts grading outward to low pressure amphibolite facies in belt margins and
surrounding plutonic gneisses.
*.•»' Proleroioic Plwn.roxoic locks
—•-• Subfxownc. boundwy
UtCHEAN SUBPflOWNCE TYPE
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18 TECTONIC SETTING AND EVOLUTION OF LATE ARCHEAN
K.D. Card
The contacts between greenstone belts and enclosing plutonic rocks, and between
the greenstone-rich subprovinces and adjacent plutonic subprovinces are generally either
intrusive or tectonic. An unconformity between greenstones and older granitoid rocks
has been demonstrated only at Steeprock, Ontario(5) and although younger volcanics and
older plutonic rocks are juxtaposed in a number of places, faults, mylonites, or shear
zones invariably intervene. Dextral transcurrent faults trending EW and NW and sinistral
faults trending NE form subprovince boundaries in part, as do NE and EW trending
thrusts. One notable product of this faulting, the Kapuskasing Structural Zone, exposes
granulites considered to represent upthrusted lower crust (7,8). Late alkalic volcanic -
fluviatile sediment sequences are spatially related to major transcurrent faults and may
represent deposition in pull-apart basins formed by alternating periods of transtension
and transpression in strike-slip zones.
Interpretation of geophysical data
shows changes in depth to the Conrad
Discontinuity and to the Moho from one
subprovince to another, indicating
significant structural relief across their
faulted boundaries (*). Greenstone belts of
Abitibi and Wabigoon subprovinces generally
extend to depths of only 5 to 10 km(3)
whereas metasedimentary gneisses of
English River Subprovince and plutonic
rocks of Winnipeg River Subprovince may
extend to depths of 10 to 20 km(4).
Juxtaposition of high-pressure granulites of
the Kapuskasing zone with low-pressure
greenschist-amphibolite facies rocks of
Abitibi Subprovince implies structural relief
of 15 to 20 km across the boundary thrust
(7,8).
Metasedimentary
(English River, Quetico,
subprovinces
Pontiac etc.)
LEGEND
Fault
Tnrual twit
Tranacurraflt lautt
Normal fault
Province boundary
consist mainly of turbidite wacke and pelite metamorphosed at grades ranging from low
greenschist at belt margins to upper amphibolite and locally, low-pressure granulite in
belt interiors. Anatectic, s-type granitic rocks are prevalent in the migmatitic, high-
grade interiors of the metasedimentary belts.
Most metasedimentary subprovinces have a linear aspect attributable to
transcurrent boundary faults and isoclinal folds with subhorizontal to subertical axes,
late structures superimposed on earlier complex, recumbent folds and dome-basin
structures. In areas where contacts between metasedimentary and volcano-plutonic
subprovinces are unfaulted, there appear to be rapid facies transitions from sedimentary
to dominantly volcanic sequences. Preliminary isotopic age data also indicate that the
sedimentary and volcanic sequences of some adjacent subprovinces are broadly coeval.
U-Pb zircon dates demonstrate that volcanic, plutonic, deformational, and
metamorphic events of relatively brief duration affected large parts of Superior
Province and that there are detectable differences in ages of these events from one area
to another(6). In the northwest (Sachigo, Berens, Uchi subprovinces) major volcanism and
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.4"
TECfofjIC^SETTING AND EVOLUTION OF LATE ARCHEAN
CARD, K.D.
accompanying plutonism occurred at about
3.0 to 2.9 Ga, 2.85 to 2.80 Ga, and 2.75 to
2.7 Ga. These voclanic episodes were
followed by major deformation,
metamorphism, and plutonism about 2.73 to
2.7 Ga. In the south (Wabigoon, Wawa,
Abitibi subprovinces) volcanism and
plutonism occurred mainly between 2.75 and
2.69 Ga, followed by major deformation,
metamorphism, and plutonism at about 2.70
to 2.66 Ga. There is evidence for somewhat
younger (2.65 to 2.63 Ga) metamorphic-
plutonic events, or of later closure of
isotopic systems, in the high-grade rocks of
the metasedimentary belts and of the
Kapuskasing zone.
In summary, Superior Province
consists mainly of Late Archean
supracrustal and plutonic rocks with Middle
Archean gneisses in the south and possibly in
the north. The Late Archean supracrustal
sequences are possibly mainly of island-arc and inter-arc affinity, although continental
rift zone settings have also been postulated(2). Abundant plutonic rocks include early
synvolcamc intrusions and later synorogenic and post-orogenic intrusions derived in part
from the mantle and in part from crustal melting caused by thermal blanketing of newly-
thickened continental crust combined with high mantle heat flux.
The contemporaneity of magmatic and deformational events along the lengths of
the belts, coupled with the structural evidence of major compression and transcurrent
faulting, is consistent with a subduction-dominated tectonic regime for assembly of the
Superior Province orogen. Successive lateral and vertical accretion of volcanic arcs and
related sedimentary accumulations, accompanied and followed by voluminous plutonism,
resulted in multi-stage crustal thickening and stabilization of the Superior craton prior
to emplacement of mafic dyke swarms and Early Proterozoic marginal rifting.
REFERENCES
1. Attoh, K. (1981); Geological Survey of Canada, Paper 81-1B, p. *9-5«f.
2. Ayres, L.D. and Thurston, P.C. (1985): Geological Association of Canada, Special
Pa n«*r / f i n ^ / i / i ^ Q / l * *^ *-•»**
reaper £O) p*
 <2*TlT"Oov/«
3. Gupta, J.K., Thurston, P.C. and Dusanowskyj, T.H. (1982): Precambrian Research,
v. 16, p. 233-255.
*' Hfji D*H* and Brisbin> w-c- <1982)' Candian Journal of Earth Sciences, v. 19, p.20<*9-
19
5. Joliffe, A.W. (1955): Economic Geology, v. 50, p. 373-398.
6. Krogh, T., Corfu, F. and Davis, D.W. (1985): G.A.C. Spec. Paper 28, p. 358.
7. Percival, J.A. and Card, K.D. (1983): Geology, v. II, p. 323-326.
8. Percival, J.A. and Card, K.D. (1985): G.A.C. Spec. Paper 28, p. 179-192.
9. Poulsen, K.H., Borradaile, G.J., and Kehlenbeck, M.M. (1980): Canadian Journal of
Earth Sciences, v. 17, p. 1358-1369.
lO.Thurston, P.C. and Breaks, F.W. (1978): Geological Survey of Canada, Paper 78-10,
p. "9—62.
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Tectonic setting and evolution of late Archaean greenstone belts of Superior Province, Canada

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