Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

Abstract
The Sioux Lookout orogenic (“greenstone”) belt, a relatively small part of the western Wabigoon
subprovince in northwest Ontario, is composed of the following tectonized stratigraphic units, from north
to south: 1) the Northern Volcanic belt; 2) the Patara Formation; 3) the “Big Vermilion–Daredevil unit”; 4)
the Ament Bay Formation; 5) the Daredevil Formation stratotype; 6) the Central Volcanic belt; 7) the
Minnitaki Group; and 8) the Southern Volvanic belt. The Sioux Lookout belt is bounded to the north by the
Winnipeg River subprovince and to the south by the Basket Lake batholith.
Tectono--stratigraphic analysis of the metamorphosed and variably deformed Archean supracrustal
rocks of the Sioux Lookout belt suggests the accumulation of lower, older, subaqueous basaltic successions
(Stage 1 magnesian tholeiites of MORB to arc character presumably were deposited in a narrow ocean;
Stage 2 iron tholeiites of E--MORB character, ca. 2733 Ma old, represent a juvenile arc setting), followed
by development of a local andesitic volcanic centre (Stage 3, ca. 2732 Ma: a partly rifted arc?), all of which
were then capped by sequences of more felsic volcaniclastic and sedimentary rocks (Stage 4 calc--alkalic
late arc dacites and rhyolites, including adakites, ca. 2714 Ma old). Associated sediments such as turbidite
deposits (e.g., Minnitaki Group, Stage 4) were influenced by synchronous intermediate--felsic volcanism
(interbedded tuffs, volcanic pebble--conglomerate, magnetite iron formation), and within one horizon, distal
coarse clastic sedimentary facies and iron formation can be traced along strike to their probable proximal
volcanic dome source.
A relatively small, 3 km thick, partly subaerial, andesitic stratovolcano succession (pyroclastic and
sedimentary facies, lava flow facies, synvolcanic dikes) has a local (nonregional) distribution, suggestive of
a fault--bounded volcanic basin such as a graben. This well preserved andesitic succession and its
comagmatic dioritic subvolcanic intrusion (Stage 3) were coeval with similar rocks 60 km to the east at
Sturgeon Lake (caldera complex pyroclastic rocks and the Beidelman Bay subvolcanic pluton). Such a
correlation implies that the Sturgeon Lake caldera--hosted VMS deposits formed during Stage 3 tectonism
(interpreted as a rifted arc phase). The sporadic presence of similar ca. 2733 Ma andesitic rocks at
widespread sites in western Wabigoon subprovince suggests that a laterally discontinuous system of coeval
rifts, graben and local andesitic volcanic centres formed at that time.
Geochronological relationships and sub--regional scale structural–stratigraphic map patterns suggest
that approximately northwest–southeast compression segmented the original basin (arc units on early
basalt, buried and flanked by younger volcanically sourced detritus) into an imbricate stack of numerous
thrust slices that generally dip and young to the southeast (Stage 5: a foreland--vergent fold--and--thrust
belt). Fluvial polymict conglomerates (Ament Bay Formation) accumulated in a narrow, linear
(strike--parallel), intra--orogenic “molasse” piggyback basin and were overthrust by older volcanic rocks
forming the bases of thrust slices (relative ages confirmed by published U/Pb zircon dates).
Many of the major faults in the Sioux Lookout orogenic belt appear to be sigmoidal sinistral strike--slip
faults (Stage 6), some of which may be reactivated thrust faults (i.e., Stage 6 superimposed on Stage 5).
Granitoid plutons appear to have intruded along some of these major faults; one small, undated plutonic
complex has sanukitoid characteristics. Sub--regional map patterns suggest that late stage sinistral--sense
strike--slip (to oblique--slip) motions produced features such as: 1) strike--slip duplexes (multi--scale
clusters of 1–10 km long sigmoidal fault--bounded stratal lenses); 2) a pull--apart basin: the Patara
Formation has a 20 km long, lazy S--shaped outline, and its basin fill sequence coarsens up to basalt--clast
fluvial conglomerate (the basalt and accessory BIF clasts being derived from the adjacent “Northern
Volcanic belt”); 3) releasing--bend (dilational jog) plutons (roughly sigmoidal in map view); and 4) a large
“Riedel flake structure” (7 km diameter, rotated counter--clockwise). The regional--scale sinistral faulting is
thought to be a record of movements near the west margin of the “Wabigoon Indenter”, and the structural
style of the northern half of the Sioux Lookout orogenic belt can be considered as the southwest
continuation of the northeast--trending Miniss River Fault.
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